7
9
25274 75349
0 7>
CIRCUIT
CELLAR
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T H E M A G A Z I N E F O R C O M P U T E R A P P L I C AT I O N S
$4.95 U.S. ($5.95 Canada)
#168 July 2004
GRAPHICS & VIDEO
Waveform Monitor
Smart Touchscreen User Interface
Color Video Frame Grabber
Multimedia Thermometer
Digital Oscilloscopes
•
2 Channel Digital Oscilloscope
•
100 MSa/s
max single shot rate
•
32K samples per channel
•
Advanced Triggering
•
Only 9 oz and 6.3” x 3.75” x 1.25”
•
Small, Lightweight, and Portable
•
Parallel Port
interface to PC
•
Advanced Math options
•
FFT Spectrum Analyzer options
DSO-2102S
$525
DSO-2102M
$650
Each includes
Oscilloscope
,
Probes, Interface Cable, Power
Adapter, and software for
Win95/98, WinNT, Win2000
and DOS.
•
40 to 160 channels
•
up to 500 MSa/s
•
Variable Threshold
•
8 External Clocks
•
16 Level Triggering
•
up to 512K samples/ch
•
Optional Parallel Interface
•
Optional 100 MSa/s Pattern Generator
LA4240-32K (200MHz, 40CH)
$1350
LA4280-32K (200MHz, 80CH)
$2000
LA4540-128K (500MHz, 40CH)
$1900
LA4580-128K (500MHz, 80CH)
$2800
LA45160-128K (500MHz, 160CH)
$7000
Logic Analyzers
• 24 Channel Logic Analyzer
• 100MSa/S max sample rate
• Variable Threshold Voltage
• Large 128k Buffer
• Small, Lightweight and Portable
• Only 4 oz and 4.75” x 2.75” x 1”
• Parallel Port Interface to PC
• Trigger Out
• Windows 95/98 Software
LA2124-128K (100MSa/s, 24CH)
Clips, Wires, Interface Cable, AC
Adapter and Software
$800
All prices include Pods and Software
P
resentation is everything. You don’t have to look any further than
your local mall or supermarket to see the major effect of display.
Manufacturers spend millions creating packages that sell—packages
that clearly and concisely relay essential information about the product
they contain and look good doing it. You might call it user-friendly pack-
aging. The package is an extension of the advertising campaign that has
been painstakingly researched and tested to ensure sales. In fact, the
way in which products are displayed has become so important that col-
leges now offer degrees in supermarket display and packaging.
When designing electronic displays, the same principles apply. The
goal is to create a display that is eye-catching, readable, easy to under-
stand, and most likely easy to program (if you’re making the display for
a client). We have a couple of articles this month that discuss excellent
display projects ideally suited for use by consumers. Alberto Ricci Bitti’s
DVD-Thermometer (p. 12) and Bruce M. Pride’s Smart-E-Touch (p. 56)
are definitely attractive designs that meet the demand for user-friendly
technology. Anyone who followed the Motorola Flash Innovation 2003
Design Contest will be especially interested in these contest winners.
Alberto designed a system to display temperature on a TV with full-
color graphics, which can be still or animated. Graphics that correlate to
various temperatures (for example, a chilly Linux penguin with the back-
drop of snow-capped mountains for 15°F) are stored on a DVD. Alberto
imagines his DVD-Thermometer being used in public spaces like malls
or retail shops, perhaps incorporating advertising. The DVD-
Thermometer’s versatility lends itself to a variety of other uses as well.
You can use the system to gather and display any type of data.
Bruce’s Smart-E-Touch is an intelligent touchscreen user interface
that’s perfect for just about any setting. The design relies on a Motorola
Nitron microcontroller to control and monitor Motorola e-field sensors.
Using the e-field sensors enabled Bruce to build the user interface direct-
ly on the circuit board, which makes the system extremely reliable, not to
mention easy to create.
In addition to these great projects, we have a number of other equal-
ly terrific graphics and display projects. The video frame grabber
designed by Eric Gagnon digitizes a single image of full-field color video
into a field buffer, and offers a cost-effective solution to traditional off-the-
shelf systems (p. 22). The waveform capture and display system creat-
ed by Dick Cappels is designed around three Atmel microcontrollers (p.
42). This simple project, which consists of a graphic display unit and a
capture and control unit, is a great addition to your workbench. Last, but
not least, we have a graphics library written in C for the Zilog Z8 Encore!
family designed by Brian Millier (p. 62). Brian’s library enables Z8 micro-
controllers to utilize the full functionality of graphic LCD panels.
4
Issue 168 July 2004
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6
Issue 168 July 2004
CIRCUIT CELLAR
®
www.circuitcellar.com
July 2004: Graphics & Video
TASK MANAGER
Presentation is Everything
FEATURES
COLUMNS
DEPARTMENTS
Uncomplicated dsPIC Implementation
Linear’s LTC6903/4 Programmable Oscillator
Jeff Bachiochi
Adaptable Multimedia Thermometer
Alberto Ricci Bitti
Flash Innovation 2003 Design Contest Winner
Full-Field Color Video Frame Grabber
Build an SMT Reflow Oven Controller
Robert Lacoste
Renesas Design 2003 Grand Prize Winner
Designing with the Nios (Part 2)
System Enhancement
George Martin
Triple AVR Waveform Capture and Display
Three Controllers Make a Waveform Monitor
Dick Cappels
An Intelligent User Interface
Bruce M. Pride
Flash Innovation 2003 Design Contest Winner
Graphics LCD Library for the Z8 Encore!
DVD-Thermometer (p. 12)
Smart User Interface (p. 56)
Waveform Monitor (p. 42)
converts base-band analog video into digital video, sup-
porting up to 10 component video inputs including com-
posite (NTSC, PAL and SECAM), S-Video, and component
(YPbPr) video signals.
The device also includes an analog video output for TV
video out signals and connection to other external devices.
Video signal processing has been optimized to produce high-
quality video from weak broadcast signals or nonstandard
VCR video. Additionally, the decoder outputs ITU-R BT.656
and supports Macrovision copy protection as well as advanced
VBI functionality. The TVP5147 includes two 10-bit, 30-msps
ADCs. Each analog channel contains an analog circuit,
which, prior to each converter, clamps the input to a ref-
erence voltage and applies a programmable gain and off-
set. The TVP5147 digital video decoder includes a 2-D five-
line adaptive comb filter.
Evaluation boards, software tools, and application notes
are available for the TVP5147. The device costs $9 in
1,000-piece quantities.
Texas Instruments, Inc.
www.ti.com
8
Issue 168 July 2004
CIRCUIT CELLAR
®
www.circuitcellar.com
NEW PRODUCT NEWS
HIGH-QUALITY DIGITAL VIDEO DECODER
The new TVP5147 is a high-performance digital video
decoder (NTSC/PAL/SECAM) that digitizes and decodes
all popular base-band analog video formats into digital
component video. The device is a mixed-signal video
decoder optimized for use in the U.S., Japan, and China. It
Edited by John Gorsky
HIGH-RESOLUTION DISPLAY
Half and full VGA-size LCDs put the power of high-reso-
lution graphics and detailed screen content in the hands
of embedded developers. High-resolution LCDs make the
GUI ideal for developing innovative point-of-sale termi-
nals and introducing unique touchscreens for high-end
consumer electronics.
Creating attractive GUI screen content is as easy and
as time-saving as with the Graphics OS in silicon; how-
ever, the final output can now be more elaborate and
ornate. These higher-resolution displays enable more
content per GUI page as well. This solution comes with
its own microprocessor, graphics library, and LCD con-
troller so that the host microprocessor or microcon-
troller is never burdened by the GUI’s operation.
The half VGA-size LCD is 5.4
″
with a resolution of 320 ×
480 pixels. The full VGA-size LCD is 7.5
″
with a 480 ×
680 resolution. The new displays complement a line of
modules that supports a 5.7
″
QVGA LCD with a resolu-
tion of 240 × 320 pixels.
The half VGA-size LCD is available for $150 each for
production volumes
of 10,000 units or
more. The full-size
VGA LCD is available
for $250 each for pro-
duction volumes of
10,000 units or more.
Amulet Technologies
(408) 244-0363
www.amulet
technologies.com
LOW-CURRENT, SINGLE-DIGIT, SEVEN-SEGMENT
DISPLAYS
The new TDSR family of high-intensity, low-current,
single-digit, seven-segment displays provide excellent read-
ability even in direct sunlight. The displays, which use
AllnGaP technology in the color red, offer high performance
for extremely low-power applications, delivering a typical
luminous intensity of 1,500 µcd while being driven with
just 1 mA of current.
Featuring a wide
viewing angle, the
new displays can be
used in panel meters,
test and measure-
ment equipment,
point-of-sale termi-
nals, control units,
and battery-operated
systems requiring
extremely low-power consumption.
The six TDSR displays offer designers a choice of com-
mon anode (TDSRxx50) or common cathode (TDSRxx60)
versions with minimum and maximum luminous intensity
ratings from 180 to 3,600 µcd, respectively, enabling opera-
tion in direct sunlight or in “black front” designs using
tinted filter glass. A maximum forward current of 10 mA is
allowed for an ambient temperature range of –40° to 85°C
without current derating.
Pricing in 10,000-piece quantities is $60 per 100 pieces.
www.circuitcellar.com
CIRCUIT CELLAR
®
Issue 168 July 2004
9
NEW PRODUCT NEWS
INDUSTRIAL GRADE FLAT-PANEL DISPLAY INTERFACE
The new industrial grade AD9883A is the industry’s
first fully integrated analog interface to operate over the
industrial temperature range of –40° to 85°C. The device
enables superior image quality in rugged environments
that require wider temperature ranges and robust design
features. The AD9883ABST is ideally suited to automotive
and aerospace applications where displays operate in rigor-
ous, taxing environments and need to withstand extreme
fluctuations in temperature.
The AD988A is optimized for XGA and SXGA LCDs,
HDTV, and Advanced TV applications. The AD9883ABST
features three high-bandwidth, 8-bit, 140-msps ADCs and an
ultralow jitter PLL (less than 500 ps at 140 MHz), ensuring
maximum robustness and image quality. Its architecture is
compatible with LCD, plasma, projection, and other types of
digital display technologies. The IC performs other functions:
composite sync processing, mid-scale clamping, input signal
detection, automatic power-down control, and programmable
gain/offset control—all of which help maximize ease-of-use
and robustness in graphics and video applications.
The AD9883ABST flat-panel interface, which is available
in 80-pin LQFP packaging, is sold in both industrial and
commercial temperature-grade versions.
The part costs $8.50 per unit in 1,000-piece quantities.
www.circuitcellar.com
CIRCUIT CELLAR
®
Issue 168 July 2004
11
You may contact the quizmasters at eq@circuitcellar.com
CIRCUIT CELLAR
—
Test Y
Your E
EQ
Problem 3
—
Each phase of a three-phase power
system is a sine wave displaced by 120° from the pre-
vious one. If you connect a resistor across two of the
phases, what is the waveform of the current that flows
in it?
Problem 4
—
Demonstrate the answer to Problem 3
mathematically.
Contributed by David Tweed
Edited by David Tweed
Problem 1
—
A residential power meter contains two
coils that together produce a torque on the rotor disc
that is proportional to the instantaneous power flowing
in the load. The mechanical inertia of the disc aver-
ages this torque over several cycles.
Power factor correction (PFC) circuits come in two
flavors, one that corrects for reactive (inductive or
capacitive) loads, and one that corrects for nonsinu-
soidal current waveforms.
If the load is an inductive load, such as a lightly
loaded motor, what is the effect of adding a PFC of the
first type to it on the power meter reading? In other
words, does the meter run faster, favoring the power
company, or does it run slower, favoring the customer?
Problem 2
—
If the load is a nonlinear load—such as
a large capacitor-input power supply that draws cur-
rent in relatively narrow spikes at the peaks of the AC
voltage waveform—what is the effect of adding a PFC
of the second type to it on the power meter reading?
mind. As a result, all of the modules
reside in separate files that are ready to
be ported to other applications by chang-
ing only a few
#define commands.
Now I’ll describe how I managed to
transform a DVD player into an eye-
catching video thermometer. Even if you
aren’t interested in taking temperatures,
you can adapt these basic techniques to
display any sort of data. These techniques
are worth adding to your bag of tricks,
just in case. Along the way, you can col-
lect some valuable, reusable software
modules, like a versatile programmable
pulse generator, an I
2
C bus driver, and a
RECS80 remote control transmitter.
SYSTEM BASICS
Let’s examine the building blocks of
the system (see Figure 1). A specialized
digital temperature sensor IC measures
ambient temperature and converts it
into a digital word that can be read by
the microcontroller via the I
2
C bus.
The microcontroller doesn’t have a
hardware I
2
C, so the I
2
C driver is imple-
mented in software. This driver works
as the foundation for the thermometer
chip driver, which is, in turn, required
by the main application firmware.
Another software module makes the
microcontroller’s internal 16-bit timer
work as a programmable pulse generator
(PPG). The RECS-80 remote-control
encoder block assembles these pulses in
order to drive an infrared LED connected
to a port pin. The sleep manager module
ensures that all of the previous blocks
12
Issue 168 July 2004
CIRCUIT CELLAR
®
www.circuitcellar.com
I
built an innovative multimedia
thermometer that displays tempera-
ture readings on an ordinary TV with
crisp, full-color graphics. To add a lit-
tle magic to a reading, you can ani-
mate pictures and program CD-quality
audio announcements to accompany
each temperature change. The pictures
can vary according to your application.
The system is perfect for a wide range
of settings. If you use it in a retail shop
or mall, you can complement logos and
product advertisements with tempera-
ture information. On the domestic
front, photos of the kids playing at the
beach or, should the temperature drop,
making a snowman are nice gifts for
Grandma. The thermometer would also
work well at hotels, airports, sporting
venues, and tourist sites (see Photo 1).
DVD-THERMOMETER
Despite its rich feature set, my
DVD-Thermometer is an amazingly
simple design that requires few com-
ponents. Another advantage is that it’s
hundreds of dollars cheaper than typi-
cal PC-centric solutions. I made the
DVD-Thermometer at home, just for
the fun it, without special tools. All I
used were a soldering iron, a multime-
ter, a PC with ordinary CD-R burner,
and the Motorola QT demo board.
The circuit, which is battery-operated,
fits comfortably in a small plastic box
that’s located in front of an ordinary DVD
player. A switch and infrared LED peep
out of the box. The DVD player is loaded
Adaptable Multimedia
Thermometer
Alberto’s 68HC908QT4-based multimedia thermometer displays temperature readings in full
color on a TV screen. What’s more, you can add animated pictures along with high-quality
audio to announce each temperature change. If you don’t need a temperature-taking system,
you can use the design techniques described here to display any other type of data.
with a disc that’s prerecorded with pic-
tures that correspond to different temper-
atures. Acting as an intelligent infrared
remote control, the DVD-Thermometer
directs the player by selecting the scene
that matches ambient temperature.
The circuit diagram is embarrassingly
simple: it’s entirely built around an inex-
pensive eight-pin microcontroller, a digi-
tal temperature sensor, and an infrared
LED. All of the hard work has to do with
the software, which is the most impor-
tant ingredient of the project. I wrote the
program in C language, implementing
low-power techniques with code reuse,
portability, and data encapsulation in
FEATURE ARTICLE
by Alberto Ricci Bitti
Photo 1—
One of the great features of this project is
that you can customize the graphics. Here is Tux, the
Linux penguin, commenting on the temperature.
it receives from the circuit.
The adjustment is repeated
until the temperature on the
TV matches ambient temper-
ature. There is no feedback
from the DVD player telling
the microcontroller which
image it’s playing, so the
microcontroller and player
must start from a well-known
temperature/picture pair. To
ensure the DVD player always
starts from the first picture in
the sequence, the microcon-
troller stops and restarts the
disc during start-up.
REMOTE CONTROL HACKING
I started the design process by
reverse-engineering the remote control
that came with my DVD player.
picture on the TV. The DVD driver uses
the RECS-80 embedded remote control
to direct the DVD player to the appro-
priate position. The DVD player cannot
tell the difference between a real
remote control and the infrared stream
www.circuitcellar.com
CIRCUIT CELLAR
®
Issue 168 July 2004
13
are called in a timely manner
and that power isn’t wasted.
The infrared LED is placed
in front of an ordinary DVD
player connected to a TV. The
DVD player is loaded with a
special disc containing a prere-
corded sequence of pictures,
each of which shows a differ-
ent temperature value. When
played normally, the disc
shows all of the temperatures
in the thermometer range,
from the lowest to the highest.
Refer to the “Disc Production”
sidebar for more information.
When the system runs, the main
program continuously evaluates ambi-
ent temperature. If it differs from the
on-screen temperature, the system
instructs the DVD driver to change the
Temperature
sensor
Microcontroller
Main program
Thermometer
driver
I
2
C Bus
driver
Sleep manager
DVD Driver
RECS-80
Remote
control
Programable
pulse
generator
SDA
SCL
Infrared
LED
DVD Player
DVD Media
(recorded with graphics
and sound)
TV
Figure 1—
One of the building blocks is an entire DVD player that’s used as a
video generator. The remaining hardware consists of a digital temperature sen-
sor, an infrared LED, and a microcontroller. The software is the most important
ingredient because of the many levels of nested drivers.
DISC PRODUCTION
The disc is an essential part of the system because the
images, videos, animations, and sounds are recorded on it.
Skipping the artistic part of the process, let’s focus on the
technical aspects of the procedure.
Of course, you can use your favorite recording and video-
editing software to burn the disc. Strictly speaking, it will
work if it can burn a disc with a sequence of images that your
DVD can browse back and forth through by way of the Next
and Previous keys.
I searched for the easiest way to record a test disc. A DVD
burner is unnecessary. If your thermometer doesn’t require the
DVD player’s full 4.7-GB
capacity, an inexpensive CD-R
burner is sufficient. I used CD-
RW media to perform different
experiments recycling the
same disc. You must ensure
that your DVD player can read
CD-R or CD-RW discs.
I prepared a set of 62 frames,
each of which is stored in a
separate JPG file on the PC
hard drive, representing
temperatures between –10° and 50°C, plus one frame with
the message “battery low” (see Figure 1). Newer DVD
players can automatically play JPG and MP3 files located
in the root directory of a CD-R or CD-RW disc. This is
definitely the easiest way to get started with the DVD-
Thermometer. It’s worth a try.
Unfortunately, my DVD player refused to read such a disc,
so I made a second attempt at recording the CD-RW disc in
the video-CD format. This method worked well.
I prepared my disc with Nero Burning ROM 5.5, a popular
software solution for burning CD-Rs. It features a CD-R
wizard that makes the process a piece of cake.
Click on the File/New menu, and the New Compilation
window will appear. From the left banner, select the Video-
CD format. Don’t touch the defaults (except deselecting
thumbnail generation if it’s enabled. Confirm the new com-
pilation by clicking on the New button. The program contin-
ues with the familiar selection of the files to be included on
the disc, with an Explorer-like directory tree interface. Drag
and drop all of your 62 JPG files on the new project. Pay
attention to the order: the first file you drop must be the first
file that appears in the final
sequence. Multiple selection
and cut and paste work well,
provided that you select the
original files in the right
order. I suggest you give the
files new names so they can
be sorted alphabetically.
Then, sort the folder by file
name and do a multiple
select by dragging the mouse
from top to bottom.
That’s it. If you click on the Burn icon, the program
will format the disc according to the Video-CD specifica-
tions and create all the necessary support files required
by the DVD player. I recommend that you choose the
lowest possible burn speed. When you’re finished, test it
on the DVD player to verify that all of the images are in
the correct sequence, passing from one another, by press-
ing the Next and Previous keys. This is also the best
time to read the user manual in order to discover the pro-
cedure for disabling the on-screen display.
Figure 1—
I implemented readings from –10° to 50°C in 1°C steps. Therefore, I set
DVD_LAST_PICTURE to 61 (62 pictures in total). The last picture appears when the
microcontroller detects an insufficient battery level (approximately 4.3 V on my proto-
type). You may download this sample picture set from the Circuit Cellar ftp site.
14
Issue 168 July 2004
CIRCUIT CELLAR
®
www.circuitcellar.com
weekend and wasn’t able get one until
the following Monday. So, I used the
PC sound card to analyze the signal.
I connected an infrared LED (the same
one I used in the final circuit) to the mic
input in order to use it as a sort of photo-
voltaic sensor. I placed the remote con-
trol in front of it (as close as possible) and
watched the signal with a sound-editing
program. The waveform suffered because
of the rude measurement technique, but
it was clearly discernible as one of the
popular codes described in Other’s article.
Fortunately, I didn’t need to open it.
All I wanted to know was how it
encodes its various keys. The market
is shared among a handful of different
infrared modulation schemes, but I
was lucky enough to find that the
remote control conformed to one of
the standards whose documentation
was readily available in A. N. Other’s
article “IR Remote Control Codes”
(Elektor Electronics, March/April 2001).
Although an oscilloscope is prefer-
able, I started the design on a rainy
The good news is that the remote control
or my Irradio DVD player conforms to
the RECS80 format. The bad news is that
the RECS80 waveform is difficult to
encode because it involves synthesizing
pulses whose duration spans many orders
of magnitude, from microseconds to
hundreds of milliseconds.
A complete waveform anatomy is doc-
umented in Philips’s “SAA3004 Remote
Control Transmitter” datasheet. In brief,
a RECS80 code word is a sequence of
short pulse bursts. The basic burst
requires six cycles of the 38-kHz carrier
frequency with a 33% duty cycle. This
requires asymmetrical, nail-shaped
pulses just 8.77 µs wide.
Bits are encoded using PPM modula-
tion. In PPM code, the time between
two consecutive bursts represents a sin-
gle bit of a code word. In the case of
RECS80, an interval of 5.06 ms repre-
sents a zero, whereas a 7.6-ms interval
means a one. An entire code word
counts 12 pulses, encoding 11 bits of
information including a bit that toggles
each time a new key is pressed. To
ensure that the code is received without
errors, the same word is repeated many
times, with a repetition period of 0.12 s.
Note that because encoded PPM code
words differ in length, the duration of
the pause between repetitions must
change accordingly to compensate.
As you observe your remote control
waveform, I suggest you take note of
the key codes involved in the DVD
scene selection process. I guess there
are standard tables somewhere, but
because there are only four keys to
use, I found it convenient to transcript
their binary values watching the emit-
ted waveform. After the start pulse,
every short pause means a zero,
whereas a longer one means a one.
Photo 2 highlights the keys of inter-
est. The Next key, which allows you
to skip to the next scene in the DVD
sequence, is opposite the Previous key,
which allows you to go back. The
Next/Previous pair is all you need to
search for a specific picture.
A few other keys are useful during
start-up because the software uses
them as a sort of reset, forcing the
player to always start from the first
scene on DVD disc. In the case of my
DVD player, these keys include Stop,
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CIRCUIT CELLAR
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15
which stops disc playing, and Play,
which starts playing the disc from the
beginning after the disc has stopped.
HARDWARE SIMPLICITY
With just eight parts and a battery,
the hardware could not be simpler (see
Figure 2). Circuit optimization is a
corollary coming straight from the level
of integration of its main components.
The temperature sensor IC is either
a DS1621 or DS1631. This eight-pin-
ner includes a complete digital tem-
perature sensor/thermostat with I
2
C
output. With the appropriate com-
mands, the temperature can be read
from the serial interface with a guar-
anteed accuracy of 0.5°C and a resolu-
tion of up to 0.1°C. It does not require
calibration, which is advantageous
because you can trust your ther-
mometer without messing with ice
and boiling water. Pin 3 works as a
thermostat, but this feature is unused
in this application. Pins 5 through 7
determine the low bits of the device’s
I
2
C address. (Up to eight sensors can be
connected in parallel on the same I
2
C
bus.) Setting the connections as shown
in Figure 2 configures the device to
respond at address 0x90.
I used the MC68HC908QT4, a tiny
eight-pin device with 4 KB of flash
memory and 128 bytes of RAM. A
nice characteristic is that all of its
pins (except power) can be used as
I/O, because proper reset is guaran-
teed by means of internal brownout
circuitry and a flash-calibrated inter-
nal oscillator provides a reasonably
stable clock source. Timing precision
is important to get correct remote
control waveforms. The 1% specifica-
tion for the internal oscillator is not
far from the accuracy of ceramic res-
onators, which are commonly
employed in remote controls.
Two I/O pins serve as I
2
C bus data
and clock. All MC68HC908QT4 pins
feature programmable pull-ups, so exter-
nal resistors on the I
2
C bus are unneces-
sary. Another I/O port drives the IR
LED (ports can source up to 25 mA)
through a current-limiting resistor. Peak
current is set to 8 mA—much lower
than a real remote control—because the
device is designed to operate in close
proximity to the DVD player. Excessive
LED power may interfere with other IR
devices in the same location (e.g., air
conditioners, TVs, VCRs, DVD play-
ers, and devices commonly found in
homes, offices, and shops).
The circuit will not add another
cable to your AV tangle: it’s battery
powered. Four AA cells supply a total
of 6 V. A voltage regulator is unneces-
sary; but, if you feel uncomfortable
powering chips close to their maximum
tolerable voltage, you can always add
one. I found that an electrolytic capac-
itor and two filter capacitors are
enough to provide clean power.
An extremely useful feature of the
MC68HC908QT4 is hardware support
for one-pin ICD (in-circuit debug),
which makes an emulator unnecessary
for most developers. An MC68HC908QT4
chip programmed with a tiny user
monitor allows you to load applica-
tion programs, place breakpoints, and
watch variables with almost no over-
head for the program running. ICD
mode requires just one pin (PTA0) for
data transfer, a push button to keep
PTA2 grounded during reset, and a
trivial interface for the PC serial port.
(Refer to the Motorola application
notes listed in the Resources section
of this article for a circuit schematic
that’s compatible with the DVD-
Thermometer and detailed program-
ming explanations.)
If this is your first project with an
HC908 microcontroller, it is simpler
to start with a part with the user mon-
itor already programmed and good
hardware (e.g., the inexpensive
M68DEMO908QT4 starter kit). In
this case, I suggest you replace the
sleep() routine with a classic
“empty-for” delay loop, because the
stop instruction should not be used
with the demo board.
SOFTWARE ESSENTIALS
Because this project requires a mini-
mal amount of hardware, most of the
hard work is done in software. I devel-
oped the software with the free
Metrowerks Codewarrior integrated
development environment. The IDE
includes an assembler, ANSI-C com-
piler, simulator, chip programmer, in-
circuit emulator/debugger, and even
Processor Expert, which is an auto-
matic C code generator.
You may download the code from
the Circuit Cellar ftp site. I selected C
language to make the code reusable
and more flexible. By the way, I had
some I
2
C sources coming from a previ-
ous application on a different micro-
controller.
I have a quite formal program-
ming style. When time allows, I do
my best to write structured code,
using #defines to name every con-
stant, ban global variables, and par-
tition the code in .c modules and .h
header files. Source file names
closely reflect the elements in the
Photo 2—
The microcontroller emulates four remote
control keys: Stop, Play, Next, and Previous. The
RECS80
routine can generate any key code given its
binary value.
Figure 2—
There are eight parts plus the battery. Most of the hard work has to do with the software. The only active
parts are a digital temperature sensor and an 8-bit microcontroller. As long as batteries are used, a power regulator
is unnecessary.
To ensure that disc
playing starts from
the first scene,
DVD_init() stops
and restarts the
player, emulating
the respective
remote control keys.
The distinct
remote.c file imple-
ments the RECS80
remote control
encoder. To make
the IR LED flash the
play command, call
remote_control
(KEY_PLAY).
I defined codes for
the Stop, Play, Next,
and Previous keys.
The routine can gen-
erate any other code
if given its binary
value. It automatical-
ly swaps the toggle bit each time it is
called. Each code is repeated five times
to ensure it is received correctly.
I’m extremely satisfied
by the RECS80 encoder
implementation (see
Listing 1). The secret is
using a programmable
pulse generator (PPG)
that works in the back-
ground under interrupt
control; it does all the
magic transparently and
allows for a straightfor-
ward definition of the
encoding scheme. If your
DVD player uses an
infrared code other than
RECS80, implementing
the new scheme should-
n’t be too difficult. On
the contrary, the PPG
pulse_gen.c file is rather
complex, so I will
describe it separately.
The DS1621.c file,
which is the thermome-
ter driver, is in charge
of initializing the chip
registers and the I
2
C
bus as well as taking
temperature readings. It
carries out these opera-
tions issuing short
16
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CIRCUIT CELLAR
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block diagram (see Photo 3).
The main application (main.c) includes
initialization code for port register set up
and void interrupt routines. During the
start-up phase, it calls the initialization
procedures of the DVD driver and ther-
mometer respectively. Each driver han-
dles its initialization hierarchically (e.g.,
after it’s initialized, the thermometer
routine
DS1621_init() calls
I2C_init() to initialize the I
2
C pins).
After the system is up, the main con-
trol loop begins. First, check if the bat-
tery has enough juice by reading the
LVI register. If not, use the remaining
energy to select the “battery low” mes-
sage from the DVD pictures. (I arbi-
trarily placed it as the last image in the
sequence.) Otherwise, obtain the ambi-
ent temperature, subtract the lowest
temperature possible to get an index
that starts from zero, and use the
index to select the image.
DVD.c provides the scene selec-
tion facility. For instance, call
DVD_select( 25 ) to show the twen-
ty-fifth scene. Figure 3 illustrates the
selection algorithm. The DVD module
keeps track of which image is currently
displayed, starting from zero during ini-
tialization. If the image that will be
selected follows or precedes the current
one, the disc is advanced, emulating the
Next or Previous remote control code.
sequences of I
2
C commands. I tried to
make them as easy as possible to read,
but understanding the DS1621/DS1631
datasheet, as well as familiarizing
yourself with I
2
C bus concepts, is defi-
nitely necessary. If you don’t have the
time to read the datasheet, just call
DS1621_temperature() to get the
current temperature (°C). If you need
higher resolution, uncomment the
lines in the code to get 0.1°C readings.
Notably, the software doesn’t need to
convert Celsius to Fahrenheit for use
of the thermometer outside Europe.
All you need is a different DVD on
which Celsius pictures are replaced by
Fahrenheit temperatures!
The software emulates an I
2
C bus tog-
gling two I/O pins. When speed is not a
concern, software implementations like
this one are perfectly adequate. I ported
the I2C.c code from a bigger processor.
All that was required was to provide
methods to set and clear the SDA and
SCL pins, to change data direction, and
to activate the pull-ups. All of this was
done with macros, so I accomplished
Select DVD picture
Picture
greater than
last picture?
Picture
less than
first picture?
Same picture
as currently
on screen?
Picture
greater than
on-screen picture?
Picture
less than
on-screen picture?
Function end
N
N
Y
N
N
N
Y
Transmit “prev” remote
control key
picture = picture –1
Transmit “next” remote
control key
picture = picture + 1
Y
Go in Stop
mode
for 1 s
Picture = last picture
Picture = first picture
Y
Y
Figure 3—
The DVD selection routine operates iteratively. It keeps track of
the image currently on the screen and passes to a new one (one step at a
time). This technique requires the DVD to respond only to two commands,
next and previous.
Photo 3—
The C file names in the Codewarrior project window reflect the blocks
constituting the application. Each module encapsulates a reusable driver for func-
tions like DVD selection, I
2
C bus communication, temperature readout, and
remote control generation. The code compiles in approximately 2 KB and runs in
User Monitor mode.
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CIRCUIT CELLAR
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19
the task in minutes. I didn’t need to
adjust the waiting loop, although you
might have to if you port the code to
fast (or slow!) processors. After it was
recompiled, the driver worked like
magic, providing the I
2
C primitives
for communicating with two-wire
peripherals:
I2C_start, I2C_stop,
I2C_tx_byte, and I2C_rx_byte.
The last file, sleep.c, provides a func-
tion to put the processor in Stop mode for
a given number of seconds. The sleep
function is called from different modules;
therefore, I placed it in a separate file.
Because the delays are based on the wake-
up timer, the times are approximate. To
have the processor take a 1-min., low-
power nap, just call
sleep(60);.
PULSE GENERATOR
The usual way of generating remote-
control pulses with small controllers
is by bit banging (i.e., flipping pins
directly in a carefully timed software
loop). This solution is far from perfect.
The main defects are its poor resolu-
tion and inherent dependency on code.
Even touching a single line of code
inside the loop can completely ruin
the output waveform.
The excellent 16-bit timer module of
the MC68HC908QT4 processor called
for a more elegant solution than crude
bit banging. The result is the versatile
programmable pulse generator (PPG)
contained in the pulse_gen.c source file.
It can generate pulse bursts whose carri-
er frequency and duty cycle are set with
a simple command. The number of puls-
es in a burst can be set as well. This is
ideal for handling the 8.77-µs LED flash-
es. You can disable the output but keep
the timing in order to generate pauses
with the same resolution to handle the
5.06-ms, 7.6-ms, and 0.12-s gaps required
by the RECS80 code.
The PPG is easy to set up and use
(see Listing 2). It is interrupt-based
and synchronizes itself for pending
requests, so it frees the main program
for other tasks while working in the
background. This feature is beneficial
when writing the infrared encoding
routine because it leaves plenty of
time for the processor to compute the
pulses that come next.
I started from a working example
that I created with Processor Expert,
using its PPG component (“bean” in
Processor Expert jargon). I studied the
computer-generated code along with
the datasheets in an effort to get
familiar with the timer module details
before I started writing my own code.
My incarnation of the PPG is
extremely different from the PE code.
First, you must set up the desired fre-
quency and duty cycle with a call to
ppg_set(frequency, duty);. The
purpose of this function is to set the
PWM registers; it does not modify the
PPG output. A call to
ppg_set can be
issued at any time because it automati-
Listing 1—
The PPG works in the background under interrupt control. Calls to
ppg_pulses()
synchronize automatically and return almost immediately to the
caller. As you can see, the implementation of a RECS80 remote control encoder is straightforward.
static void recs80( unsigned int code, unsigned char toggle )
{
unsigned int mask;
unsigned int pause = 4598;
ppg_set( 38000, 33 );
//Prepare PPG for 38 kHz, 33% duty cycle
if (toggle)
code |= (1 << 10);
//Add toggle bit to data word
for( mask= 1<<11; mask!=0; mask>>= 1 ) //Scan all bits in the code word
{
if (mask & code)
//Is data bit = 1?
{
ppg_pulses( 6, PPG_ENABLE_OUT ); //Yes, transmit burst
ppg_pulses( 288,PPG_DISABLE_OUT ); //and a longer silence
pause = pause - 6 - 288; //adjust repetition rate pause to compensate
} //for the time spent
else
{
//No, data bit is 0
ppg_pulses( 6, PPG_ENABLE_OUT ); //transmit burst
ppg_pulses( 192,PPG_DISABLE_OUT ); //and a short silence
pause = pause - 6 - 192; //adjust repetition rate pause to compensate
}; //for the time spent
};
ppg_pulses( pause, PPG_DISABLE_OUT ); //Add the pause to get the 121-ms repetition rate
}
Listing 2—
The programmable pulse generator defined in PPG.C is easy to set up and use. It provides
functionalities for generating square waves of an arbitrary duty cycle with specified duration as well as
pauses. Therefore, the code can be adapted to generate encoding schemes other than
RECS80
.
//Set up PPG for 38-kHz square waves, 33% duty cycle
ppg_set( 38000, 33 );
//Generate six pulses with the current frequency and duty cycle.
ppg_pulses( 6, PPG_ENABLE_OUT );
//Generate a 100-cycle long pause
ppg_pulses( 100, PPG_DISABLE_OUT );
20
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CIRCUIT CELLAR
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cally waits for the completion of a pre-
vious cycle if the PPG is running. The
PPG is running whenever
TSC_TSTOP
is zero.
Starting pulse generation requires a
call to
ppg_pulses(count, mode);.
This function triggers the PPG to gener-
ate count pulses (up to 65535). The sec-
ond parameter tells the PPG to toggle
the output if set to one and to leave the
output unchanged if set to zero. Setting
the mode to zero is an effective way to
generate pauses synchronized to the
main PPG frequency. It that case,
count
specifies the pause length expressed as
cycles of the PPG frequency.
It is remarkable that, if no previous
requests are pending,
ppg_pulses
returns to the main application almost
immediately. The purpose of this
function is to load a cache buffer with
the pulse parameters, and to start the
timer if it was not already running. It
then returns to the calling application,
which gets plenty of time to prepare
the next pulse burst. Successive calls
to
ppg_pulses join and synchronize
seamlessly. This feature allows for the
straightforward coding of the remote
control encoding routine in Listing 1.
After the timer runs, waveform gen-
eration is handled by the timer over-
flow interrupt service routine. It’s more
complex than it looks. Timing is crucial
because a 38-kHz pulse takes only 26.3
µs, which leaves little time for counting
pulses and flipping bits. The situation
gets even worse when working in User
Monitor mode because of the overhead
involved with the vector’s redirection.
At every interrupt, the handler checks
the pulse ticks counter to determine if
it is the last pulse. If not, it decrements
the counter and returns. Otherwise, it
checks the cache buffer to see whether
or not more pulses are scheduled. In the
former case, it reloads the tick counter
and sets the mode of operation for the
output pin. In the latter case, it waits for
the current pulse to end, testing the sta-
tus of the output port pin, and resets the
output mode to no-pulse generation. It
finally stops the timer, which stops
overflow interrupts as well. Fortunately,
you don’t have to know all of these
complex details to benefit from the
PPG, which is encapsulated in a C mod-
ule and can be called transparently.
IT’S SHOWTIME
The circuit, which is quick and easy
to assemble on a piece of perf board,
doesn’t require calibration. My proto-
type is shown in Photo 4. The system
begins working immediately after plac-
ing the disc in the player and switching
on the player and the thermometer.
The DVD-Thermometer has superb
multimedia capabilities unparalleled
by current thermometers. It’s inexpen-
sive too. With DVD players starting at
$59.99, using a DVD player as a sort
of graphic front-end is reasonable. An
alternative PC-based solution costs
more, consumes more power, is much
more complex to set up and upgrade,
and is immensely less fun to design!
The basic idea can be adapted to a
variety of practical situations. Clearly,
it’s a great idea for hotels, malls, show-
rooms, stations, airports, and tourist
sites. Thermometers, per se, are unques-
tionably useful, but the multimedia use
and low cost of the DVD-Thermometer
prospects new uses, especially when the
possibility of adding advertisements is
considered. Potential applications
include talking please-take-a-number
displays, interactive product selectors
(e.g., select a certain paint or see how a
new drill works), and colorful, dramatic
displays for any sort of measurement.
You shouldn’t have a problem using
the circuit as a building block for
other projects. The C software is
structured and modular, and the PPG
module performances in particular are
excellent. Its cache-based algorithm
allows for an unusually straightfor-
ward implementation of the remote
Photo 4—
I assembled the DVD-Thermometer prototype
on perf board. The circuit fits a small plastic box, with the
power switch and infrared LED peeping out one side. To
measure the external temperature, separate the LED to be
a few meters apart and connect it with twisted-pair wire.
PROJECT FILES
To download the code, go to ftp.circuit
cellar.com/pub/Circuit_Cellar/2004/168.
SOURCES
Nero Burning ROM
Ahead Software
www.ahead.de
68HC908QT4 Microcontroller
Motorola, Inc.
www.motorola.com
DS1621/DS1631 Digital temp sensor
Maxim Integrated Products, Inc.
www.maxim-ic.com
Alberto Ricci Bitti holds a degree in
Computer Science from the University
of Milan. He has more than 10 years of
experience designing and writing soft-
ware for embedded systems. Alberto
currently designs industrial controllers
and instrumentation for Eptar. In his
free time, Alberto enjoys competing in
design contests. He has been awarded
several prizes. You may visit Alberto’s
web site, www.riccibitti.com, or write
him at a.riccibitti@iname.com.
RESOURCES
Philips Semiconductors, “SAA3004
Remote Control Transmitter,” 1982.
J. Sibigtroth, “User Mode Monitor
Access for MC68HC908QY/QT Series
MCUs,” AN2305, 2002.
J. Suchyta, “Reprogramming the
M68DEMO908QT4,” AN2322/D, 2002.
control encoder routine, which you
can adapt to most of the encoding
schemes currently on the market. If
you write a driver for a different DVD
player brand, please contact me so I can
share it with other interested readers.
This design is also a great candidate
for classroom experiments. It’s instruc-
tive for teaching students about micro-
controllers, programming languages,
serial (I
2
C) busses, sensors, remote con-
trols, waveform generation, and the
process of creating and preparing multi-
media content. Young, open-minded stu-
dents working on simple everyday tech-
nologies can generate wonderful
results. I look forward to seeing what
they can get out of a bit of silicon!
I
cameras, digital picture frames, and
photofinish freeze-frame cameras).
More importantly, my goal is to
demonstrate the techniques by which
this is accomplished. This article is
also intended to serve as a starting
point for further exploration.
ANALOG VIDEO BASICS
Since the early 1950s, the National
Television System Committee (NTSC)
color analog video standard has been in
effect for all North American TV broad-
casts. Similar flavors of the standard (i.e.,
PAL and SECAM) also have been devel-
oped in various other parts of the world.
Figure 2 shows the structure of a typi-
cal baseband analog composite video sig-
nal. Essentially, there are four regions of
interest: the sync pulse, which is the
lowest level of the signal; the back porch
(black level) with color burst, which is
used as a synchronization reference
during the color decoding process; the
active video region; and the front porch
(black level).
The luma (Y), or intensity informa-
tion, is encoded as the
average amplitude of the
signal within the active
video portion. In addi-
tion, note that two chro-
ma (Cb/Cr), or color
components, are quadra-
ture-modulated and
added to the luma sig-
nal. In simpler terms,
the two modulated chro-
ma components are
characterized by a super-
imposed sinusoidal sig-
nal, whose phase with
respect to the reference
color burst signal (in the
back porch) represents
22
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M
y old cell phone had finally
packed it in! That wonder of modern
technology wouldn’t even allow me to
dial out. Instead of having it fixed, I
decided to go shopping for a replace-
ment. I was surprised to find out that a
large number of the newer cell phones
have built-in cameras. Not only can you
talk to your friends, but you can now
see them too. Hmmm.
On the way into the store I also noticed
several closed-circuit TV cameras remind-
ing customers that they should be on
their best behavior. In another corner of
the store, satellite TV products were on
display featuring a personal video recorder
(PVR), which you can use to time-shift
and record whatever TV shows keep you
glued to your seat. Freedom at last!
Essentially, over the past few years,
there has been an explosion in the num-
ber of video-related embedded products
on the market. This has been possible in
large part because of the availability of
inexpensive CMOS imagers, specialized
video compression ICs, and FPGA cores.
Over the years, I’ve designed a number
of embedded products
involving digital video.
These products range in
complexity from a full-
fledged, stand-alone,
FPGA-based multichannel
digital video recorder
(DVR) to a much simpler
video freeze-frame device.
For all of these designs, one
of the key requirements
always has been ultralow
cost. Surprised? In my
experience, “ultralow cost”
is usually a synonym for
“8-bit microcontroller.”
Unfortunately, digitiz-
ing video can be a daunt-
Full-Field Color Video Frame Grabber
ing task because of the volume of data
collected and the rate at which it is
generated (more than enough to choke
the fastest 8-bit microcontrollers on the
market). In this article, I will present
you with a relatively simple and inex-
pensive circuit that allows for the sin-
gle-shot digitization of a full field (720 ×
240) of color video (PAL/NTSC) into a
field buffer. This buffer is accessible by
an Atmel AVR 8-bit microcontroller,
and its contents are sent to a host PC
through the UART (see Figure 1).
Of course, the circuit probably won’t
win any frame-rate contests! Even though
the video acquisition rate supports
60 fields per second, downloading a frozen
full-resolution (720 × 240) color image
over the serial port takes about 30 s at
115.2 kbps. (A download 10 times faster
could be realized over the SPI bus.) If
you sub-sample the image at a fraction
of the resolution during downloading,
expect proportionally better results. In
many applications, acquiring a single
field of video following a trigger is all
that’s required (e.g., motion-triggered
FEATURE ARTICLE
by Eric Gagnon
Eric’s frame grabber enables the single-shot digitization of a full field of color video. Use this
circuit for digital, freeze-frame, and motion-triggered camera design projects.
Video 1
Video 2
Video 3
Video 4
SAA7113
Video
decoder
Video out
Power
regulator
5 to 9 VDC
3.3 V
AL422B
Field
buffer
ATmega8535L
8-bit MCU
4:2:2
(CLK, VS)
WE
HS
4:2:2
(RCK, RE, RRST)
VS
ACQUIRE
(RXD, TXD)
RS-232
Level
shifter
RS-232 Port
(to PC)
I
2
C (SCL, SDA)
Figure 1—
The SAA7113 video decoder digitizes one of the analog input video signals and stores the
samples in a field buffer. The ATmega8535L coordinates the acquisition and download processes.
solid dark lines in
Figure 3) and one of
all the 262.5 even-
numbered lines
(the lighter dashed
lines in Figure 3) in
rapid succession.
Half images, or
fields, are displayed
with a slight verti-
cal offset in rela-
tion to each other
in such a way that
the eye’s retinal
persistence super-
imposes them and
is fooled into see-
ing a 525-line image. Each field is sent
at 60 Hz. This trick is still in use today,
although most PC monitors and TVs
now have video deflection circuits fast
enough to display noninterlaced images
at full video rates.
A field of video is defined as the half
image composed of either the odd or even
262.5 lines of a frame. A frame of video is
defined as a full 525-line image with both
the odd and even fields super-
imposed (interleaved).
One of the problems that
arises when digitizing video of
moving objects is that the odd
and even fields are acquired at
one-sixtieth of a second apart.
When played back on a TV
screen, this is not apparent
because the eye is fooled.
However, if you freeze both
odd and even fields in a frame
buffer and overlay them,
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23
you’ll notice motion artifacts as shown
in Figure 4. Because the images aren’t
taken at the same point in time, any
moving object in the scene will result
in this slight spatiotemporal mismatch.
There are some deinterlacing tech-
niques aimed at isolating the changes
between both images and applying inter-
polation to infer details in regions of large
motion. However, the most common
approach to the problem is simply to dis-
card one of the fields and use the other
(albeit at half the vertical resolution). If
absolutely necessary, it’s always possible
to interpolate the acquired field vertically
to bring the image back to the original
size. (Although this is artificially inferred
information, it is often adequate.)
Another approach is to copy the data
as-is to the other field. In this article I
simply explain how to acquire one of the
video fields and discard the other.
DIGITIZING VIDEO SIGNALS
The digital revolution has taken the
world by storm, and video is no excep-
tion. With the creation of two standards,
ITU-R BT601 and ITU-R BT656, a new
digital video format has been estab-
lished. The former describes the essen-
tials of the video digitization process
such as minimum/maximum, reserved
values, sample rates, etc. The latter
specifies both a parallel and a serial elec-
trical interface for digital video transfer.
The parallel interface supports an 8-bit-
wide datastream (10-bit is also support-
ed) with embedded codes representing
timing and ancillary information.
Both standards apply to component
4:2:2 digital video. The 4:2:2 ratio ter-
the hue, and whose amplitude repre-
sents the saturation of the color.
FRAMES VERSUS FIELDS
Encoding the information contained
in an entire image requires the con-
catenation in time of a series of sin-
gle-line video signals. In fact, there is
one for every active video line and
also some for additional timing infor-
mation (the first 21 lines per field).
Figure 3 illustrates the scanning pat-
tern used to encode the information
contained in a typical image.
The NTSC composite video standard
calls for 525 total lines of video per
frame at 30 frames per second. In order
to reduce image flicker and increase the
refresh rate without changing the overall
video signal bandwidth or needing faster
deflection circuitry in TV receivers,
engineers came up with the interlaced
video technique. This technique doubles
the image refresh rate to 60 Hz (while
maintaining the perceived vertical reso-
lution of 525 lines) by quickly alternat-
ing between a half image composed of
all the 262.5 odd-numbered lines (the
Y component (luma) = average signal offset
1 V
Back
porch
Active video
Cb, Cr components
(chroma)
= superimposed
quadrature-
modulated
sinusoids
63.555
µ
s
Color burst
(reference)
Front porch
Sync pulse
Figure 2—
Study the structure of an analog composite NTSC video signal. The main
sections include the sync pulse, the back porch, the active video, and the front
porch. The average signal offset represents the luma (Y) and the superimposed
quadrature-modulated sinusoid, the chroma (Cb and Cr) components.
Even field
(lines 264–525)
Line 285
Line 286
Line 287
Line 288
Line 524
Line 525
Odd field
(Lines 1–263)
Line 22
The first 21 video lines
of each field
contain no active
video information
(not shown)
Vertical spatial
offset between
odd and even fields
Typical scan line
consists of a
single-line
video signal
Line 263
Line 23
Line 262
Line 261
Figure 3—
The NTSC image scanning pattern is for full video frame. The odd field scan lines are shown in solid
dark, and the even field scan lines in the light dashed pattern. Both fields are scanned in rapid succession from top
to bottom, left to right with a slight vertical offset to each other.
Figure 4—
The motion artifacts are obtained when simultaneously
displaying both the odd and even fields captured from a moving
object. Because the fields were acquired one-sixtieth a second apart
by the camera, the image of the moving van appears jagged
because of the spatiotemporal mismatch.
Vehicle at time T2
Vehicle at time T1
Even field
lines sampled
at time T2
Odd field lines
sampled
at time T1
Direction of
vehicle motion
much more familiar with the R, G, B
color space where a pixel has three sam-
ples corresponding to its red, green, and
blue color components. Fortunately,
converting between the two involves a
simple matrix operation. To transform a
YCbCr triplet to a corresponding RGB
triplet, use the following YCbCr-to-
RGB transformation equations:
[1,2]
R, G, and B are gamma-corrected val-
ues ranging between zero and 255.
SAA7113 VIDEO DECODER IC
Converting an analog video signal
into the digital domain requires an
ADC. About 5 to 10 years ago, many
semiconductor manufacturers offered
digital video decoder integrated cir-
cuits. A video decoder is essentially a
specialized ADC with a front-end used
to condition and synchronize to an
analog video signal. In recent years,
however, only a few of the dominant
players have remained and continued
to offer a good selection of parts.
Among these is Philips Semiconductor,
whose SAA7113 is truly a lean-and-
mean video decoder IC. Its ancestor is
the venerable SAA7111 known by
many in the field.
The SAA7113 is housed in a com-
pact 44-pin TQFP package and oper-
ates at 3.3 V. It accepts up to four
video input channels that are internal-
ly multiplexed to a single video ADC.
A convenient analog video output pin
buffers the currently selected analog
video channel for viewing. The video
ADC outputs data in 8-bit, multi-
plexed, 27-MHz (BT656) format on its
data bus pins. It also provides addi-
tional signal outputs such as vertical
sync, horizontal sync, and odd/even
field detect on two programmable pins.
This makes it convenient to interface
to external circuitry without needing
to recover the synchronization infor-
mation from the datastream itself.
The SAA7113 has a built-in phase-
locked loop (PLL) circuit, which enables
it to lock its on-board oscillator clock
to the incoming video signal. This pro-
vides an accurate pixel clock (27 MHz
R
G
= 1.164 Y 16 + 1.596 Cr 128
= 1.164 Y 1
×
−
(
)
×
−
(
)
×
−
6
6
0.392
Cb
128
0.813
Cr
128
= 1.164
(
)
−
×
−
(
)
−
×
−
(
)
×
B
Y
Y 16 + 2.017 Cb 128
−
(
)
×
−
(
)
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Issue 168 July 2004
CIRCUIT CELLAR
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Y sample (see Figure 5). The (Cb,Y,Cr)
triplets are cosited in space, and each
second Y sample is alone because of the
dropped Cb and Cr samples. The result-
ing aggregate sample rate, 27 MHz, is
calculated by adding the 13.5-MHz Y
component as well as both the 6.75-MHz
Cb and Cr components.
COLOR SPACES
The 8-bit Y, Cb, and Cr sample values
are used to represent a pixel in color
space. In the computer world, we are
minology simply means that during
the video sampling, the luma informa-
tion (Y) is sampled at every pixel posi-
tion (13.5-MHz sample rate), and the
chroma (Cb, Cr) components are sub-
sampled by a factor of two (6.75 MHz
each). This decimation is possible
because the eye is less sensitive to spa-
tial color resolution than spatial intensi-
ty resolution. In a BT656 4:2:2 digital
video stream, one 8-bit sample of Cb is
followed by a Y sample and then a Cr
sample, which is followed by the next
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CIRCUIT CELLAR
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25
nominal) with respect to the video sig-
nal timing and tracks any signal jitter.
I should also mention that besides
video decoders used to digitize legacy
analog video signals, there are also new
video imagers based on CMOS technol-
ogy that directly output BT656. (The
video ADC is internal to the IC.)
Therefore, in theory, you could directly
replace the video ADC front end of this
circuit and substitute a CMOS imager
if you want to build a small camera.
KEEPING PACE
As I mentioned earlier, the funda-
mental problem in capturing digital
video is the rate at which it is generat-
ed and the volume of the generated
data. A typical 8-bit MCU would not
have the SRAM resources required to
directly store a single color field of
video (300 KB) or even the processing
speed to read the data at 27 MHz. The
solution to this problem lies in buffer-
ing the video field into a field buffer.
This can be implemented in a number
of different ways. Figure 6 shows a
few possible system architectures.
For optimal performance, an FPGA-
based (or CPLD-based) external mem-
ory buffer is the architecture of
choice, especially when the FPGA is
otherwise needed in the circuit (see
Figure 6a). As such, its incremental
duties as the field buffer can be rela-
tively inexpensive. The circuitry
inside the FPGA typically includes a
counter that cycles the external mem-
ory in sync with the incoming video
data samples and writes the values at
each memory location. The vertical
and horizontal sync pulses are used to
gate (enable/disable) the clocking of
the SRAM addresses to ensure that
data is only written to memory during
valid video data sampling (and not
720 sampled pixels per video line (at 13.5 MHz)
720
Pixel 720
Y720
Cb720
Cr720
Pixel
1
Chroma components
are sampled at 8 bits after
quadrature demodulation
1
Each sampled pixel
has a [Y, Cb, Cr] triplet
4:2:2 Digital video stream (at 27 MHz)
from video decoder
Every second, Cb/Cr chroma samples
(shaded) are dropped
Y720
Cb1 Y1 Cr1 Y2 Cb3 Y3 Cr3 Y4
Y1
Y2 Y3
Y4
Cb1
Cr1
Cb2 Cb3 Cb4
Cr2 Cr3 Cr4
Pixel
2
Pixel
3
Pixel
4
8-bit
Sampling range
(for luma)
0
255
Figure 5—
Here you see the ITU-R BT656 4:2:2 digital video stream. The 27-MHz digital datastream is mainly
composed of a succession of Cb, Y, and Cr samples. Each component sample value of the (Cb, Y, Cr) triplets are
cosited in space. Because of the 4:2:2 decimation, each second Y value has no associated chroma information
(shown shaded). Embedded sync codes are also added throughout the digital stream.
26
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CIRCUIT CELLAR
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needs to be transferred to the main
processor for processing, communica-
tion, or storage. This operation can be
accomplished by implementing a DMA
controller inside the FPGA or simply by
having the FPGA switch over all the
control and data lines of the memory
to the MCU during the entire transfer.
(This is done to the detriment of the
overall system frame rate because no
new fields of video can be acquired
while the SRAM is under MCU control.)
Another way to accomplish the data
transfer is to use dual-ported memory
(see Figure 6b). The video frame grab-
ber writes to the memory on one port,
and the system can read the data from
the other port. However, full-field,
dual-ported SRAM memories are
extremely expensive. It is often more
economical to use smaller dual-ported
SRAM as single-line buffers.
If you have the luxury of a DSP or
extremely fast processor with an
external memory bus, you can simply
use DMA transfers. Assuming the
processor can read out one line of
stored video data at a faster average
rate than it’s generated, all you need is
a single-line video FIFO or dual-ported
SRAM (see Figure 6c). The video
decoder simply fills the FIFO with
data at the video rate, and it interrupts
the main processor when the buffer is
outside these bounds). At the same
time, the circuit can separate the Y,
Cb, and Cr components into different
memory destinations if desired.
After the digital video field data is
collected in the field buffer, it usually
SRAM or SDRAM
(full frame/field)
ADDR, CTRL
FPGA
or
CPLD
DATA
4:2:2
HS, VS, CLK
ADDR
DATA
MCU
FPGA
or CPLD
Dual-ported
memory
MCU
(external
memory bus)
ADDR
4:2:2
DATA
ADDR
DATA
Single-line
FIFO
4:2:2
HS, VS, CLK
DATA
CTRL
Fast MCU
or DSP
ADDR
DATA
Main memor
y
Full-field
FIFO
MCU
DATA
CTRL
4:2:2
HS, VS, CLK
HS, VS, CLK
Figure 6—
There are a few possible field buffer architectures: an FPGA managing memory buffer
(a)
, a dual-ported
memory
(b)
, a single-line FIFO
(c)
, and a full-field FIFO
(d)
.
a)
b)
c)
d)
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CIRCUIT CELLAR
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half full or at the end of each line. The
processor then immediately bursts out
the data from the buffer in less time
then it took to fill it, preferably much
faster. The data is transferred to the
processor’s main memory space,
which has to be large enough to hold
a complete field.
In this project, you don’t have access
to an FPGA or DSP, or even enough
internal memory to buffer a field.
Therefore, you need a full-field FIFO
with an interface simple enough for
the MCU (see Figure 6d). The video
decoder will fill the FIFO using the
write port. Then, the MCU will inde-
pendently read out the data from the
read port after it detects that a com-
plete field has been stored.
AL422B FIELD BUFFER FIFO
Averlogic’s AL422B is a popular
full-field FIFO. This IC is implement-
ed internally as dynamic RAM, so it
costs less than a comparable SRAM-
based product. It has a convenient and
simple FIFO interface with one input
write port and one output read port.
On the input port, the write enable
signal is used to gate the main clock,
which must be present for the DRAM
cells to get refreshed internally. The
write reset pin resets the internal write
address generator counter to zero. At
every write clock pulse, the address
generator is automatically incremented.
On the read side, the same idea
applies, but from the read perspective.
PUTTING IT TOGETHER
The circuit is shown in Figures 7
and 8. The SAA7113 is used to digitize
one of four possible analog video input
channels. The buffered analog video out-
put allows for the live viewing of the
selected video channel. The BT656-for-
mat digital output of the video decoder
is wired to the input port of the FIFO.
The SAA7113 is programmed to
output an odd/even field signal
switching after line 21 (start of active
video lines) on its RTS1 pin. This signal
goes high for the duration of an odd
field, and then goes low for the even
field. It is tied directly to the write
reset pin (*WRST) and holds the write
counter in its reset state, except when
an odd video field is present.
Remember, you only want to sample
one field to eliminate motion artifacts.
Furthermore, the SAA7113’s other pin
(RTS0) is programmed to output a hori-
zontal reference (HREF) signal, which
goes high and stays high for the duration
of the active video data portion of every
video line. Connected to the *WE pin
Figure 7—
U1 digitizes one of the four video input signals from BNC connectors J2–J6. The digitized samples are stored in U4, a full-field FIFO memory. All of the operations
of the frame grabber, as well as download functions, are coordinated by U5, an 8-bit Atmel AVR MCU.
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CIRCUIT CELLAR
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29
through an inversion, it is used
to gate the main pixel clock,
which is connected to WCK. It
simply enables the writing of
valid video data values to the
FIFO at the appropriate times.
The microcontroller also has
access to the write enable line
through a NAND gate. This
allows for the acquisition of a
single field and then the ability
to stop under MCU control.
(Otherwise, the video fields will
be continuously overwritten
before you have the chance to
read out the data.) The MCU
has full access to the FIFO’s
output port. After detecting that
a frame grab is complete, the MCU
waits for the appropriate serial port
command before reading out the video
data from the FIFO for transmission.
The fully assembled circuit board is
shown in Photo 1. Note that the design
was separated into two modular parts:
a stand-alone daughter card, which
contains the entire frame grabber cir-
cuit, and a larger motherboard that
holds the various connectors as well as
the RS-232 level shifter circuitry.
FIRMWARE AND SOFTWARE
The firmware for this project was
developed in C using ImageCraft’s
ICCAVR compiler. The MCU’s main
roles involve running a serial command
server, initializing the SAA7113 through
the I
2
C bus, and managing the field
acquisition process. The serial com-
mands are sent in plain ASCII format.
The basic ASCII frame grabber
command set includes the following:
VER returns firmware version number;
GRAB grabs a field into the field buffer;
CHAN n selects video channel n; RRST
resets the FIFO read pointer; and
SEND
sends the next n bytes of video data
from the FIFO in binary format.
A variation of the
SEND com-
mand could allow decimation of
the image to speed up the transfer
(Thumbnail mode). Additional
commands allow for board config-
uration and extra features.
The SAA7113 is a highly config-
urable part that contains a large
number of registers. At power-up,
each register must be set up cor-
rectly through I
2
C before the part
can be used. Just remember to pro-
gram the unused registers to zero
as indicated in the datasheets; oth-
erwise, it won’t work. After ini-
tialization, the desired channel is
selected and the digital video
comes spewing out!
At this point, the frame grabber is
capable of digitizing a full-color video
field and transmitting the data over a
standard RS-232 serial port in
response to simple ASCII commands.
This offers great flexibility for use in
generic embedded systems. In order
to allow for simple user experimenta-
tion, a Windows-based command and
viewing utility was developed using
Visual C++ 6.0. This utility commu-
nicates with the frame grabber
through the PC’s serial port. It can
send all the commands used to con-
trol the frame grabber as well as
Figure 8—
The live video output BNC connector J7 allows for a live preview of the currently selected video input. An RS-232 level shifter (U2) provides the required signal lev-
els for interfacing to a PC. U3 is a 3.3-V LDO regulator providing local regulation of the input voltage.
Photo 1—
This is the microcontroller frame grabber (uCFG) prototype.
The small daughter card on top contains the entire frame grabber cir-
cuitry, and the larger motherboard on the bottom contains the various
connectors as well as the RS-232 level shifter circuitry.
30
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CIRCUIT CELLAR
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download the image data for viewing.
The downloaded YCbCr data has to
be reconstructed into an RGB array
for display on the computer screen.
To perform this reconstruction, the
decimated Cb and Cr samples are first
regenerated through interpolation.
The YCbCr-to-RGB matrix transform
is then applied to convert image data
to the RGB color space. Because the
captured video field is 720 × 240,
merely displaying it 1:1 on a PC mon-
itor results in a vertically squashed
image. In order to compensate, verti-
cal interpolation of the missing field
is performed before displaying the
image. A high-resolution mode could
be implemented that would capture
the odd field, the even field of a static
scene (eliminates motion artifacts), and
provide an actual 720 × 480 resolution
image. The software can also save the
uploaded image in the .bmp format.
FIRST LIGHT
Well, enough theory. It’s time to get
your hands dirty! After building the
frame grabber circuit, I used my cam-
corder as a color video source. I sent
the command to acquire the video
field from the Windows-based GUI
application and downloaded the image.
Photo 2 shows the PC control software
in action. You can see the downloaded
image of the frame grabber taking a
picture of the frame grabber, taking a
PROJECT FILES
REFERENCES
[1] K. Jack, Video Demystified, LLH
Publications, 2001, www.video-
demystified.com.
[2] C. A. Poynton, A Technical
Introduction to Digital Video
, John
Wiley & Sons, Hoboken, NJ, 1996.
RESOURCES
International Telecommunication
Union, ITU-R series standards,
www.itu.int/home/index.html.
NTSC standard information, www.ntsc-
tv.com.
SOURCES
ATmega8535L Microcontroller
Atmel Corp.
www.atmel.com
AL422B Full-field FIFO
Averlogic Technologies, Inc.
www.averlogic.com
C Compiler
ImageCraft
www.imagecraft.com
CMOS Imagers
OmniVision
www.ovt.com
SAA7113 Video decoder
Philips Semiconductor
www.semiconductors.philips.com
Eric Gagnon, M.A.Sc., P.E., has more
than 10 years of embedded design
experience. He currently runs Digital
Creation Labs, Inc. (www.digitalcreation
labs.com). Eric has worked on projects
pertaining to the International Space
Station, industrial robotics, 3-D
machine vision, and embedded video
products. You can contact him at
egagnon@digitalcreationlabs.com.
picture of the frame
grabber… Well, you
get the picture (no pun
intended)!
LOOKING AHEAD
I’m sure you are
already thinking ahead.
How can you integrate
this simple frame grab-
ber into a product?
What about adding an
SPI-based secure digital
(SD) flash card on the
SPI interface of the
Atmel MCU and turn-
ing this into a simple
digital camera? You
could save a large num-
ber of images in raw
format to the flash card
(or with a file system).
How about implementing a simple
video compression algorithm inside
the microcontroller and sending the
image to a remote observation host
using a wireless RS-232 module? Maybe
you could implement a photo finish
freeze-frame module to take a single
picture of athletes crossing the finish
line. Or what about a motion-triggered
camera taking a snapshot of the last
person entering the premises? Using the
Cypress wireless USB chipset (wireless
SPI), you can build an all-digital wire-
less camera. Let your imagination run
wild! If you come up with some neat
ideas, I would love to hear from you.
Hopefully this article has provided
you with some useful background on
digital video as well as some hints on
how to implement a basic video frame
grabber. By extending this basic sys-
tem, it’s possible to implement high-
er-performance video capture circuits,
and perhaps combine them with some
simple image-processing algorithms. If
you’re interested in hands-on experi-
mentation, take a look at my full
evaluation kit and individual OEM
frame grabber modules (www.digital-
creationlabs.com).
Anyhow, back to my story. After
waiting in line at the store (for longer
than I should have), I ended up purchas-
ing a replacement cell phone, but not
one with a video camera. I guess I’m
cheap. Some things never change!
I
Photo 2—
The frame grabber circuit acquired this image. Also shown is a
screen capture of the Windows-based PC control and viewing software. All
communications occur over a standard RS-232 serial port.
REFLOW OVEN
The key to good reflow is a precise
multistep thermal profile, as illustrat-
ed in Figure 1. The preheating profile
allows you to slowly bring the PCB to
a temperature high enough to dry the
solder paste (approximately 100°C)
while minimizing the risk of thermal
stress on the components in the reflow
phase. After thermal stabilization, the
board is heated as quickly as possible to
the reflow temperature (approximately
250°C). The board stays above the
reflow temperature for a set period of
time (usually around 30 s) before it’s
cooled down in a controlled fashion.
What are the key characteristics of
an industrial reflow oven? Quick heat-
ing and cooling is important (the
quicker the better) in order to reduce
the thermal stress on the components.
(The pads and solder paste have a
lower thermal latency than the com-
32
Issue 168 July 2004
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F
or years surface-mount technology
(SMT) has been the manufacturing
technology of choice, particularly
because it’s less expensive than classic
through-hole processes and because of
the reduction in board size. I’m sure
you have noticed that SMT is now
mandatory for prototypes, even if a
project isn’t space-constrained. Many
new and exciting components are only
available in SMT packages. High-
speed designs (using high-speed digital
or radio frequencies) won’t work using
large through-hole packages.
It’s possible to solder the majority of
SMT components by hand, but it’s
painful and time-consuming. The
components must be soldered individ-
ually with either a small soldering
iron or a hot-air pencil. But small
Easy Reflow
When designing high-speed applications, working with SMT components and soldering by
hand can be tedious. Robert’s H8/3687-based SMT Reflow Oven Controller transforms a
conventional infrared toaster oven into an effective reflow oven that ensures thermal control.
packages like 0603 resistors are likely
to fly away when soldered with hot air
and stick to the soldering iron at the
worst time.
A solution to this problem is to use
reflow oven technology just like the
manufacturers do. A reflow oven is a
well-controlled oven that allows to
you cook a PCB with its components
and solder all of the pads simultane-
ously with solder paste. Great! The
only issue is that reflow ovens cost a
fortune, ranging from $2,000 to $1
million. The ovens cost so much
because they need strictly controlled
thermal profiles to ensure good solder-
ing and also to limit the thermal
stress on the components.
In this article I’ll show you how a
homemade controller built around a
Renesas evaluation
board can transform
an inexpensive toaster
oven into a reflow
oven. First, I’ll cover
how reflow ovens
work and explain
which toasters on the
market are best suited
to mimic them. Then,
I’ll describe my con-
troller’s hardware and
software. I’ll finish up
with some tips on
how to assemble your
own SMT boards
using the reflow oven.
Let’s get hot.
FEATURE ARTICLE
by Robert Lacoste
Preheating,
1° to 4°C/s
Heating, 2° to 4°C/s
Maximum temperature =
reflow temperature + 20°C,
or around 250°C, depending
on the solder paste.
Drying, 100° to 150°C,
1 min., 30 s to 3 min.
Cooling,
–2° to –4°C/s
Reflow
Figure 1
—
In order to achieve reliable solders and to minimize thermal stress on
components, a precise thermal profile must be used with four successive steps.
Build an SMT Reflow Oven Controller
GRAND PRIZE WINNER
Photo 1—
You must choose an appropriate toaster
oven. For the best results, find one with infrared
heaters on top that allow for a quick temperature rise.
This one, which was made in France, costs less than
$150. The thermocouple wire is on the left.
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CIRCUIT CELLAR
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Issue 168 July 2004
33
you to select either Configuration
mode or Run mode.
In Configuration mode, the LCD
successively displays each of the nine
key profile parameters (preheating slope,
drying temperature, drying duration,
heating slope, reflow temperature,
reflow duration, cooling slope, thermal
hysteresis, and differential coefficient).
It also allows you to modify the profile
parameters with the plus and minus
buttons. (It’s easy to store these values
in EEPROM, but it isn’t done in the cur-
rent version of the firmware.) In Run
mode, the LCD shows the current actu-
al and preset temperature, as well as the
current state and the remaining time to
be spent in the state (see Photo 3).
Seven steps are managed by the
state machine in order to achieve a
thermal profile that’s as close as possi-
ble to the theoretical one. The first
step involves preheating. The preset
temperature is linearly increased from
the ambient temperature up to the
drying temperature. Next, the con-
troller waits for the actual tempera-
ponents themselves.) Maintaining a
homogeneous temperature inside the
oven is also important, especially when
you’re working with large PCBs. And,
last but not least, regulation is a must
in order to guarantee that successive
reflows follow the exact same profile.
As you can imagine, I will handle the
regulation with a microcontroller-
based solution, so let’s focus on the
low-cost oven you’ll use to replace a
real reflow oven.
In order to have a quick heating
time, some industrial reflow ovens
use infrared heaters. Why not just buy
an infrared-based toaster oven like the
one shown in Photo 1? I bought mine
on the ’Net for less than $150. It has
two quartz heating elements on the
top and a classic resistor on the bot-
tom (a total of 1,100 W).
I used a thermocouple affixed on a
small PCB to take the first measure-
ments. The temperature got up to
250°C in less than 4 min., which is
definitely enough for this application.
The only real drawback was the lack
of a cooling system. I decided to use a
manual cooling method: I open the
door when the controller asks.
HOME-BUILT CONTROLLER
As for building the controller, I used
the Basic Micro EVB87 Renesas evalu-
ation board, which is fitted with an
H8/3687 microcontroller (see Figure 2).
This inexpensive microcontroller is
exactly what I needed for this project.
It has a low pin count, high RAM and
program memory for using a simple
high-level language like BASIC, and
on-chip ADC. The evaluation board
already provides an LCD and push but-
tons, so all I had to do was add two small
circuits using the prototyping area. The
first was a thermocouple interface. I used
a K-type thermocouple and an Analog
Devices AD595 monolithic thermocou-
ple amplifier with on-board cold-junction
compensation directly connected to the
H8/3687’s ADC. More than simple, isn’t
it? Lastly, I needed a way to drive an
external relay to switch the oven on and
off. It took nothing more than a BD235
transistor and a free-running diode. Of
course, you could use your favorite
microcontroller (e.g., Basic Stamp’s) for
this design, but this evaluation kit
speeds up the process (see Photo 2).
On the firmware side, I decided to
try to the BasicATOM programming
language because this application isn’t
managing microseconds. This was
easy thanks to the user-friendly devel-
opment environment provided by the
board in addition to the resident boot-
loader. It took only 4 h to write and
debug the full software, even though I
wasn’t able to use the debugger for
some reason.
The application, for which the
source code is available on the Circuit
Cellar
ftp site, is extremely easy to
read. It is structured into two inde-
pendent sections: a configuration sec-
tion and a state machine section. The
former allows you to manually modify
all profile parameters with the LCD
and a couple of push buttons. The lat-
ter manages all of the successive steps
of the thermal profile and reads the
actual oven temperature in real time.
Let’s look at the details.
First, a welcome screen is displayed.
Then, a static screen displays the cur-
rent temperature in real time and allows
Figure 2—
The reflow oven controller is simple. There are only six components in addition to the Renesas EVM87
evaluation board.
Photo 2—
The oven controller includes a Renesas
evaluation board with thermocouple and relay inter-
faces in the breadboard area.
Photo 3—
When it’s running, the controller shows the
current phase (“UpDry,” meaning “up to drying tempera-
ture”), and the preset and actual temperatures, as well
as the remaining time to spend in this phase.
34
Issue 168 July 2004
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ture to be equal to the drying tempera-
ture. The third step involves drying.
The preset temperature stays constant
during the drying time.
Heating follows this. Basically, the
preset temperature is linearly
increased from the drying to the
reflow temperature. Then, the con-
troller waits for the actual tempera-
ture to be equal to the reflow tempera-
ture. Next, the preset temperature
stays constant during the reflow time.
Finally, the preset temperature moves
linearly down to the initial ambient
temperature, and you open the oven
door to help the process.
In addition to state
machine management, a
thermal control loop is
implemented in the soft-
ware using a PD-like algo-
rithm. An estimated future temperature
is calculated using the actual tempera-
ture plus a multiple of the differential of
the temperature. The estimated future
temperature is compared to the preset
temperature in order to switch the
heater elements on or off. This allows
you to account for the thermal latency
of the captors and heaters and to get a
more stable temperature. A small hys-
teresis factor is also used in the compar-
isons in order to extend the life of the
heater. Figure 3 shows the actual ther-
mal profile compared to the preset val-
ues generated by the controller.
OVEN IN USE
Now that you know why you need a
reflow oven and how to build one,
how will you use it? First, you need to
get some additional equipment. You’ll
need a good magnifier (a video magni-
fier would be best) and a syringe of
solder paste. It’s easier if you have a
pneumatic fluid dispenser
(which is easy to find on
the ’Net, but you will
also need shop air)
because using the syringe
can be difficult. You will
also need a flux cleaning
spray, a small classic iron
(15 W or less), and, ideal-
ly, a hot-air iron for
repairs. Don’t worry, the
Internet is a good source
for low-cost equipment.
Even if you don’t have a video magni-
fier, you can be successful if you have
a good optical lens and good eyes!
A clean blank PCB is extremely
important. I know from experience
that it’s nearly impossible to achieve
good reflow results with home-etched
PCBs. Solder masks are really helpful
in keeping the solder paste on the
pads. I don’t know how to get solder
masks on home-etched PCBs, so find
a good PCB manufacturer and order
your PCB with solder masks.
You have to put solder paste on the
pads with the syringe either manually
or with the fluid dispenser. Then, you
must place each component on the
PCB on the solder paste spots. These
two steps are time-consuming, but
things speed up with experience!
Remember that it is far quicker to use
solder paste than to try to solder by
hand. After all of components are
placed, you can put the board in your
reflow oven and press the Start but-
ton. After 10 min. or so, your PCB
will be cooked. Beware, it can still be
quite hot. Use flux removal fluid and
a toothbrush to clean the board (see
Photo 4). Refer to the Circuit Cellar
ftp site for a detailed illustration of
the process.
By the way, you can use this process
for boards with components on both
sides if you put light components on
one side. Reflow only that side first,
and then flip the board and repeat the
process: apply solder paste, place the
components, and then reflow. The
components on the bottom side will
300
250
200
150
100
50
0
0
100
200
300
400
500
600
Thermal profile
T
e
mper
ature (
°C)
Time (s)
t
SET
t
ACTUAL
Figure 3—
The red curve is the set point defined by the software, where-
as the yellow curve shows the actual temperature as measured inside
the oven. The full cycle takes around 10 min.
With a $150 toaster oven and a
low-cost controller, you can
achieve reflows with good results.
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CIRCUIT CELLAR
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Issue 168 July 2004
35
usually stay soldered thanks to the
surface tension of the solder.
Finally, you should check the elec-
trical connections and make the neces-
sary repairs. If you follow the process
closely, you shouldn’t have too much
trouble. My last 160 mm × 100 mm
board had more than 300 fine-pitch
components on both sides, but I had
only a couple of bad joints and one
solder bridge on a 0.65-mm pitch IC.
THREE-DAY SOLUTION
This project proves that SMT tech-
nology is neither boring nor too expen-
sive for the occasional prototype circuit
maker. With a $150 toaster oven and a
low-cost controller, you can achieve
reflows with good results. The only
other things you need are a good lens
and some basic equipment that’s readily
available on the used market. And, yes,
you’ll need some patience too.
This project also shows that BASIC
is efficient language for low-speed,
real-time applications, especially those
that are associated with microcon-
trollers with plenty of RAM and pro-
gram memory in order to fully use the
power of such languages (e.g., the
Renesas H8/3687).
Last but not least, this project
demonstrates that embedded develop-
ment doesn’t necessarily have to be a
huge undertaking to give good results.
It took me only three days to finish
this project. I spent one day drawing
the concept and algorithms on paper.
The last two days were spent building
the prototype, writing the software,
and optimizing it.
The next step will be to experiment
with BGA packages, but that should
work too. Happy reflowing!
I
RESOURCES
Analog Devices, Inc., “Monolithic
Thermocouple Amplifiers with Cold
Junction Compensation: AS594/
AD595,” rev. C, 1999.
———“3687 Starter Kit: Quick Start
Guide,” 2003, www.basicmicro.com/
downloads/Renesas/3687/QStart.pdf.
Research International, “Reflow
Technology Handbook,” www.research
-intl.com/reflowtechbook.htm.
SOURCES
AD594/AD595 Thermocouple ampli-
fiers
Analog Devices, Inc.
(800) 262-5643
www.analog.com
EVB87 Renesas evaluation board
Basic Micro, Inc.
(248) 427-0400
ww.basicmicro.com
H8/3687 Microcontroller
Renesas Technology Corp.
(408) 382-7500
www.renesas.com
PROJECT FILES
To download the code and additional
photos, go to ftp.circuitcellar.com/pub
/Circuit_Cellar/2004/168.
Photo 4—
Here’s an example of a cooked PCB after
reflow and cleaning. Not too bad, huh?
Robert Lacoste lives near Paris,
France. He has 15 years of experience
working on innovative real-time soft-
ware and embedded systems.
Specialized in cost-optimized mixed
signal designs, he has won more than
a dozen international design con-
tests. Robert currently manages his
own design and consulting company.
You can reach him at rlacoste@
alciom.com and visit his web site at
www.alciom.com.
and detailed in Figure 2. Its purpose is
to limit the system’s maximum veloci-
ty. The velocity is calculated and then
subtracted from the error signal. When
the error signal is at its 2047-count
limit, the motion speeds up until the
velocity reaches a large enough value
to limit additional speed increases.
If you recall, velocity is change in
position divided by change in time. The
velocity feedback design has P0 to latch
the present position and P1 to latch the
previous position. The latching is per-
formed by counting 4,096 clock ticks
(4,096 × 50 ns = 0.203450 ms). A little
state machine transfers the position data
into the appropriate registers and the
proper times using states S0, S1, and S3.
Note that S2 is not used, so don’t look too
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L
ast month I started describing a
two-axis motion control system featur-
ing an Altera Nios embedded CPU. In
fact, I put the entire system inside one
of Altera’s FPGA devices. I left off with
V00-00. I had the CPU and one axis
designed. Also, I did an initial placement
and achieved my design goal of 20-MHz
operation. This month I’ll explain how
to enhance the axis component by
adding velocity feedback and building a
two-axis system. I’ll also detail the
software you’ll need to run the design.
Well, I was extremely pleased with
the V00-00 version of this project.
Typically, you would review this with
all the interested parties in your organ-
ization (a design review). And out of
that hypothetical meeting, suppose
service said that you need to monitor
the temperatures of motors and other
circuitry, and that marketing is really
close to having an answer about the
network requirements. What to do?
I found Analog Device’s AD7476/7/6
serial A/D converters, which have 8, 10,
or 12 bits, respectively, of resolution at a
1-msps rate. So, I added a four-device SPI
serial interface to the Nios processor. And
for the network requirements, I added an
interface to the LAN91C111 on an Altera
development board. So, now I have a
schematic and software, if marketing is
ever able to firm up their requirements.
Photo 1 shows the new SOPC builder.
Note that the SPI interface requires
seven pins, three of which are shared
for data in, data out, and clock, and
four of which are chip selects, one for
each device. The LAN91C11 interface
requires just three additional pins
because there is already a bus for off-
Designing with the Nios (Part 2)
Last month, George laid the groundwork for a Nios-based two-axis motion control system.
Now he discusses velocity feedback and preps you on the software you’ll need to drive it.
chip memory. The pin count can be a
limiting factor in any FPGA design.
Having too many pins pushes you into
a larger, more expensive package.
The place and route achieved 31.02
MHz in the selected device. The maxi-
mum clock frequency times got better
even though I added more logic ele-
ments. Don’t count on this to be the
norm. By looking at the timing reports
I see that the longest path is in the
axis-error calculation, which isn’t sur-
prising. I could speed up this path and
increase the maximum clock frequen-
cy by adding registers in the path. But,
as it stands, I’ve met the design goal.
I also added a velocity feedback ele-
ment to the design. The velocity feed-
back block diagram is shown in Figure 1
FEATURE ARTICLE
by George Martin
System Enhancement
Photo 1—
This screen shot of the SOPC builder shows the CPU configuration details. Screen peripherals are
added to the CPU from this. The implementation details of the peripherals are also defined.
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CIRCUIT CELLAR
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37
quencies. If the resonant point in your
mechanical system is below the high-fre-
quency cut off defined by the output driv-
ers, then the mechanics will oscillate
whenever the offending frequency is out-
put. And because the step output contains
all frequencies, your system will oscillate.
After you know the resonant fre-
quency, you could define a digital fil-
ter to remove it from the output sig-
nal. The great part about this solution
is that each system probably has a
slightly different resonant point, and
you can tune the digital filters for each
system. Production will love that.
SOFTWARE
I built a set of software routines to
help you understand the Nios design.
long for that reference. I was planning to
add registers for settling time on the cal-
culations, and S2 provides another 50 ns
of delay for the calculations to complete.
So, I have
∆
P over
∆
T, and that’s
available on the output pins. This num-
ber needs to be scaled. I could scale it by
changing the 4096 clock divider in the
velocity feedback count block. I could
increase or decrease the time period for
the measurement. If that’s not suffi-
cient, I could change the type of counter
to a count down and achieve any time
period. If that still isn’t enough, I could
add a multiplier to the VEL[31..0] sig-
nal to scale it to the appropriate value.
Figure 1 shows an optional filter
block just before the output signal. I
didn’t include this as part of my design.
This would be a great place to add a digi-
tal filter to attenuate any unwanted fre-
quencies in the output signal. Most sys-
tems have resonant frequencies. The
mechanical designers try to eliminate
them, but you usually will be faced with
working around them. If you need a visu-
al example, imagine a fly fishing rod. As
you slowly move your arm and the rod,
the tip of the rod exhibits no lag in its
positioning. But, as you start accelerating
you arm and the rod, the rod’s tip starts
to lag. If you move your arm back and
forth at just the correct frequency, the tip
of the rod will be moving in the opposite
direction of your arm.
You’ve just found the system’s resonant
frequency. The controller’s output can be
thought of as a step, containing all the fre-
Figure 1—
Aiming for single-axis control with velocity feedback? The A and B inputs derive the current position. The CPU sets the target position register. The errout output is
used to drive the servo motors.
Figure 2—
For the velocity feedback circuit, the positions are captured in latches P0 and P1. The difference is latched and represents the velocity. The up-counter determines
the rate of position sampling.
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I know I’ve got some explaining to do.
First, let’s take a look at the constants
__TURBOC__ and RUNNING_IN_NIOS.
When the Turbo C compiler is
invoked,
__TURBOC__ is defined and
RUNNING_IN_NIOS is not defined. If
__TURBOC__ is not defined, then
RUNNING_IN_NIOS is defined.
Look at the beginning of
CCCONST.h, which you may down-
load from the Circuit Cellar ftp site.
This file enables code to run in Turbo
C or any other compiler, without writ-
ing to an address that will cause prob-
lems on the PC.
uart0 is defined at
some constant address in excalibur.h,
so accessing it in Turbo C will probably
crash the PC’s operating system. I know
this because I’ve done it by accident.
Next, the
BYTE keyword is defined
in CCCONST.h. When different com-
pilers and CPUs are used, the value of
definitions such as
int can change.
So, I created keywords including
BYTE,
These routines create a test bed for code
development and will actually run the
hardware. I used Turbo C++ V. 3.0 in a
DOS window to write, compile, and
debug code. I know this is an old
method, but it’s great for getting start-
ed, especially when the hardware
doesn’t exist yet. Eventually, you will
use the GNU tool chain to build the
software for the Nios CPU.
UART
The interface to each of the periph-
erals selected for the design is generat-
ed as a set of registers. Take the UART
for example. The register set for the
UART is defined as a structure in the
excalibur.h file (see Listing 1).
The structure
np_uart (“np” stands
for Nios peripheral) has six 32-bit registers
that interface to the hardware. These reg-
isters are memory-mapped I/O. The first
register is
np_uartrxdata, which con-
tains the data received in the UART. The
actual data is only 8 bits, but 32 bits are
reserved for this and every register. Other
symbols are defined too:
na_uart_0 is a
pointer to an
np_uart located at the
0x00040000 address;
na_uart_0_base is
the base address value for the UART 0
register set; and
na_uart_0_irq is
the interrupt request number.
The excalibur.h file also contains bit
definitions for the UART and all periph-
eral control purposes. So, the software
can be completely written using the
excalibur.h file. To make hardware
design changes, all you need is the latest
copy of excalibur.h. If you’ve ever bought
a C compiler dedicated to a microproces-
sor, it probably came with a header file
that described the CPU; those header
files look a lot like excalibur.h.
In the ARMCOM.c file posted on the
Circuit Cellar
ftp site, you will find the
routines that directly touch the Nios
hardware as opposed to transferring
data in and out of the serial buffers in
memory (CCCOM.c and CCCOM1.c).
The
PutCom0(BYTE c) routine puts a
BYTE (8-bit entity) in the UART’s
transmit buffer (see Listing 2). First, a
pointer to the UART register structure
is declared (
np_uart *uart), and then
the pointer is set to point to the
uart_0 register set in memory-mapped
I/O. And finally, the data is placed in
the Tx register (see Listing 2).
INT16, INT32, and UINT8 to define
what I intend to do.
If I use a different CPU or compiler,
I will redefine these values for each
new implementation. Take a look at
the
INT16 for the Turbo C (16-bit) com-
piler versus the Nios (32-bit) compiler.
Lastly, note that if you are compiling
for Turbo C and running in a DOS win-
dow, the output for
Com0 goes to the
stdout, which is the console on the PC.
The serial communications routines
are in the ARMCOM1.c, CCCOM.c,
and CCCOM1.c files. The incoming
characters generate an interrupt in
which characters are read from the
hardware and placed in a buffer. Input
line-feed characters are ignored. I’ve
had trouble when systems send a car-
riage return and then line feed while
others send a line-feed followed by a
carriage return. So, I just started ignor-
ing line feeds and life became simpler.
Also, I processed complete lines of
Listing 1—
The NIOS UART data structure is defined by
np_uart
. The hardware mapping of the UART is
defined by the
(np_uart *)
construct.
// UART Registers
typedef volatile struct {
int np_uartrxdata; // Read-only, 8 bits
int np_uarttxdata; // Write-only, 8 bits
int np_uartstatus; // Read-only, 8 bits
int np_uartcontrol; // Read/Write, 9 bits
int np_uartdivisor; // Read/Write, 16 bits, optional
int np_uartendofpacket; // Read/Write, end-of-packet character
} np_uart
#define na_uart_0 ((np_uart *) 0x00040000) // altera_avalon_uart
#define na_uart_0_base 0x00040000
#define na_uart_0_irq 16
Listing 2—
This
Put_Com
example will run a PC and Nios. If compiled with
__TURBOC__
defined, then
the character goes to the PC’s console device. If compiled with
RUNNINGIN_NIOS
defined, then the
character is placed in the UART buffer at location 0x40000 (see Listing 1).
// This routine puts a character into Nios Com0: port
void PutCom0(BYTE c) {
np_uart *uart;
// Create a ptr to a Nios UART
// data structure
uart = (np_uart *)na_uart_0; // Set that pointer to Com0 hardware
// Output the data
#ifdef RUNNING_IN_NIOS
uart->np_uarttxdata = c;
#endif
#ifdef __TURBOC__
// put_com(c);
#endif
} /* end of void PutCom0(BYTE) */
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CIRCUIT CELLAR
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could just write to a memo-
ry-mapped register offset
from some base address. My
approach takes a couple of
extra instructions for each
operation, but it works
without any hassle.
In the CCPOS.C file, I
defined the bit functions for
the Read_Status[15..0] inputs
and the CTRL[15..0] outputs.
On the input side, I have Loop
Status (OK/fault), Home,
Limit, and Fault indicators for each axis.
On the output side, I have Latch
Position, Clear Position, Set Position,
Clear Target, Latch Target, and Close
Loop for each axis. I don’t use every one
of these bits because several are redun-
dant. I included all of them to show you
what’s possible in this type of design.
The
SetTarget, GetPosition,
and
SetPosition routines are fairly
self-explanatory. These are used to
clear control bits, output/input the
data, and then set any control bits
(see Figure 3). The control bits are
positive-edge active.
data as they were received. I
define an end-of-line charac-
ter for each serial port. When
that character is received, a
line counter is incremented.
All the main routine looks at
is that line counter. When
the input line counter does
not match the output line
counter, I know a complete
line has been received and I
start to process that line.
I selected a Nios UART
with a fixed data rate and other param-
eters. If you look through these serial
routines, you’ll see how to interface to
the Nios UART hardware. You can
then extrapolate to other variations
you might add to the CPU. Altera also
provides documents and code examples
for the peripherals.
TIMER
The CCTIM1.c file contains the
timer routines. You can set the timer
period and install the timer interrupt.
The timer variables are incremented for
each interrupt. If you have a timer inter-
rupt period of 1 ms, every 1,000 inter-
rupts are equal to 1 s in the real world.
I created a timer test option in the
main program loop. With this option,
you can install and remove the timer
interrupt, and set the interrupt period.
PIO INPUT AND OUTPUT
I used a parallel I/O scheme to inter-
face to each axis. Basically, I set a value
for the Nios PIO and that worked its
way down through the hardware. This
is the safest approach I could think of.
It’s slower than designing each axis as a
device, like the UART, for which you
Figure 3—
This circuit is looking for a change from zero to one. After the change
occurs, the circuit’s output goes to a one for one clock period.
George Martin began his career in the
aerospace industry in 1969. After five
years at a real job, he set out on his own
and cofounded a design and manufactur-
ing firm (www.embedded-designer.com).
George’s typical designs include servo-
motion control, graphical input and
output, data acquisition, and remote
control systems. He’s a charter member
of the Ciarcia Design Works Team. He’s
currently working on a mobile commu-
nications system for the military. In his
spare time, George has become a nation-
ally ranked revolver shooter. Contact
him at george.martin@worldnet.att.net.
RESOURCES
Altera Corp., “Nios UART,” DS-NIO-
SUART-3.0, 2003.
Nios peripheral library, www.altera.com/
products/devices/nios/features/nio-
peripherals.html.
SOURCES
Nios embedded processor, SOPC
Builder, QuartusII design software
Altera Corp.
www.altera.com
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an
int, which is a 32-bit entity. You
need to test this assumption when
you’re debugging the hardware.
You now have an axis and a step size.
The motion system should be resting,
and the position and target registers
should be equal values. However, note
that the target register is constant, and
the position register varies slightly
because outside disturbances cause the
mechanism to move.
I have defined target registers for each
axis. You need to add the step to the tar-
get register and set that new value in the
hardware. This causes the hardware to
generate an error signal that drives the
axis motor in a direction to reduce the
error. You can monitor the position until
the motor stops and its close enough
to the target. If all is well, you’re done.
A word of caution here: Lots of things
can go wrong. The mechanism could
run into something and never get to the
target position. The step size could be
inacurate and you might move too far
past your limit. The move could be so
slow that you timeout before reaching
your goal. The motor could overheat
and shut down. Furthermore, you could
reach your goal but be unstable, oscillat-
ing back and forth across the target. I
didn’t consider this when I wrote the
code. I just looked for a way to reach the
target. This gets you started on the right
foot. You also might want to modify the
design to address the potential problems.
You’ll notice that the
RunMotion rou-
tine assumed the axis was at some sort
of acceptable, stable position. How did
you get there? Well, you went through a
zeroing process, which moved the axis to
the home position, which was zeroed
along with the target registers. Actually,
each axis has a home, zero, and limit
position defined by its construction.
Home is a position that you can move
to rapidly. Some overshooting is permit-
ted. Zero is a position found under stable,
repeatable conditions. For example, mov-
ing to zero occurs at a rate until an opti-
cal sensor is tripped. Limit is a fault-type
condition that indicates you’ve gone too
far. Sometimes software reacts to the
limit condition, and sometimes the hard-
ware takes over and shorts the motor,
causing the axis to stop abruptly. You can
recover from a limit condition by moving
the axis by hand. If you try to move
quickly, the motor will act as a generator
and the short will act like a brake.
A
ZeroAxis() routine takes care of
this homing operation. Its implementa-
tion requires information from outside
sensors. Keep in mind that I used hypo-
thetical data for this article rather than
gathering data from sensors. Therefore,
you’ll want to pay more attention to
the comments than the actual code. I
suggest adding a
GoHome(INT16 Axis)
routine that would close the loop and
start stepping relatively small amounts
toward the home direction until the
home sensor is activated.
NEXT
Next month I’d like to show you a
complete system with... Wait a minute.
Why don’t you take over? It doesn’t
have to be a Nios; any FPGA vendor’s
system would be good. Come on, show
us where you’re going today.
I
The
InitAxis routine opens the
control loop for the axis. Then, it sets
both the position and target registers
to zero and closes the loop.
I left out looking for errors in the
control loop. Potential errors include a
high motor temperature reading, detec-
tion of limit input, or a timeout on a
move command. Each system is so dif-
ferent that showing one implementa-
tion is a waste of ink.
Another feature of the Nios PIO input
registers is that an interrupt can be gen-
erated for an event on any input bit. If a
safety interlock is activated, you might
want to open the loop immediately.
MOTION CONTROL
You have two options for motion test-
ing in the main control loop. The first is
to zero both axes. The second is to move
one of them a predetermined distance.
Let’s start with the second command.
The format is as follows: G n dddd<CR>.
“G” is the unique command prefix,
“n” is the axis, “dddd” is the command
amount, and “<CR>” is the end-of-com-
mand character. This data is entered
into the serial port and processed using
the call
count = ReadLineCom1(buf).
The return value is the number of
characters returned in
buf. If there are
any, the main loop attempts to process
the returned characters. The first char-
acter is the test option selected. If that
first character is “G” or “g,” then con-
trol is transferred to
RunMotion and
that routine is passed a pointer to the
rest of the command line.
The
RunMotion routine takes the
pointer and extracts the axis number
and the size of the step. It does so by
first ignoring (skipping over) spaces in
the command string. Then, a call goes
to the C
atoi() routine, which returns
the integer value of the characters start-
ing at the pointer value passed to it.
That return value is the axis number.
Skipping over the axis number and
any subsequent spaces places the point-
er at the start of the step size. The
atol() function is used to extract the
value. Be careful here. You need to rep-
resent position and step sizes in 32-bit
entities. In Turbo C,
atol is used to
get a 32-bit return value. In Nios, I
selected a 32-bit CPU, so I expect the
compiler to interpret
atoi() to return
PROJECT FILES
To download the code, go to ftp.circuit
cellar.com/pub/Circuit_Cellar/2004/168.
DS requires only approximately
60 words of program memory. It was
designed for this kind of interface, but
my only debugging tool is awkward to
use, and I was more interested in see-
ing waveforms than writing another
debugging tool. If one or more of the
processors had been tied up in a time-
intensive task while needing to attend
to the firmware-only communications
channel, I would have used this one.
The serial peripheral interface (SPI)
does not require any critical timing. It
only needs a small amount of code, and
would have been a good candidate,
except that it would have required a
third wire, which is undesirable for the
interface between the graphic display
unit and the capture and control unit
because of the cable between them.
When it came time to actually write
the code, I settled on asynchronous
serial ASCII for all the communica-
tions channels, including the one that
was to be implemented in firmware.
The second serial channel in the
waveform capture controller is easy to
implement using the compact code I
found on the ’Net. As for a debugging
tool, I realized that if I would stick to
printable ASCII codes, I could use
the terminal program on my PC
and directly interact with the con-
trollers. And that’s what I did.
DISPLAY MODULE
The graphic display unit’s inputs
are power and asynchronous serial
data. Its output is handshake data
to its host, and it displays informa-
tion by writing black dots on a
42
Issue 168 July 2004
CIRCUIT CELLAR
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A
fter building the minimum mass
waveform capture circuit that I
described last October (“Minimum Mass
Waveform Capture,” Circuit Cellar,
issue 159, 2003), I couldn’t resist the
urge to build an oscilloscope-like instru-
ment with it. The waveform capture
engine was already designed, so all I
needed were a little analog processing, a
simple graphic display, a user interface,
and some interchip communications to
tie it all together.
My waveform capture and display sys-
tem is composed of two circuit assemblies
incorporating a total of three Atmel
AT90S2313 microcontrollers (see Figure 1).
One assembly is the graphic display
unit, which includes a Seiko-Epson
G1216B1N000 64 × 128 graphic LCD, an
adjustable negative bias supply for the
display, and a microcontroller-based seri-
al interface. I put the graphic display unit
in a separate clear plastic box so I could
position it for easy viewing; it’s inde-
pendent of the rest of the electronics.
Because the unit is in its own box, I can
use it with other projects. The display
and control panel are shown in Photo 1.
The other assembly is the capture
and control unit, which includes the
waveform capture electronics that
supply the waveform and trigger
signals to the waveform capture
controller. It also has all of the user
controls, including those for signal
gain and offset, triggering, timebase
adjustments, a four-digit LED time-
base display, and a trigger indicator
LED, all of which are minded by the
user I/O controller. All the commu-
nication between the capture con-
Triple AVR Waveform Capture and Display
Dick built a simple waveform capture and display system around three AT90S2313 micro-
controllers. Although it’s not a replacement for a real-time oscilloscope, he relies on the sys-
tem for a variety of tasks such as monitoring the I/O on a microcontroller.
trol unit and the other two controllers
is 0- to 5-V asynchronous serial.
WHY ASYNC SERIAL?
Each of the three controllers has a
hardware UART on it, but the wave-
form capture controller has to support
two bidirectional communications chan-
nels (one to the graphic display unit and
the other to the user I/O controller), and
that means one or more of the commu-
nications channels must be imple-
mented in firmware only. I looked at
Philips’s I
2
C protocol a couple of times.
It’s certainly well proven and designed
for exactly this kind of interchip com-
munication. But, I was concerned about
the amount of code space a firmware
implementation would take and the
severe demands on the slave controller’s
response time. Most of all, I dreaded the
hassle of debugging something I had
little experience with and no tools for.
The DS protocol turned out to be a
better choice for this project than I
2
C.
It’s a two-wire serial protocol that I
wrote for some earlier projects in
which I
2
C wasn’t suitable because of
its demands on the slave controller’s
response time.
FEATURE ARTICLE
by Dick Cappels
Photo 1
—
It works! I removed the display from its plastic box for
this picture. The controls are arranged in a manner similar to
that found on old Tektronix analog oscilloscopes. The red light
below the timebase display is the trigger signal indicator.
Three Controllers Make a Waveform Monitor
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CIRCUIT CELLAR
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Issue 168 July 2004
43
ASCII Input mode.
$82 switches to
Binary Input mode, and
$83 restarts
the firmware and controller.
The controller’s main loop watches
for data coming into the UART. When
two successive address bytes are
received, they are used as x,y coordi-
nates and a dot is written at that
point. Then, it sends an ASCII asterisk
character back to the host to indicate
that the operation has completed. Any
byte received with a value higher than
127 is interpreted as a command, and
then the execution jumps back to the
start of the main loop. If an unsupport-
ed command is received, it sends an
ASCII exclamation point before return-
ing to the main loop. The acknowledge
(ACK,
$06) and negative-acknowledge
(NAK,
$15) are traditionally used as
handshakes, but I used the asterisk and
exclamation points to make sure they
would print on all ASCII terminals.
CAPTURE AND CONTROL
Two circuit boards are inside the cap-
ture and control unit’s plastic enclosure.
The main board holds two microcon-
trollers: the waveform capture controller
that digitizes the analog waveforms and
the user interface controller that attends
to the user input buttons and the LED
timebase display. Analog circuitry pro-
vides minimal signal processing for the
analog waveform and triggering. The
other board is the control panel board
on which the user controls are mount-
ed. The circuitry for
both boards is shown
in Figure 3.
Early in the project,
I decided to use two
separate controllers
for the waveform cap-
ture and user I/O
functions so I could
put them at opposite
ends of the circuit
board to minimize the
noise pickup by the
analog circuits (see
Photo 2). As it turned
out, using two sepa-
rate controllers also
helped simplify the
development process;
it forced me to use
small modular pro-
light background at locations speci-
fied by the host.
The power for the graphic display
unit comes through a 7805 regulator
in a TO-220 package. The 7805 supplies
5 V to the Seiko module, the microcon-
troller, and a voltage multiplier and
adjustable regulator that provides the
negative bias for the display. To make
the negative supply, the buffered output
of an NE555 oscillator is passed through
a five-stage, capacitor-diode, half-wave
voltage multiplier to provide a negative
power supply for the op-amp, which
provides a regulated bias supply that is
adjusted with a contrast potentiometer.
As for the original breadboard, I was
disappointed to see that the NE555’s
output dropped to less than 3 V
PP
under load. The datasheet confirmed
that this is expected, so I added a two-
transistor saturating buffer so I could
get the necessary negative voltage
with the minimum of multiplier stages.
The transistors in the buffer saturate,
causing the voltage multiplier to be
driven within a couple hundred milli-
volts of ground and the 5-V line.
The inverting serial interface is a
clever circuit that I first saw in
Atmel’s AVR910 application note. It
takes its negative power supply from
its host, which makes it well suited
for projects like this. The interface’s
output swings negative if it’s connect-
ed to my PC’s EIA-232 port for debug-
ging with the terminal program. It
safely swings down to
only a little above
ground if it’s connect-
ed to the 0- to 5-V
COMS input on the
waveform capture con-
troller. There are no
switches, jumpers, or
fancy sensing circuits;
it just works that way!
The interface firm-
ware for Seiko’s 64 ×
128 LCD was compli-
cated because the dis-
play appears as two 64 ×
64 displays side by side,
while the interface was
to make it appear as a
single continuous dis-
play. There are separate
chip-select pins for the
two halves, and the AT90S2313 con-
troller was a pin short, so an inverter
was added to make two chip-select lines
out of one (see Q3 in Figure 2).
When power is first applied, the
AT90S2313 LCD interface controller
resets the LCD, erases the screen, and
draws a line from the upper-left corner
to the middle of the bottom edge and
then another to the upper-right corner,
which looks similar to a large V span-
ning the display. This is a visual indica-
tion that the power supply, display
module, and serial controller are up and
running. At that point, the graphic
interface controller enters its main loop
where it waits for data from its host.
Data from the host is either com-
mands or parts of the x,y screen
address at which a dot is written. An
internal flag tells the UART character
receive routine whether to receive
data in 2-byte ASCII hex format,
which is useful for debugging, or in
single-byte binary format, which
needs only 2 bytes for a dot address
instead of 4 and doubles throughput
when writing to the screen.
Because the highest dot address is
127, I was able to reserve those values
with the most significant bit set (cor-
responding to decimal values of 128 or
higher) as commands, while values of
127 or lower were treated as x,y
address data. As for the command
codes,
$80 erases the display and
sends an asterisk, and
$81 switches to
8 to 14 VDC
Power
return
5-V
Regulator
A
T
90S2313
Microcontroller
Signal
Attenuator
A
T
90S2313
Microcontroller
Serial ASCII
Ser
ial ASCII
Amplifier
Signal
Ground
External
trigger
Trigger
circuit
Waveform capture
controller
User interface
controller
4 × 7
LED display
8 8 8 8
User input buttons
Capture and control unit
AT90S2313
Microcontroller
Adjustable
negative
supply
64 × 128
LCD Module
LCD Interface
controller
Graphic display unit
5-V
Regulator
Figure 1
—
The waveform capture and control system is composed of a capture and control unit and a
graphic display unit connected to each other with a 50-cm, five-conductor flat cable.
44
Issue 168 July 2004
CIRCUIT CELLAR
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small capacitor should be selected and
placed in position C1 or C2 to compen-
sate for the differences in capacitance,
which include wiring capacitances and
the input capacitance of the FET buffer
following it. With my layout, I needed
5 pf for C2 and C1 was not used.
Source follower Q1 provides high-
input impedance to the attenuator and
shifts the signal to positive so that sig-
nals below ground are within the
grams rather than one large,
complex one.
The unregulated 8 to 14 VDC
coming into the assembly is
shunted by a 14-V varistor to
protect against line transients.
After passing through an on/off
switch, the unregulated voltage
directly powers the op-amps and
the 78L05 voltage regulator.
Then, it’s sent off the board
through the display connector to
the graphic display unit.
The input waveform is capaci-
tively coupled to the top of a
10:1 attenuator. The waveform
capture controller’s full-scale input is
1 V, so the 10: attenuator raised the
full screen sensitivity to 10 V, which
is more than enough for most of my
needs. Closing the switch across the
coupling capacitors makes the input
direct coupled. The ratio of the capaci-
tive reactance across each of the atten-
uator resistors should be the same
ratio as the resistance values in order
to make square waves look square. A
input range of the waveform
capture CPU. Either the indi-
vidual FET used for Q1, the
value of R1, or both may have
to be selected to obtain the nec-
essary 1.5- to 3.5-V gate to
source voltage. A gain stage
made with A1A and A1B allows
for the selection of a bandwidth-
limited version of the signal to
give 10× gain for the full-screen
sensitivity of 100 mV. The fine
and coarse offset potentiometers
are adjusted so that the 0-VDC
point on the displayed waveform
doesn’t shift when the gain
stage is switched in or out. The
2N2907 on the output of A1A protects
the comparator input of the waveform
capture CPU from excessive voltage.
Trigger signals, whether they’re from
the separate trigger input or the incom-
ing waveform, can be either AC or DC
coupled; they are compared in compara-
tor A2A with the trigger level set by the
trigger-level potentiometer. The result-
ing pulses are applied to the trigger input
Photo 2
—
The main board is at the top. The control board is at the bot-
tom. The waveform capture controller is on the right-hand end of the
main board, and the user I/O controller and the LED timebase display are
at the left-hand end. The analog circuitry is in the middle.
Figure 2—
The op-amp regulates the contrast voltage at –2.2 times the voltage set on the wiper of the contrast potentiometer. The NPN and PNP transistors can be 2N2222
and 2N2907 (or similar devices), respectively.
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CIRCUIT CELLAR
®
Issue 168 July 2004
45
of the waveform capture controller and
to a simple one-shot made with com-
parator A2B, which drives the trigger
signal indicator LED with voltage fol-
lower A1C. As a result, the LED is illu-
minated whenever triggering signals are
sent to the waveform capture controller.
The waveform is digitized by suc-
cessively comparing the input wave-
form at specific time intervals relative
to the trigger signal with a reference
voltage that is stepped from 1 V to
nearly ground. Vertical centering of
the waveform in the display is accom-
plished by changing the reference volt-
age by adding current from Q1 (which
is set by the vertical centering poten-
tiometer) to the resistor divider on the
controller’s PWM output. The PWM
output divides the PWM signal down
to 1-V full-scale.
WAVEFORM CAPTURE CONTROLLER
The firmware for the waveform cap-
ture controller is a modified version of
the firmware posted on the Circuit
Cellar
ftp site for my October 2003
article (ftp.circuitcellar.com/pub/
Circuit_Cellar/2003/159.). During
power-up initialization, the waveform
capture controller’s firmware waits
until the serial data line from the
graphic display unit goes high, which
indicates that the LCD interface con-
troller is ready to receive data. This
keeps the system start-up synchro-
nized when the graphic display unit has
a separate power supply. After that, it
sends an erase command to the dis-
play and waits for an acknowledgment
before sending a “Switch to ASCII
data” command to the graphic display
unit. It then sends a character to the
user interface controller to display the
default timebase setting.
Having set up the system, it goes
into its main loop where it continu-
ously cycles through the process of
collecting a waveform, sending the
waveform data to the graphic display
unit, and checking to see if a character
from the user I/O controller was
received by the UART. In each meas-
urement cycle, 100 waveform samples
are collected at 6-bit resolution to cor-
respond to the 64 × 128 display for-
mat. Some of the extra columns are
used as vertical scale markers, which
are written to the display each meas-
urement cycle.
Captured waveforms are sent to the
graphic control unit. The handshakes
are received at 19,200 bps, 0 to 5 V with
EIA-232 polarity on regular I/O pins, by
Figure 3
—
Take a look at the circuitry on the front panel board and the main circuit board. CPDNG is a special ground bus on the front panel board. The 39-k
Ω
resistor on the
base of Q2 was added to stop RF oscillations.
46
Issue 168 July 2004
CIRCUIT CELLAR
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firmware asynchronous serial commu-
nications routines. Communication
with the user interface controller
occurs at 9,600 bps, 0 to 5 V with the
signal polarity inverted from EIA-232
and uses the hardware UART. The
hardware UART is used for user I/O
to make sure user input is not missed,
even during the time-intensive tasks
of capturing waveforms and sending
them to the graphic control unit.
When the waveform capture con-
troller receives a button code from the
user interface controller, it performs
the indicated internal functions, such
as adjusting the timebase, and returns
one or two characters depending on the
button code. If the timebase changes, it
sends a symbol corresponding to the
current timebase setting. If the button
code is a toggle, such as the one to tog-
gle trigger polarity between positive
and negative edges, it also sends back
the four status flags that show the
states of the four toggle controls.
Although the status flags weren’t
used by the current version of the
user interface controller, they were
handy when debugging with the termi-
nal program. I can use them to update
status LEDs if I update the user inter-
face controller code in the future. The
ASCII Character
Waveform capture unit
Internal action taken when
Action taken when received
response
received by waveform
by user I/O controller
capture unit
Function code
0
Timebase value
Set sampling to 1 µs
Display “1–6”
1
Timebase value
Set sampling to 2 µs
Display “2–6”
2
Timebase value
Set sampling to 5 µs
Display “5–6”
3
Timebase value
Set sampling to 10 µs
Display “10–6”
4
Timebase value
Set sampling to 20 µs
Display “20–6”
5
Timebase value
Set sampling to 50 µs
Display “50–6”
6
Timebase value
Set sampling to 100 µs
Display “1–4”
7
Timebase value
Set sampling to 200 µs
Display “2–4”
8
Timebase value
Set sampling to 500 µs
Display “5–4”
9
Timebase value
Set sampling to 1,000 µs
Display “1–3”
:
Timebase value
Set sampling to 2,000 µs
Display “2–3”
;
Timebase value
Set sampling to 5,000 µs
Display “5–3”
<
Timebase value
Set sampling to 10,000 µs
Display “10–3”
Button code
A,a
Status flags
Toggle triggered/free run
None
C,c
Status flags
Toggle capture enable on/off None
D,d
Status flags, Timebase value Decrement timebase
None
E,e
Status flags
Toggle auto erase
None
F,f
Status flags
Toggle trigger polarity
None
I,i
Status flags, Timebase value Increment timebase
None
Table 1—
These ASCII codes can be received by either the waveform capture controller to set capture parameters
or by the user interface controller to set the timebase display. The user interface controller can only generate the A,
C, D, E, I, and F characters. It does not use the status flags, but characters “0” through “<” and the status flags are
helpful for debugging the waveform capture controller using a serial terminal.
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CIRCUIT CELLAR
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47
functions and button codes are shown
in Table 1.
The status flags returned by the
waveform capture controller are gen-
erated by masking off the irrelevant
bits in the register that the flags are
stored in and adding the
$50 hex
value to it. None of the button or func-
tion codes have values higher than
the
$50 hex value, so any byte with a
hex value of
$50 or higher can be inter-
preted either on an ASCII terminal or
by the user interface controller (if I
add features) as containing flags.
The flags have significance when
they are set: flag 0 indicates that eras-
ing the display before rewriting is
enabled; flag 1 indicates that trigger-
ing is enabled; flag 2 indicates that
triggering occurs on the positive edge;
and flag 3 indicates that the timebase
is in free-run mode.
USER INTERFACE CONTROLLER
The last and easiest part to design
was the user interface controller.
Having thoroughly worked out and
tested the interface to the waveform
capture controller using a terminal,
designing the user I/O controller and
circuitry, as well as writing and
debugging the firmware, was a piece
of cake.
Inside the user interface controller,
a round-robin multitasker, triggered
by Timer0 interrupts, refreshes one of
the four LED characters or checks the
input buttons at each interrupt so
that all four characters are displayed
and the buttons are checked every
five interrupts. The controller’s main
loop continuously checks for a char-
acter arriving in the UART’s receive
buffer. After a character is received, it
checks the character against charac-
ters corresponding to the 13 possible
timebase settings. When a match
occurs, the appropriate bit patterns
are loaded into registers that hold
refresh data for the LED display.
Using this technique, everything
other than the characters relating to
the timebase settings are ignored.
Thus, the status flags are ignored
when they are sent from the wave-
form capture controller. The timebase
display shows the selected waveform
sample rate in a 1-2-5 sequence from
1–6 to 10–3, which represent 1 µs and
10 ms per sample, respectively. The
display codes are shown in Table 1.
Updating the timebase display in
response to data returned from the
waveform capture controller instead
of directly from the user-control but-
tons causes a perceptible lag between
pressing a timebase button and seeing
the display change. That’s because
the waveform capture controller does-
n’t send the updated timebase value
until after the completion of the
waveform capture cycle in which the
character has been received. I had to
do it this way. When I tried setting
the display according to the button
codes sent to the waveform capture
unit, the display was hopelessly out
of sync as soon as the waveform cap-
ture controller missed a button code.
The LED is a common cathode.
The anodes are driven by the user
input controller’s I/O port B through
1-k
Ω
current-setting resistors. When
the multitasker gets to the time slot
assigned to sensing the input buttons,
port B changes to inputs, the buttons’
states are read, and then the port pins
turn back to outputs. An additional
1-k
Ω
resistor between the buttons
and ground protects port B from
over-current when a button is
pressed while its corresponding I/O
pin is driven high.
The user input buttons include the
following: Triggered/Free-Run Toggle,
which enables you to vertically center
and measure DC voltages, even if no
trigger signal is available; Positive
Transition/Negative Transition
Triggering Toggle; Capture Enable/
Disable Toggle, which stops sampling
and freezes the display; and Erase
Before Write Toggle, which keeps the
display from erasing a captured wave-
form before writing the new one. This
last feature makes the display look
more solid because the display doesn’t
disappear before it’s rewritten in each
waveform capture cycle.
BUTTONING UP
The graphic display unit resides in
a clear plastic box with one 50-cm
flat cable that carries serial data and
power to connect it to the outside
world. The waveform capture unit is
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CIRCUIT CELLAR
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mounted in a clear plastic pencil box
that’s the same color as the LEDs,
which can be seen through the top of
the box. Note that the user controls
are mounted on the top of the box.
Along with a ground lead, the trig-
ger and signal inputs exit the side of
the case close to where the flat cable
to the graphic display unit and the
power cable to the 12-V wall wart
transformer exit the case. Captive
micrograbbers and a subminiature
alligator clip enable you to attach the
signal, trigger, and ground leads. All
of the cables plug into the main
board. Note that when the top of the
pencil box is closed, all of the cables
are captive.
Since building this little waveform
monitor, I have come to rely on it for
a variety of indication and measure-
ment tasks. Although it isn’t a sub-
stitute for a real-time oscilloscope,
it’s just fine for things like seeing
what the I/O on a microcontroller is
really doing, and even sorting through
a bag of mystery inductors from the
surplus store.
I
National Semiconductor Corp., “LM124/
LM224/LM324/LM2902 Low Power
Quad Operational Amplifiers,”
SA009299, 2000.
———“LM193/LM293/LM393/LM2903
Low Power Low Offset Voltage Dual
Comparators,” DS005709, 2000.
Philips, “I
2
C Bus Specification Version
2.1,” 9398 393 40011, 2000.
Seiko Instruments, “Liquid Crystal
Display Module G1216B100N,”
MG1216B1N000-002, 1993.
Dick Cappels enjoys tinkering with
and writing about analog circuits and
microcontrollers. He has published
several papers on the topic of elec-
tronic displays in computer systems
and is currently active in the Society
for Information Display. Dick holds
18 U.S. patents in areas including dis-
play technology, color management,
and capacitive proximity sensing. You
may reach him at projects@cappels.org.
PROJECT FILES
To download the code, go to ftp.circuit
cellar.com/pub/Circuit_Cellar/2004/168.
RESOURCES
Atmel Corp., “AVR910: In-System
Programming,” rev. 0943C, 2000.
———“8-bit AVR Microcontroller
with 2K Bytes of In-System Program-
mable Flash: AT90S2313,” 0839I, 2002.
D. Cappels, “DS Two-Wire Protocol,”
users.cableaz.com/~cappels/dproj/
dspage/ds.htm.
SOURCES
AT90S2313 Microcontroller
Atmel Corp.
(408) 441-0311
www.atmel.com
LM324 Op-amp, LM3939 Comparator
National Semiconductor Corp.
www.national.com
G1216B1N000 LCD
Seiko Instruments, Inc.
(310) 517-7771
www.siielectroniccomponents.com
50
Issue 168 July 2004
CIRCUIT CELLAR
®
www.circuitcellar.com
I
speak regularly with the electrical
and application engineers that support
the ICs and modules that I talk about
in my columns. The reason for this is
simple: I get into their heads. As a
result, I have a better understanding
as to how to work the ICs and elec-
tronic modules into applications that
may interest and help you. Getting
freaky with an application doesn’t
always make sense for you, the reader.
So, I always try to use a device for its
intended purpose. I’ll do that this time
around, but with a slight twist.
dsPICDEM (BEFORE)
As you can see in Photo 1, I have a
Microchip dsPICDEM 28-pin demo
board loaded with a dsPIC30F2010 engi-
neering sample. Engineering samples are
like rental cars: you get one, run the
heck out of it, and buy your own if the
experience is positive. I tend to work on
the bleeding edge of most demo boards,
so I took this one for a test drive. Now
troller communications channel, I con-
nected the dsPIC30F2010 and the
PIC18F452 coprocessor serially, using
the dsPIC30F2010’s UART interface and
the PIC18F452’s USART interface. By
directly connecting the USARTs, I pro-
duced a simple and relatively high-speed
communications link with a minimum
of hardware and firmware effort.
Next, I added a NICholas 10/100
microcontroller NIC, which is solely
supported by the PIC18F452. The idea
here is to provide a fast Ethernet
interface to the motor control func-
tionality of the dsPIC30F2010 on
the dsPICDEM demo board. After the
Ethernet interface is established, you
can get at the dsPIC30F2010 and its
internal organs in all sorts of ways.
dsPICDEM (AFTER)
Photo 2 is a view of the modified
dsPICDEM 28-pin demo board. The
NICholas Ethernet NIC stacked on
the dsPICDEM is based on the ASIX
AX88796L 10/100 Ethernet engine IC,
which does all of the work that makes
Ethernet things go.
Uncomplicated dsPIC
Implementation
APPLIED PCs
by Fred Eady
I’ll let you know how it felt because
you’ll probably get the real thing on
your dsPICDEM 28-pin demo board.
At first glance, the dsPIC30F2010
datasheet gave me the impression that
the dsPIC30F2010 was designed to be
an integral part of a motor control or
power control system. The six PWM
output channels can go either way,
while the quadrature encoder interface
supports the motor control side of my
datasheet interpretation. Although the
dsPIC30F2010 sports an internal DSP
engine, its pins are heavily multiplexed
to incorporate all of the motor control-
oriented functionality. That’s a good
thing if all you want to do is dictate the
actions of a motor or supply numerous
PWM signals to a power controller/
voltage regulator subsystem.
No matter what, you still have to be
able to talk to the dsPIC30F2010 to con-
trol it dynamically. The dsPIC30F2010
is capable of communicating with its
three-wire SPI module, I
2
C, or serially
using its internal UART module. The
downside of taking the dsPIC30F2010
out of solitary confinement is that
you’ll either have to give up some
motor control pins to the communi-
cations interface of your choice or
bring in some help.
There isn’t much on the dsPICDEM,
but considering all of the stuff inside
the dsPIC30F2010, not much else is
required if motor control is your goal.
Just add an H-Bridge, tie in your
encoder, and write some PWM driver
code. That’s nice, but I had something
else in mind (see Figure 1). First, I
added a coprocessor to the dsPICDEM
in the form of a PIC18F452. To facili-
tate a microcontroller-to-microcon-
As soon as Fred got his hands on a Microchip dsPICDEM 28-pin demo board, he modified it.
This month he proves that you don’t have to be a DSP wiz to get started with the dsPIC.
Photo 1—
This is one unassuming development board,
but don’t let the lack of componentry fool you. There’s a
lot of bang in this little package.
Photo 2—
Check out my dsPICDEM 28-pin demo board
after a trip to the Florida room chop shop.
www.circuitcellar.com
CIRCUIT CELLAR
®
Issue 168 July 2004
51
The AX88796L is an NE2000 regis-
ter-compatible, local CPU bus
Ethernet controller with an embedded
10/100-Mbps PHY transceiver. Using
the IEEE802.3 and IEEE802.3u stan-
dards, the AX88796L provides 10- and
100-Mbps Ethernet functionality in
full- or half-duplex operation.
Note that 8 KB of 16-bit SRAM is
included on the AX88796L, as is an 8-
or 16-bit local CPU interface that can
accommodate a wide range of micro-
controllers. The AX88796L can be
configured to run in either 8- or 16-bit
mode by simply setting a bit in its
data configuration register (DCR).
Specifying the CPU interface is just as
easy. The AX88796L can be config-
ured to run in one of four local bus
CPU modes by jumpering pins CPU[0]
and CPU[1] to logic levels that
instruct it to interface to one of the
following environments: an ISA, an
80186, a 68000, or an 805X. The
NICholas loaded on the dsPICDEM
demo board is jumpered for ISA opera-
tion, which equates to both CPU[0]
and CPU[1] being pulled low.
The AX88796L, which is housed in
a 128-pin LQFP low-profile package,
typically consumes less than 100 mA
from a 3.3-V power source. In addition
to smashing packets through a wire,
the AX88796L’s I/O is 5-V tolerant,
which means you really don’t have to
convert the dsPICDEM’s 5-VDC
power supply to 3.3 VDC to accom-
modate the Ethernet controller. The
AX88796 L’s clock lines are the only
I/O lines that are not 5-V tolerant. A
25-MHz crystal or oscillator can be
used as the AX88796L’s clock source.
To keep the part count down and
maintain the smallest footprint possi-
ble, the AX88796L development board’s
Ethernet magnetics are contained with-
in a single LAN Mate unit. The LAN
Mate NU1S041C-434 complies with
the IEEE 802.3u standards. The module
used on the NICholas is a 1:1 center-
tapped configuration that contains all
of the necessary electronics needed to
interface to standard Cat 5 twisted-pair
media. Three-link activity and speed
status LEDs are also incorporated in
the LAN Mate module.
The NICholas’s dual-row header
pins out five of the AX88796L’s address
lines that I used to gain access to the
AX88796L’s internal NE2000 (MAC)
register set (SA0–SA4). The remaining
address lines are hard-wired at the
dsPICDEM’s dual-row female NIC sock-
et to select a base address of 0x200,
which is the default base address for
the AX88796L. By using the default
address, the three base address pins—
I/O_BASE[0], I/O_BASE[1], and
I/O_BASE[2]—can be left in a disconnect-
ed state. This allows the AX88796L’s
base address pins to follow their internal
pull-up and pull-down circuitry and
select the base addresses of 0x200
(I/O_BASE[0]=0, I/O_BASE[1]=1 and
I/O_BASE[2]=1). I hand-wired this
Figure 1—
No surprises here. Imagine what you can do with six remotely controlled PWM outputs.
52
Issue 168 July 2004
CIRCUIT CELLAR
®
www.circuitcellar.com
baby in, so every extra pin I don’t
have to connect a wire to is goodness.
In terms of raw processing power,
the dsPIC30F2010’s MIP rate falls
with the voltage, which gave me one
more reason not to rip up the
dsPICDEM and modify the on-board,
5-V power supply just to accommo-
date the NICholas. So, to keep the
AX88796L 3.3-VDC power supply cir-
cuitry as simple as possible, and to
keep the dsPIC30F2010’s performance
potential high, I powered the
NICholas using a Zetex ZSR330 volt-
age regulator, which is fed by the
dsPICDEM’s 5-VDC power supply.
The ZSR330 doesn’t require any exter-
nal components for stability.
The DPDT switch you see mounted
on the dsPICDEM in Photo 2 is there
in place of the jumpers and shorting
blocks you see there in Photo 1. I
used the MPLAB ICD 2 as the
dsPIC30F2010 debugging tool. After
some initial serial routine failures
with BASIC code that executed in the
debugger correctly, I finally figured
out that the UART transmit and
receive lines are doing double-duty as
the MPLAB ICD 2 debugger commu-
nication lines. To circumvent this sit-
uation, I used a dsPIC30F2010 config-
uration fuse setting that shifted the
MPLAB ICD 2 debugger lines to an
alternate set of MPLAB ICD 2 com-
munication pins.
The dsPIC30F2010 has four sets of
alternate MPLAB ICD 2 interface
pins. The primary set of pins, EMUC
and EMUD, is multiplexed with the
primary UART transmit and receive
pin pair. The EMUC1 and EMUD1
pins are positioned on the alternate
UART transmit and receive pair locat-
ed on port C. The third pair of pins,
EMUC2 and EMUD2, is found on port
D, and the final pair of pins, EMUC3
and EMUD3, resides on port B.
Having a MPLAB ICD 2 interfaces
on different ports gives you the flexi-
bility to hang an MPLAB ICD 2
debugger on pins that you aren’t try-
ing to use in the debugging process. In
this case, the jumpers and the configu-
ration fuse setting switched the
MPLAB ICD 2 from EMUD and
EMUC to EMUD1 and EMUC1 to
allow the serial port pins originally
assigned to the MPLAB ICD 2 to be
used as transmit and receive lines in
the debug process. I added the DPDT
switch because the MPLAB ICD 2
must use the EMUD and EMUC inter-
face to initially load the compiled
code and debug executive with the
alternate MPLAB ICD 2 interface fuse
setting into the dsPIC30F2010.
After the debugger load is complete,
the jumpers have to be moved to con-
nect the MPLAB ICD 2 to the
EMUD1 and EMUC1 interface pair
and free the EMUD and EMUC pins
to be used as the UART transmit and
receive pins. I moved the jumpers
back and forth once before deciding to
mount the DPDT switch in their
place. Another way to avoid moving
jumpers or adding switches is to code
the dsPIC30F2010 to use the alternate
UART transmit and receive pins. To
do that with the dsPICDEM requires
custom trace cutting and rewiring.
Adding the DPDT switch is the easi-
est way out.
dsPIC30F2010 CODING
I always like to start out with a new
microcontroller by conquering the
serial interface. After I get the serial
port of that microcontroller working,
other things just seem to fall into
place. My first experiences with the
dsPIC30F2010 UART were unhappy
ones because I didn’t realize that the
MPLAB ICD 2 was messing up the
outgoing data. However, after I finally
caught on to what was transpiring,
using the dsPIC30F2010’s UART was
as simple as Listing 1 shows it to be.
Although simple, the code in
Listing 1 is instrumental in that this
is the way the dsPIC30F2010 and the
PIC18F452 communicate. Taking the
code line by line, the dsPIC30F2010’s
UART is enabled by setting the most
significant bit of the U1MODE register.
The dsPIC30F2010’s UART data rate is
calculated with the following formula:
F
CY
is the instruction clock rate. BRG
is the value to load into
U1BRG for
data rate clock generation.
The U1BRG statement in Listing 1
solves for BRG and loads the data rate
generator value in the
U1BRG register.
The last action performed before turn-
ing the UART loose involves enabling
the UART transmitter by setting the
UTXEN bit in the U1STA register.
The code between the
while(1)
braces is a simple serial echo mecha-
nism. The dsPIC30F2010 UART has
its own 4-byte receive-and-transmit
buffer. When a character enters the
receive buffer, the URXDA bit is set.
If there is no transmission in progress,
Data
F
CY
rate =
BRG
+
1
16
×
(
)
Listing 1—
After I got this little piece of code working, I was on my way to bigger and better things.
main(void)
{
U1MODE |= 0x8000;
U1BRG = (((7372800/9600) /16) - 1);
U1STA |= 0x0400;
while(1){
while(!U1STAbits.URXDA);
while(!U1STAbits.TRMT);
U1TXREG = U1RXREG;
}
}
Listing 2—
I really had to dig to get enough information to make this little code snippet work correctly.
main(void)
{
int x;
PTPER = 0x1FFF;
PDC1 = 0x1FFF;
PTCON = 0x8000;
while(1){
++x;}
}
www.circuitcellar.com
CIRCUIT CELLAR
®
Issue 168 July 2004
53
the TRMT bit is clear. The code waits
for a character to enter the receive
buffer, and when all is clear on the
transmit side, it moves the incoming
character from the receive buffer into
the transmit buffer for transmission.
The dsPIC30F2010 has an extensive
PWM module. The dsPIC30F2010
datasheet provides information about
how to set up the PWM registers, so I
turned to Microchip’s “dsPIC30F Family
Reference Manual” for the scoop on
the bits inside the dsPIC30F2010’s PWM
register set. As you can see in Listing 2,
the PWM module is just as easy to set
up and use as the UART.
The PWM operation is based on a
15-bit up/down counter, or PWM time
base, implemented by the PTMR spe-
cial function register. The most signif-
icant bit of the PTMR is read-only and
indicates the direction of the timebase
count (up or down). Using the PWM
timebase to count up and down allows
for the generation of center-aligned
PWM signals. To set the PWM period,
a 15-bit value is written to the PTPER
special function register. When the
count in the PTMR reaches the count
established by the PTPER, the PWM
timebase either resets to zero or starts
counting in the reverse direction.
In Listing 2, PWM1 is set to run
in Free-Running mode. The
dsPIC30F2010’s PWM running in Free-
Running mode produces an edge-
aligned PWM signal, which means the
PTMR value counts up to the PTPER
value and resets the timebase to zero.
The time it takes for the PTMR value
to reach the PTPER value is the period
of the PWM signal. It’s computed as
follows:
where T
PWM
is the PWM period, and T
CY
is the dsPIC30F2010 clock cycle time.
T
CY
in Listing 2 is the reciprocal of
the crystal, or clock frequency, multi-
plied by four:
With a value of 0x1FFF (8191 decimal)
and a PTMR prescale value of one, the
PWM period is 4.44 ms, which results
in a PWM frequency of 225 Hz.
1
7.37 MHz
4
×
T
PTPER
PWM
=
T
PTPER + 1
prescale value
CY
×
(
)
produce PWM signals, the rest of the
code for the dsPIC30F2010 calls on
the serial routine to receive a PWM
period or duty cycle value from the
PIC18F452 coprocessor, as well as
some auxiliary code to apply the
received data to the appropriate PWM
special function register. Listing 3 is a
way of routing the data to the correct
special function register to implement
the change in the requested PWM sig-
nal. As you can see, the ASCII charac-
ter P or D, followed by 2 bytes of spe-
The PDC1 special function register
determines the duty cycle of the
dsPIC30F2010’s PWM1 signal. The
value in the PDC1 special function
register determines how long the PWM
signal will remain high during each
cycle. The value of PDC1 in Listing 2
sets a 50% duty cycle. Figure 2 is a
graphical interpretation of how
PTPER, PTMR, and PDC1 interplay to
form a PWM signal.
With some dsPIC30F2010 code to
communicate with the PIC18F452 and
PROJECT FILES
To download the code, go to ftp.circuit
cellar.com/pub/Circuit_Cellar/2004/168.
Fred Eady has more than 20 years of
experience as a systems engineer. He
has worked with computers and com-
munication systems large and small,
simple and complex. His forte is
embedded-systems design and com-
munications. Fred may be reached at
fred@edtp.com.
SOURCES
NICholas
EDTP Electronics
www.edtp.com
ZSR330 Voltage regulator
Zetex
(+44) 161 622 4444
www.zetex.com
54
Issue 168 July 2004
CIRCUIT CELLAR
®
www.circuitcellar.com
cial function register data
received from the PIC18F452
coprocessor via the asynchro-
nous serial link, can be used
to modify the dsPIC30F2010’s
PWM output signal.
CODING THE PIC18F452
The PIC18F452’s job is to
initialize the AX88796L and
perform the interface func-
tion between the
dsPIC30F2010 and the
Ethernet link. If the
PIC18F452’s code is written
in a modular fashion, it really
doesn’t matter how you
decide to encode the messages
that ultimately will be delivered to the
dsPIC30F2010. For instance, Visual Basic
allows you to put together a quick and
nasty UDP engine. The UDP code mod-
ule contained within the PIC18F452
picks up the UDP data, performs the
necessary data conversions, and passes
the message to the dsPIC30F2010 via
the serial link established between the
two microcontrollers.
If Visual Basic isn’t your thing, you
can code a Telnet module for the
PIC18F452 that essentially does the
same thing as the UDP module. The
basic idea is to capture the data using
the AX88796L Ethernet interface,
process it, and pass it to the
dsPIC30F2010 via the PIC18F452’s seri-
al port. Doing so simply entails gather-
ing data from one code module inside
the PIC18F452 and passing it to anoth-
er code module that either prepares the
data for output or sends it on its way.
MELDING MODULES
The AX88796L is NE2000-compati-
ble, which means you can reuse (with
minor modifications) code written for
other NE2000-compatible Ethernet
controllers such as the Realtek
RTL8019AS. Visual Basic, which pro-
vides plenty of space when it comes
to Internet protocols, is a fast and
easy way to build a useful user inter-
RESOURCES
Microchip Technology, Inc.,
“dsPIC30F Family Reference Manual,”
DS80169C, 2004.
face. Even though I didn’t do
any deep DSP stuff, I’ve
shown you just how easy it
is to get started with the
dsPIC using the dsPICDEM
28-pin demo board.
I want you to have as
much fun as I did. So, I bun-
dled up all of the dsPICDEM
Ethernet, Visual Basic, and
dsPIC30F2010 code that I
wrote so you can download it
from the Circuit Cellar ftp
site and use it in a project of
your own. The dsPIC is
much like the PICs you’re
used to, but some of the
newer features will have you
digging into the datasheets. After you
get familiar with the differences,
you’ll find that dsPIC stuff isn’t com-
plicated, it’s embedded.
I
Listing 3—
You can build on this to include changing just about any special function register-related
dsPIC30F2010 function.
#define make16(varhigh,varlow) ((varhigh & 0xFF)* 0x100) +
(varlow & 0xFF)
switch (recvchar)
{
case ‘P’:
//Change PWM period
while(!U1STAbits.URXDA);
low_byte = U1RXREG;
while(!U1STAbits.URXDA);
high_byte = U1RXREG;
PTPER = make16(high_byte,low_byte);
break;
case ‘D’:
//Change PWM duty cycle
while(!U1STAbits.URXDA);
low_byte = U1RXREG;
while(!U1STAbits.URXDA);
high_byte = U1RXREG;
PDC1 = make16(high_byte,low_byte);
break;
default:
break;
}
PTPER
PDC1
PDC2
0
PWM1H
PWM2H
Duty cycle
Period
PTMR value
New duty cycle loaded from PDCX
Figure 2—
After all that reading, I see this. Bing!
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Virtual System Modelling
As the design concept evolved, I
realized that I could actually design
the entire UI right on a PCB with the
e-field circuitry. There are no unwant-
ed or flimsy connections (e.g., a touch-
screen flex connector), and the PCB
traces are my electrodes. This keeps
the manufacturing simple and it
enhances reliability.
The design uses two e-field sensors
to form an XY grid on both sides of the
board. A custom user interface template
(or overlay) is placed on the back of the
board and exposed through the enclo-
sure for interaction. When you touch
the overlay, the capacitance changes
on various e-field electrode XY points
underneath, which are then deciphered
by the Nitron. The final result is my
Smart-E-Touch, which is a simple, com-
pact design that has many possibilities.
Now I’ll cover the technical details.
SYSTEM OVERVIEW
The Smart-E-Touch is an intelligent,
touchscreen-based UI design that uses
a Nitron microcontroller to control
and monitor a pair of e-field sensors.
The sensors, in turn, monitor an XY
grid on the PCB for the actual capaci-
tive sensing of the touchscreen
through a custom UI overlay. After
the Nitron has translated a touch from
the e-field sensor data, it then outputs
an XY coordinate to its RS-232 serial
port. A main system controller, PC,
PLC, or microboard then receives the
coordinates and can act upon them.
The unique thing about this touch-
screen is that it’s built right on the
actual circuit board where the touch-
56
Issue 168 July 2004
CIRCUIT CELLAR
®
www.circuitcellar.com
U
ser interface (UI) designs are a
critical part of a system’s design.
Having worked on a variety of UI
designs, I’ve seen the simplest LCD
and keypad overlay combo all the way
up to the extreme of a full-blown
WebPAD that harvested more process-
ing power than my last desktop PC.
The front end of the system is what
you use to interact with your product
or device. For this reason, it must be
carefully tailored toward the applica-
tion for which the device will be used.
If you make it too simple or too com-
plex, your device may not get used to
its fullest potential.
As technology improves, UIs will
evolve. For example, an LCD with a
touchscreen bolted on top is a com-
mon alternative for custom keypad
switch overlays and LEDs, which can
give your product more pizzazz and
flexibility but bring inherent complex-
ity and cost. Once again, it comes
down to the level of complexity need-
ed to interface with your product, and
whether you can take the inherent
cost burden.
E-FIELD FOR UI DESIGN
Technological improvements can
provide the catalyst for new design
ideas. It was definitely the case when I
was introduced to the new Motorola
MC33794 e-field sensor. Since then,
the sensor has garnered a lot of public-
ity. It also has appeared in several
Circuit Cellar
articles, so I won’t go
into too much depth about how it
works. Let’s focus on the basics. You
can connect up to nine electrodes (e.g.,
Smart-E-Touch
wire and PCB trace) to one chip and
sense a change in capacitance on each
individual line. Touching one elec-
trode with your finger drastically
affects the capacitance measurement,
and the chip easily senses it.
At the time of the Motorola Flash
Innovation 2003 Design Contest, I
knew that hooking up an e-field sen-
sor to a Nitron microcontroller would
result in a really cool project, especial-
ly if it had something to do with sens-
ing human touch. So, how does all of
this tie into UI design? Well, given my
love (or lack thereof) for UI designs, I
thought it would be interesting to
design a simple, low-cost UI based on
e-field technology. I decided that the
design’s main focus would be on the
simplicity of interfacing and placing
low burden on the system controller
monitoring the UI.
FEATURE ARTICLE
by Bruce M. Pride
Bruce’s Smart-E-Touch is an intelligent, touchscreen-based UI design in which a
68HC908QY4 microcontroller controls a pair of Motorola e-field sensors. This inexpen-
sive system is the perfect alternative to the pricier touchscreen UIs on the market.
Photo 1—
The Smart-E-Touch design is entirely inte-
grated on the PCB, so you can easily bolt it into an
enclosure. Use custom PCB form factors to accommo-
date the enclosure for a perfect fit.
An Intelligent User Interface
Photo 2b, a 9 × 9 XY electrode grid was
created on the back of the circuit board
to provide the UI sensing. The grid was
also brought through to the front of the
board with vias so as to increase the
amount of copper for each electrode. As
you can see in Photo 2c, I shined a light
through the board to show you the UI
and the XY grid together. This should
give you an idea of the UI’s resolution.
Although the grid resolution is
coarse and only provides 81 total sens-
ing points, it was just fine for this UI
design. Most of the buttons on my UI
overlay have at least four XY intersec-
tion points on or near them that pro-
vide ample sensing capabilities for the
buttons. These 18 PCB trace elec-
trodes are then hooked up to the two
MC33794 e-field sensors. Nine elec-
trodes go to the X e-field sensor and
nine go to the Y e-field sensor.
The e-field sensors require little
external circuitry to operate. All you
need is power and a few discrete resis-
tors and capacitors (see Figure 2). By
connecting your microcontroller to the
e-field’s 4-bit logic input, you can select
which of the nine e-field channels you
want to measure. After a selection is
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CIRCUIT CELLAR
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Issue 168 July 2004
57
made, the electrode voltage is output
on the e-field sensor’s LEVEL pin for
easy reading by an A/D converter.
The e-field sensor provides other
features that I didn’t use for this proj-
ect. The internal lamp driver and ISO
communications interface could prove
handy for some designs.
Now that I’ve covered the UI, elec-
trodes, and e-field sensors, let’s take a
look at the processing power behind the
design. I used a Nitron 68HC908QY4
8-bit flash memory microcontroller in
a 16-pin TSSOP SMT package.
Although a TSSOP is tricky to solder,
I needed a small device package given
the size constraints. The Nitron pro-
vides the 100-ms sample interrupt via
its internal timer, the selection inter-
face for selecting which electrode to
measure, and uses its A/D converter
to measure the current selected elec-
trode. The Nitron then converts the
analog electrode voltage data to a
coordinate representing a specific loca-
tion on the UI. After X and Y coordi-
nates have been put together, it out-
puts them to its bit-banged RS-232 port
at 19.2 kbps via a MAX232 transceiver.
This gives the Smart-E-Touch a fairly
high-speed serial port from a low-cost
microcontroller.
The Nitron provides 4 KB of flash
memory and 128 bytes of RAM, which
was more than enough for the Smart-
E-Touch code. The Nitron doesn’t
require an external crystal because it
has a built-in digitally controlled
oscillator. I used it for this project and
programmed it to 10 MHz.
The Nitron is programmed via the
16-pin MONO8 port, which allows for
a direct connection to a P&E
Microcomputer Systems Cyclone pro-
grammer. Reset is provided via the
control circuitry is located. An interest-
ing option for this concept is that rather
than integrating a separate touch-
screen assembly into a design, you can
design the touchscreen on the main
system controller board. This makes
for a more cost-effective, reliable, and
adaptable user interface design.
So, given the Smart-E-Touch’s capa-
bilities, how can it be used in system
development? Depending on the appli-
cation, the Smart-E-Touch is a natural
replacement for touchscreens and cus-
tom-made switch overlays. A simple UI,
like the one shown in Photo 1, could be
used with or without an LCD and pro-
vide a great deal of functionality. Now
let’s look at the various pieces of the
design so you can see just how simple
and elegant the Smart-E-Touch really is.
SYSTEM BLOCKS
To keep things simple and the cost
down, the Smart-E-Touch is made up
of a minimal number of components
(see Figure 1). Starting with the user
interface, note that you interface with
the Smart-E-Touch via an overlay
placed on the back of its board. I
designed a simple UI in Excel, printed
it on paper, and then cut it to fit.
Although this wasn’t a production solu-
tion, it was the simplest way to come
up with a nonconductive overlay with-
out breaking the bank. I’m sure there
are many more durable materials out
there that can be placed over an e-field
electrode that would allow a change of
capacitance to occur when touched.
Next, the overlay is placed over the
XY electrode matrix (see Photo 2a).
Underneath the overlay is the key to
the entire system. As you can see in
Y Electrodes
Motorola
MC33794
e-field
sensor
E-field interface
X Electrodes
STOP
GO
Smart-E-Touch
Motorola MC33794
e-field sensor
E-field
interface
Motorola
Nitron
microcontroller
ASCII
Serial data
Maxim
RS-232
transceiver
RS-232 ASCII
Serial data
PCB
PC
or
XY PCB
touch panel
RS-232-enabled
monitoring device
Figure 1—
The Nitron microcontroller lies at the heart of the design, controlling the e-field sensors and sending
ASCII “touch” data out through the RS-232 transceiver.
Photo 2a—
The user overlay on the back of the board forced the requirement of having no circuitry on the back,
and it mandated that I use surface-mount components.
b—
The XY electrodes were designed right on the PCB as
rectangular strips of copper. Each electrode is isolated from the others because only one electrode is read at a time
and the others are grounded.
c—
With the overlay on the back of the board, you can see the XY points by shining
light through the board. There are 81 possible data points, or coordinates, on the board.
a)
b)
c)
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CIRCUIT CELLAR
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59
CodeWarrior IDE for the 68HC08 with
Processor Expert using the C program-
ming language.
There are several benefits to using
this IDE. Because it supports C lan-
guage, learning the processor’s native
assembly language isn’t required. This
saved time and made developing the
Smart-E-Touch easier because I was
able to use various C modules from
previous projects. The built-in
Processor Expert program saved an
enormous amount of time given that
this point-and-click GUI automatical-
ly implemented all of the Nitron’s
peripheral drivers in C language! It
also provides boot-up code for the
chip, which lets you concentrate more
on your application than on manually
setting up the microcontroller.
LT1117CM-5 voltage regulator, further
reducing the Smart-E-Touch’s cost!
The PCB, which is a single-sided,
mixed-technology (surface-mount and
through-hole) circuit board, was devel-
oped using the ExpressPCB software
package. The board was designed so
that no components are located on the
back because the user interface overlay
must be attached to the back of the
board, which then sticks out through
the enclosure or machine panel open-
ing. The board routing is dense, given
the surface-mount components, but is
required because of size constraints.
FIRMWARE
Before covering the Smart-E-Touch’s
software design, I’ll describe how the
software was developed in the
Nitron’s internal power-on-reset circuit.
This eliminates the need for an external
reset circuit. The debug LED, which is
attached to a Nitron port bit, proved
extremely useful during debugging for
watching the scan interrupt occur.
Powering the system is straightfor-
ward. You must supply 12 VDC to the
power input connector, which is used
by the e-field sensors and also convert-
ed to 5 VDC by the LT1117CM-5 low-
dropout voltage regulator. The Nitron
and the MAX232 require the 5 VDC.
The e-field sensors also convert the
12 VDC input to 5 VDC internally,
and output it to one of their pins. I
accounted for this and added a few 0-
Ω
resistors to the board to provide for
access to this feature in the future.
This eliminates the need for the
Figure 2—
As you can see, the circuitry is straightforward. The Nitron has just enough I/O pins to hook to both e-field four-input selects, both e-field analog level pins, the
MAX232, and the infamous debug LED!
60
Issue 168 July 2004
CIRCUIT CELLAR
®
www.circuitcellar.com
The overall goal of the firmware is
to control the e-field sensors, scan
them at a timed interval, translate the
sensor data into XY coordinates, and
then send the ASCII XY coordinate
data out the serial port. As you can
see in Figure 3, after the Nitron boots
up and is initialized, the 100-ms peri-
odic interrupt timer is enabled, which
then gives the firmware a timed inter-
val, or heartbeat, to perform the
required operations. Inside this heart-
beat interrupt, the Nitron scans the
two XY matrix e-field sensors via its
A/D converter inside every heartbeat
interrupt and figures out the XY coor-
dinate of any touch sensed by an X or
Y electrode (see Listing 1).
A valid touch is defined by any
value below the analog-to-digital
MaxTouchLevel threshold, which is
110 for this setup. When an electrode
isn’t touched, the 8-bit analog-to-digi-
tal value sits at approximately 255, or
a high. When a PCB metal electrode is
directly touched, the value goes down
close to zero. This value is much
higher, around 100, when touching
through the user interface overlay.
When no electrode has been sensed
on either the X or Y matrix, the coordi-
nate is marked with an asterisk. This
tells the host connected to the Smart-
E-Touch serial port that no valid coor-
dinate has been sensed. After the scan-
ning is done, the Nitron converts the
coordinates to ASCII and sends them
out the serial port at 19.2 kbps (e.g.,
X3, Y4 or X*,Y7 or X*,Y*). Refer to
Photo 3 for a real data set sample taken
from HyperTerminal. This entire
process happens every 100 ms. The
Smart-E-Touch software framework
allows for plenty of future expansion
and features
.
SYSTEM TEST AND RESULTS
I compiled the code, downloaded it
to the board, and verified the e-field
sensor’s functionality. Valid X and Y
coordinates were successfully sensed
when I touched the PCB electrodes.
When X or Y electrodes weren’t
touched, the Smart-E-Touch correctly
interpreted these as “no touches,”
which were then assigned an asterisk.
The X and Y coordinates were suc-
cessfully sent out of the serial port to
Listing 1—
The Nitron C code selects an e-field electrode, reads the analog value of it, and then compares it
to the maximum touch threshold. This straightforward code comprises the bulk of the Nitron code, showing
just how easy it is to interface to an e-field sensor.
//Check each E-field sensor electrode, 18 total electrodes
**********************************************************************
PTB = ELECTRODE_1;
//Select e-field electrode
Delay1mS();
//Wait 1 ms after selecting new electrode
ReadAdcValue (&Xsensor, &Ysensor); //Read electrode analog value
ElectrodeX1 = Xsensor;
//Save A to D value of electrode X-1
ElectrodeY1 = Ysensor;
//Save A to D value of electrode X-1
.
.
//Duplicate code for electrodes 2
.
//through 8
PTB = ELECTRODE_9;
Delay1mS();
//Wait 1 ms after selecting new electrode
ReadAdcValue (&Xsensor, &Ysensor); //Read electrode analog value
ElectrodeX9 = Xsensor;
//Save A to D value of electrode Y-9
ElectrodeY9 = Ysensor;
//Save A to D value of electrode Y-9
**********************************************************************
//Find the X position
**********************************************************************
if (ElectrodeX1 < MaxTouchLevel)
//Electrode X-1 AtoD value <
//touch max value?
{
Xcoordinate = ‘1’;
//Yes, X-coordinate 1 has sensed a touch
}
else
//No, look at next X-coordinate electrode
if (ElectrodeX2 < MaxTouchLevel)
//Electrode X-1 AtoD value <
//touch setpoint?
{
Xcoordinate = ‘2’;
//Yes, X-coordinate 2 has sensed a touch
}
.
//Duplicate code for Electrodes 3
.
//through 8
.
else
//No, look at next X-coordinate electrode
if (ElectrodeX9 < MaxTouchLevel)
//Electrode X-9 AtoD value <
//touch setpoint?
{
Xcoordinate = ‘9’;
//Yes, X-coordinate 9 has sensed a touch
}
else
//No valid touch has been sensed
{
Xcoordinate = ‘*’;
//X-coordinate set to none
}
**********************************************************************
// Find the Y position
**********************************************************************
if (ElectrodeY1 < MaxTouchLevel)
//Electrode Y-1 AtoD value
//touch max value?
{
Ycoordinate = ‘1’;
//Yes, Y-coordinate 1 has sensed a touch
}
else
//No, look at next Y-coordinate electrode
if (ElectrodeY2 < MaxTouchLevel)
//Electrode Y-1 AtoD value <
//touch setpoint?
{
Ycoordinate = ‘2’;
//Yes, Y-coordinate 2 has sensed a touch
}
.
//Duplicate code for Electrodes 3
.
//through 8
.
else
//No, look at next Y-coordinate electrode
if (ElectrodeY9 < MaxTouchLevel)
//Electrode Y-9 AtoD value <
//touch setpoint?
{
Ycoordinate = ‘9’;
//Yes, Y-coordinate 9 has sensed a touch
}
else
// No valid touch has been sensed
{
Ycoordinate = ‘*’;
//Y-coordinate set to none
}
**********************************************************************
//Output X,Y coordinates to host via 19.2-kbps RS-232 port connection.
//9 × 9 electrode matrix gives a total of 81 possible points.
**********************************************************************
DebugPortTxData(‘X’);
//Send an “X” out the serial port
DebugPortTxData(Xcoordinate);
//Send the X-coordinate
DebugPortTxData(‘,’);
//Send a “,”
DebugPortTxData(‘Y’);
//Send a “Y”
DebugPortTxData(Ycoordinate);
//Send the Y-coordinate
}
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CIRCUIT CELLAR
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Issue 168 July 2004
61
as the user interface. This would
allow the Smart-E-Touch to be used as
a real system user interface.
Another option is to refine the e-
field sensor’s scanning algorithm to
achieve better sensitivity. You also
might need to modify the e-field’s
minimum and maximum reference
capacitors to get better sensitivity.
Adding a wireless transmitter to the
Smart-E-Touch would remove the need
for a serial port. You’ll find this valuable
if you design a remote user interface.
You also could provide more detailed
data out of the serial port. The new
data can be the actual analog-to-digital
reading and the values of the e-field ref-
erence capacitors, which are the mini-
mum and maximum e-field values.
Finally, consider writing a Visual
Basic application (or something equiv-
alent) that you can use instead of a
terminal emulator. This would make
the user interface more flexible. The
program would actually do something
on the screen when a Smart-E-Touch
button is pressed.
SMART ALTERNATIVE
The Smart-E-Touch is an intelligent,
touchscreen-based UI design that’s a
great alternative to current UI options
currently on the market. In addition
HyperTerminal at 19.2 kbps (see
Photo 3). When you look at the data
set, start at the top of the Hyper-
Terminal screen shot. Note that the
first XY coordinate is X1,Y1. This
coordinate is followed by others, from
X2,Y2 all the way down to X9,Y9. I got
this pattern by placing my finger on the
bottom corner of the XY matrix and
then slowly moving it diagonally up to
the top corner of the screen. This is
why both coordinates increment
together. It proves just how well the
Smart-E-Touch tracks a moving finger!
You’ll also notice the asterisk
mixed in with the Y coordinates. This
happened when my finger crossed
over to the next Y electrode; there
was a point where no electrode was
being touched. When this occurs, it’s
detected as a “no touch,” which is
exactly how the system should work.
This proves that the sampling rate for
the system is sampling more than
enough data points. The Smart-E-
Touch concept is now a proven success!
SYSTEM ENHANCEMENTS
The Smart-E-Touch has a lot of
potential. You could change the cir-
cuit board form factor to fit nicely
inside an off-the-shelf enclosure simi-
lar to Photo 1 with a durable overlay
PROJECT FILES
To download the code, go to ftp.circuit
cellar.com/pub/Circuit_Cellar/2004/168.
Editor’s note: Bruce’s Smart-E-Touch
project won Grand Prize in the 2003
Motorola E-Field Sensor Contest and
First Prize in the Motorola Flash
Innovation 2003 Design Contest.
Bruce M. Pride holds an A.S. and B.S.
in electronics and currently works as
a Senior Electrical Engineer. He enjoys
all aspects of embedded system hard-
ware and software design, particular-
ly designs with 32-bit microproces-
sors, FPGAs, and various microcon-
trollers. In his spare time, he enjoys
spending time with his family, playing
acoustic guitar, competing in
Circuit
Cellar design contests, and consulting
via his side business, Pride Embedded
LLC. You may reach him at
BrucePride@PridEEmbedded.com.
Start
Reset and initialization
Enable the 100-ms touch
scan interrupt timer
100-ms touch
scan timer
interrupt?
Scan X e-field sensor,
scan Y e-field sensor
Translate data into XY
coordinates if less than
threshold voltage set point,
or else coordinate equal “*”.
Convert XY coordinates to
ASCII data
Send data out serial port
at 19.2 kbps
N
Y
Figure 3—
As you study the system’s firmware flow dia-
gram, keep in mind that the code doesn’t execute until
a timer interrupt or heartbeat has occurred.
SOURCES
PCB Design tool
ExpressPCB
www.expresspcb.com
LT1117CM-5 Regulator
Linear Technology
(408) 432-1900
www.linear-tech.com
MC33794 E-field sensor,
68HC908QY4 Nitron microcontroller
Motorola, Inc.
(847) 576-5000
www.motorola.com
Cyclone programmer
P&E Microcomputer Systems, Inc.
(617) 353-9206
www.pemicro.com
Photo 3—
Take a look at the HyperTerminal snapshot
showing the XY coordinate data from the serial port. If
you notice the incrementing coordinates, you’ll realize
that the data is a result of dragging a finger diagonally
across one side of the board to the other.
to its reliability, the Smart-E-Touch is
inexpensive to build and easy to use.
Given the design’s adaptability, you
can use it in applications to replace
mechanical switches, touchscreens,
membrane keypads, and entire switch
panels. To my knowledge, there aren’t
any touchscreen designs currently on
the market similar to the Smart-E-
Touch. This truly proves that the
Nitron-controlled, e-field sensor-enabled
Smart-E-Touch is a unique design.
I
For a 128 × 240 screen with a font size
of 6, the RAM needed is 40 columns by
16 rows, or 640 bytes. The third region
is the user-defined character font. Each
user-defined character requires 8 bytes.
A maximum of 128 characters can be
defined for a total of 1024 bytes.
Unless you use custom-defined char-
acters, this buffer space is not needed,
of course. The sum of the three afore-
mentioned values is 6784 bytes,
which is less than the 8 KB mounted
on the LCD panels.
Initializing the T6963 controller
requires you to first define the starting
RAM locations for each bitmap area
being used. Thus, if you are using
both text and graphics, you must
define starting addresses in RAM for
each of those areas. Further initializa-
tion chores consist of setting up a
graphics-size parameter as well as the
number of text columns on the screen.
Although I mentioned that the text
and graphics bitmaps are independent,
there can be some interaction between
them, if desired. You do this by select-
ing various values with the mode set
command. The different modes are
shown in Table 1.
If you use either the XOR or the
AND modes, then pixels placed on
the screen by text fonts will interact
62
Issue 168 July 2004
CIRCUIT CELLAR
®
www.circuitcellar.com
I
recently spent a considerable amount
of time becoming familiar with Zilog’s
new Z8 Encore! family. I was impressed
with several unique features of the
chip’s hardware, and the development
kit that I received worked right out of
the box, with an extremely short learn-
ing curve. I decided to port some of the
BASCOM/assembly language peripheral
driver routines, which I routinely use
with Atmel’s AVR family, over to Z8
Encore! C libraries. I often use graphic
LCDs in my projects, so these drivers
seemed like a good place to start.
With the proliferation of Palm Pilots
and fancy cell phones, there is certainly
no shortage of small graphic LCD pan-
els available, many of them in color.
Although quite nice, they tend to be
somewhat small and require tiny inter-
face connectors. The panels themselves
are often sub-VGA resolution, requiring
a relatively complex controller ASIC to
drive them and involving strict timing
issues that must be carefully addressed.
In the past, considering the above com-
plications, I’ve settled on LCD panels that
incorporate at least a low-level controller.
I normally choose those incorporating the
common Toshiba T6963 controller,
because it has a good architecture and is
found in reasonably priced LCD panels
having screen resolutions up to 128 × 240.
In this article I’ll briefly describe the
capabilities of LCD panels containing this
controller. I’ll also show you a text/graph-
ics library, which I wrote in C language,
that gives Z8 Encore! programs access to
the full functionality of the graphic LCD
panels. Photo 1 shows my prototype con-
troller along with its LCD panel.
T6963 LCD PANEL
The T6963 LCD controller’s most
Graphics LCD Library for the Z8 Encore!
Brian has created a graphics library you can use for your future projects. It provides access
for Encore! programs to graphics LCD panels driven by the T6963 LCD driver chip.
important feature, in my opinion, is that
it implements text and bitmap graphics
functions concurrently and independent-
ly. Text mode contains a complete ready-
to-use, ROM-based character set. (You
don’t have to download your own bitmap
images for each of the ASCII characters.)
Text characters placed on the screen
reside on their own bit plane. This means
they can coexist on the screen with any
graphics images, without any interference
between them). To put it another way,
you perform separate “text clear screen”
and “graphics clear screen” operations to
selectively erase text and graphics content
independently. This greatly simplifies
operations such as plotting dynamic
graphs with fixed, labeled axes.
The T6963 controller can handle up
to 64 KB of static RAM, but the LCD
panels that I’ve used have only 8 KB
mounted, which is sufficient for a
screen resolution up to 128 × 240. You
must partition this RAM memory space
as needed into three distinct regions.
The first region is the graphics bit
plane. When using the 6 × 8 font size,
the maximum RAM space needed to
handle a 128 × 240 screen is:
The second region is the text bit plane.
128
240
= 5,120 bytes
×
6
FEATURE ARTICLE
by Brian Millier
1
0
0
0
CG
MD2
MD1
MD0
Mode description
1
0
0
0
X
0
0
0
OR text and graphics pixels
1
0
0
0
X
0
0
1
XOR text and graphics pixels
1
0
0
0
X
0
1
1
AND text and graphics pixels
1
0
0
0
X
1
0
0
Text Attribute mode
1
0
0
0
0
X
X
X
Internal ROM character generator
1
0
0
0
1
X
X
X
RAM character generator
Table 1—
The way in which the text and graphics bit planes interact is defined by three bits in the Mode Set com-
mand: MD0–MD2. As well, the choice of the internal character generator ROM, or a RAM-based character genera-
tor, is determined by the CG bit.
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CIRCUIT CELLAR
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63
cellaneous characters consisting of
vowels with various accents and a few
currency symbols.
Although it’s possible to replace the
entire ROM-based font with user-
defined characters, it makes sense for
most users to use the ROM-based font
for the standard ASCII characters and
to define only the extra characters
needed. These get mapped to charac-
ters represented by byte values of
0x80 and higher.
Each custom character is represent-
ed by 8 bytes, 1 byte per row of pixels
in the font, starting at the top. The
most significant bit of the byte repre-
sents the leftmost pixel. My utility
application also contains a routine to
assist in generating the 8 bytes needed
to represent any given bit pattern. It
allows you to download this pattern
into the T6963 CGRam for testing.
To define a custom character from a
Z8 Encore! user program and load it
into the T6963 CGRam area, call the
LoadCGRam library routine. This pass-
es it the byte value that will subse-
quently represent that character (these
user-defined characters start at 0x80),
followed by a pointer to a character
string containing the 8 bytes needed to
represent the desired pattern. To display
a custom character on the LCD panel,
simply place the byte plus 32 into any
string that you send to the display.
Why “plus 32”? It turns out that the
T6963 defines all of its ROM-based
characters as their normal ASCII value
minus 32. Basically, this removes the
32 ASCII control characters normally
residing in characters 0 through 31.
This allows room for those 32 miscel-
laneous characters to reside in charac-
ter positions 96 through 127. As a
result of this anomaly, the
PrintChar
routine in the library must subtract 32
from the ASCII values passed to it,
before displaying them, if you are to
with graphics pixels located in the
same position. For example, if you use
AND mode, only those pixels lit by
both a text operation and a graphics
operation will show up on the screen. I
wanted completely independent text and
graphics, so I used the OR mode in my
initialization routine. I won’t describe
Text Attribute mode because it pre-
cludes the use of the graphics functions
and instead uses the graphics bit plane to
set text attributes on a character-by-
character basis. The next operation in
initializing the controller is to issue a
Display Mode command with the
desired parameters, as shown in Table 2.
Finally, you can set up the shape of
the text cursor, if you want to use it,
by issuing the Cursor Pattern Select
command. This command is made up
of an upper nibble (1010) and the
lower nibble containing a value denot-
ing the number of lines needed in the
cursor pattern (from one to eight).
All of the required T6963 initializa-
tion is performed by the
InitScreen
library routine. It’s well commented,
and you can make changes to the
display configuration by changing
the parameters.
In general, all commands sent to the
T6963 follow the following format:
LSB of parameter, MSB of parameter,
and the actual command itself. The
first 2 bytes must be sent to the
T6963’s data register and the last to its
command register. The only difference
between these two operations is in the
level set on the T6963’s C/*D line.
All such operations are performed by
the C library routines
T6963dat and
T6963cmd respectively. A third rou-
tine,
T6963WaitReady, which is called
by both of the aforementioned routines,
is needed to monitor the busy status of
the T6863 (i.e., waiting until it’s ready
before sending data/commands). An
additional routine,
T6963adt, also sends
data to the T6963’s data register; it han-
dles the special case in which large
blocks of data must be sent sequentially,
without intervening commands.
Only these four routines and the
init_port routine contain references
to the actual port pins used on the Z8
Encore!. If you were to use different
ports, or even a different microcontroller,
these routines would be the only ones
associated with the T6963 that you’d
need to modify to match your hardware.
The PWM routine that generates the
drive signal for the V
EE
generator would
need to be modified too, but that’s not
strictly a T6963-related routine.
TEXT FUNCTIONS
The library’s text functions are fairly
basic.
ClrText() clears all text from
the LCD screen (without disturbing any
graphics) and locates the character point-
er position to row one, column one.
Locate (Row,column) places the text
position pointer to the row and column
passed. These parameters are “1” based
(i.e., the “home” position is row one, col-
umn one).
PrintChar(char) places the
character passed to it on the screen at
the current text pointer position.
WriteStr(*str) writes an ASCIIZ
string on the display, starting at the cur-
rent text pointer position. The pointer
to that string is passed to this routine.
TEXT FONT CHARACTERS
Like many LCD panel controllers,
the T6963 allows you to define your
own character font to supplement its
128-character, ROM-based font. The
ROM-based font contains the standard
ASCII 96 characters as well as 32 mis-
Photo 1—
I built my unit on a vector board. There’s
plenty of free space for circuitry beyond what’s needed
for the LCD controller itself. I left Zilog’s smart cable
hooked up so you can see how the Z8 Encore! is pro-
grammed/debugged.
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Description
1
0
0
1
0
0
0
0
Display off
1
0
0
1
X
X
1
0
Cursor on, no blink
1
0
0
1
X
X
1
1
Cursor on, blink on
1
0
0
1
0
1
X
X
Text on, graphics off
1
0
0
1
1
0
X
X
Text off, graphics on
1
0
0
1
1
1
X
X
Text on, graphics on
Table 2—
Text, graphics, or both bit planes can be enabled by setting the proper bits in the Display Mode com-
mand. Also, the cursor can be turned on and off, and the blink attribute set here as well.
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the display without disturbing the text
that’s being displayed.
Pset(X,Y,color)
places a dark pixel on the screen at x, y
if the color parameter is one; otherwise,
it erases that pixel.
DrawLine(X
0
,Y
0
,X
1
,
Y
1
,color)draws the line (X
0
,Y
0
) – (X
1
,Y
1
)
if color is one; otherwise, it erases the
pixels between the two coordinates.
DrawRectangle(X
0
,Y
0
,X
1
,Y
1
,color)
draws a rectangle with opposing corners
at (X
0
,Y
0
,X
1
,Y
1
) if the color parameter is
one; otherwise, it erases such a rectan-
gle.
DrawCircle(X
C
,Y
C
,rad)draws a
circle centered at (X
C
, Y
C
) with a
radius of
rad.
There are more specialized graphics
routines. I’ll describe them in the fol-
lowing sections.
BITMAP IMAGES
Working with instrumentation in the
Chemistry Department at Dalhousie
University, I primarily need for a
graphics display to show plots of data
being collected. However, I know that
many people need to be able to display
pictures in the form of bitmap images,
so I’ve written routines to handle this.
see what’s expected. However, in the
case of the user-defined characters,
you must add 32 to the character you
pass the
PrintChar routine in order to
end up displaying the character with
the code you’ve already predefined with
a given character value.
I think Toshiba screwed up by organ-
izing things this way. It would have
made more sense to place the miscella-
neous accented vowels and currency
symbols in ASCII codes 0 through 31
and leave the normal ASCII 96 charac-
ters alone with their standard values.
BASIC GRAPHICS FUNCTIONS
The basic graphics functions include
the point, line, and circle drawing rou-
tines as well as a screen clear func-
tion. My earlier AVR assembly lan-
guage T6963 graphics routines used
my own custom line-drawing algo-
rithm, but it seemed more sensible
here to use Bresenham’s line and circle
algorithms, which are readily available
as C routines.
There are four basic functions.
ClrGraph() clears all graphics from
The first thing to consider is where
to store the bitmap images. Zilog’s
Z8F6401 contains 64 KB of flash memo-
ry, much more than the microcontrollers
I’ve used in the past. The assembly lan-
guage graphics library, which I had writ-
ten for the ’8515 AVR device, takes up
less than half of the 8-KB flash memo-
ry in that device. However, this graph-
ics library, written in C for the Z8
Encore!, is approximately 28 KB long.
Allowing additional space for a reason-
ably sophisticated program meant plac-
ing the bitmap images somewhere else.
The obvious choice is to use some
form of serial flash memory, because
it can be connected to the Z8 Encore!
using only a few lines. This is impor-
tant, given that the T6963 display
interface already consumes 12 I/O
lines. Earlier, I had used Atmel 32-KB
SPI flash memory devices for this pur-
pose, but this time around I went with
Microchip’s 24LC256 I
2
C flash memo-
ry device (also 32K × 8). Although
accessing this I
2
C memory device is
slower than a comparable SPI memory
chip, there are advantages. It requires
only two I/O lines, and you can easily
connect other I
2
C peripherals with the
same lines. Additionally, the Z8
Encore! C compiler library contains
I
2
C routines, and the 24LC256 is inex-
pensive and readily available.
To compensate for the limited space
available in the 24LC256 I
2
C flash
memory device, as well as its less-
than-stellar access speed, I decided to
use data compression prior to storing
the bitmap images. Because the dis-
play is only monochrome, it’s possible
to achieve a high data compression
ratio (from two to 10 times) of the
image using only a simple run-length
encoding scheme. You can easily store
30 full-screen images (on a 128 × 240
display) in the 24LC256, and these
images are painted on the screen in a
fraction of a second.
The images are stored in the I
2
C
flash memory by way of a simple file
organization scheme. A number refer-
ences each image, and there is a small
directory maintained in the I
2
C flash
memory that points to the starting
address of each image, as well as its
size. In addition, this directory con-
tains an eight-character name to iden-
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rows starting from the bottom
and going toward the top of the
screen (i.e., they follow the
standard Cartesian coordinate
convention). The T6963 starts
at the top of the LCD panel
and works down.
Bitmap images generally
have a white background
with a black image. Zeros rep-
resent black areas. LCD panels are usu-
ally viewed with the background unlit
and the image itself lit. A lit pixel is
black (or blue, etc.) and, in terms of the
T6963 controller, is represented by a
one. Therefore, bitmap pixel data must
be inverted before being sent to the
T6963 controller.
Given the need for the aforemen-
tioned data manipulation, as well as
the decision to run-length encode the
data, I wrote a PC utility application
that performs the data conversion. It
then downloads the compressed
image data to the Z8 Encore! via an
RS-232 serial data link. A routine in
the Z8 Encore! graphics library
receives this serial datastream and
tify that particular I
2
C flash
memory chip. The bitmap
image file utility program uses
it to identify which memory
chip is currently being pro-
grammed with image data.
How do you generate these
bitmap images in the first
place? I decided that the easiest
method was to use standard PC
drawing programs that can output
Windows bitmap (.BMP) files. The
Windows bitmap format contains a sig-
nificant amount of information in its
header, most of which is unnecessary in
this context and can be ignored.
However, the parameters that are
important here are shown in Table 3.
There are a number of differences in
the way in which bitmap files are
organized in comparison to the way in
which the data must be presented to
the T6963 controller. Bitmap images are
stored as groups of 4 bytes (long inte-
gers) representing image pixels horizon-
tally from left to right. For images that
are not a multiple of 32 pixels horizon-
tally, this long integer is zero-padded to
allow for the smaller number of pixels
in the source image.
Depending on the font size selected
in the T6963 controller, you have to
break up the packets into either 8- or
6-bit groups before sending them to
the controller. You also have to pad
your own pixel data when the hori-
zontal pixel size of the image is not a
multiple of either eight or six pixels.
Because of the significant differences
in handling the bitmap image function
for the 6 × 8 font size mode versus the
8 × 8 font size, I chose to implement
only the bitmap function for the 6 × 8
font size. I decided this presented the
most useful display option.
Bitmap images store the pixels in
Offset
Size
Type
Description of the variable
1
2
Character
“BM” to indicate that the file is a bitmap
11
4
Long integers
Pointer to start of bitmap data
19
4
Long integers
Width of the image in pixels
23
4
Long integers
Height of the image in pixels
Table 3—
Although most of the header information contained in a Windows
bitmap file is not needed in order to convert it to a file format that the T6963 can
use, the locations of the few necessary parameters are shown in this table.
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CIRCUIT CELLAR
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67
library that performs this function. In
operation, you first fill the ScopeCurrent
array with up to 240 data points. You
then call the ScopeDraw procedure with
the following parameters: horizontal off-
set (in pixels), vertical offset (in pixels),
horizontal size (number of data points),
vertical clipping size (in pixels), and
erase flag (set to one if you want to
erase the previously plotted data set).
This routine allows you to present a
dynamic data plot positioned any-
where on the screen with a defined ver-
tical and horizontal size. Although the
timing is dependent on the actual data
being plotted, the routine takes less
than 1 s to plot a full screen. The rou-
tine requires 480 bytes of RAM for a
full 240-pixel display, but it can be
reduced if a narrower display is all
that’s needed (by defining smaller
arrays for ScopeCurrent and ScopeLast).
This procedure is fairly straightfor-
ward. For each data point (n), the line-
draw routine is called using n,Y(n) and
n + 1,Y(n + 1) for the endpoints (plus
horizontal and vertical offsets speci-
fied). If necessary, the Y values are
clipped at the vertical clipping size
before plotting. If the erase flag is set
to one, on a sample-to-sample basis,
the line-draw routine is first called
with a color of zero to erase the old
line (using the data from the previous-
ly plotted screen and stored in the
ScopeLast array). Then, the line-draw
routine is called with a color of one to
plot the line segment corresponding to
the new sample. After each new line
segment is plotted, the current sam-
ple’s Y value is transferred, in prepara-
tion for the next plot, into the corre-
sponding index position in the
ScopeLast array. This provides a dis-
play that remains on the screen, with
the erasure of the old
data occurring only in
the small area of the
screen where the data is
actively being plotted. In
use, it is extremely
smooth, like that of an
oscilloscope display.
Some of my earlier
projects involving the
T6963-based LCD panel
used the AT90S8515
microcontroller, which
has only 512 bytes of RAM, much less
than the 4 KB present in the Z8F6401.
To achieve this function without eating
up virtually all of the ’8515’s RAM, I
used a more efficient raster-based algo-
rithm, which is coded in assembly lan-
guage, rather than the sequential line
erase/draw routines that I chose for
this library. However, given the
greater RAM resources and Z8 Encore!
execution speed, and wishing to use C,
the more straightforward algorithm
seemed appropriate.
WIRE IT UP
T6963-based LCD panels interface
to their host via an 8-bit, bidirectional
data bus using the Intel standard *RD,
*WR, and CS control signals. This is
generally implemented using a dual
10-pin connection, which allows the
use of standard ribbon-cable and IDC
header connectors.
Although the T6963 LCD controller
can control up to 64 KB of static RAM
for the text and graphics display
buffers, this memory is not randomly
accessible by the host, because the
interface does not include an address
bus per se. Instead, there is a single
C/*D line which steers the data from
the host into either a control register
or a data register. In terms of hard-
ware, the eight data lines and a few
control lines are all that the host
microcontroller has to actively han-
dle. There are a few additional config-
uration control lines that I generally
choose to handle with jumpers.
The T6963 controller chip’s bidirec-
tional data bus is meant for a micro-
processor featuring a full bus for exter-
nal memory, peripherals, and so on. In
earlier T6963-based LCD projects, I
used an AT90S8515 microcontroller,
stores it in the I
2
C flash
memory. Other routines
read/maintain the image
file directory, which is also
contained in the I
2
C flash
memory.
Although the text/graph-
ics routines reside in a
stand-alone library
(T6963.c), the sample pro-
gram that I’ve included
with this article contains
code in the main section,
which interfaces directly with the PC
utility program via the COMM port.
This allows you to initially download
bitmap images into the flash memory,
as well as a few other useful functions.
After the images are stored in the non-
volatile flash memory, the PC utility
program is no longer needed.
StoreBitmap() and DrawBitmap(
image#,Xoffset,Yoffset) are the
two library routines used for the
bitmap functions. The former is specif-
ically meant to work in conjunction
with the PC utility program I wrote.
It’s used to initially store compressed
bitmap images in the 24LC256 I
2
C
flash memory device. The routine is
called from the main section in the
sample program after receiving a 0xFE
command from the serial port (issued
by the PC utility program). The
DrawBitmap(image#,Xoffset,Yoff
set) routine draws a bitmap image
specified by an image number at the
specified screen offset. Don’t call this
with a nonexistent image number or
the results will be unpredictable!
Photo 2 is a screen shot of the utility
program. Note that the program can
also act as an exerciser to test the opera-
tion of the various library routines.
OSCILLOSCOPE DISPLAY
For my purposes, it’s essential to be
able to quickly plot a set of data
points on the screen. For data that
changes quickly, the ideal solution is
the left-to-right painting of data points
on the screen without an intervening
full-screen erasure. (CRT-based oscillo-
scopes do this.) Intelligent graphics
controller assemblies with serial data
links are available commercially, but I
have not seen any with this capability.
I included a routine in this graphics
Photo 2—
I wrote the PC utility program so you can download bitmap images to the flash
memory.
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which contains a full bus. I wasn’t able
to get the LCD panel to work properly
with the ’8515 using Bus mode, even
after calling for the insertion of wait
states. It appears that the T6963’s
access time is too slow to use with
most current microprocessor buses.
In my experience, it seems that the
best way to control the T6963 is with
a simple port access. You allocate one
of the microcontroller’s 8-bit ports for
the data lines and connect lines of a
second port up to the T6963’s control
lines. The latter lines are toggled up
and down in software to read and
write to the T6963 controller. This
works fine, although it takes a bit more
code to implement. Because the T6963
controller is a low-speed CMOS device,
the time it takes to perform the various
graphics commands is much greater
than the time spent doing either port or
bus access, so this is moot. Assuming
that the microcontroller port pins that
connect to the LCD panel’s *RD and
*WR lines are not shared by other
devices, the *CS line on the LCD panel
can be permanently tied low, thereby
saving one microcontroller I/O port pin.
The host program must be able to
interrogate the T6963 controller to
determine when it has finished per-
forming the last command it received.
Note that this is done by reading a
T6963 status register and examining
two individual busy bits (which
depend on the type of command opera-
tion being performed). It would have
been nicer if the interface had includ-
ed a dedicated ready line that could be
tied to a host IRQ line, freeing up the
host to perform other tasks while the
LCD panel was busy drawing. However,
the T6963 is not designed that way.
Status checking is handled by the
T6963WaitReady library function.
Unlike the AVR microcontroller
chip, which I’ve used with these LCD
panels, the Z8 Encore! runs off a 3.3-V
power source, whereas T6963-based
LCD panels require a 5-V source. I
anticipated some problems here given
the different logic levels involved.
Actually, things worked out well. The
Z8 Encore! has 5-V tolerant inputs on
its port pins, which means that the 5-V
logic signals that the T6963 outputs
on its data lines during a read opera-
tion can be handled directly by the Z8
Encore!. Checking out the T6963 con-
troller’s datasheet, I found that it
requires a minimum of 2.7 V for a
logic-high signal and a maximum of
0.8 V for a logic low, both of which are
provided by the Z8 Encore! ports.
Therefore, the two devices can be
interconnected without any glue logic.
The T6963 controller itself is capa-
ble of generating character fonts in
widths of five, six, seven, or eight pixels
via two font-select lines (FS0 and FS1).
However, many LCD panels, including
the one that I used, tie the FS0 line low,
and bring only the FS1 line out to the
20-pin I/O header. This limits the font
selection to six or eight pixels wide.
Deciding which font width to use is a
trade-off. Regardless of your choice, the
actual pattern used to generate the char-
acters is the same. The 8 × 8 font just
has more space between the characters,
which means it’s not any clearer or well
formed. Therefore, it seems logical to
use the 6 × 8 font exclusively because it
yields, for example, 40 columns of text
rather than 30 on a 128 × 240 display.
I should note that the choice of the
text font size also affects the operation
of the graphics part of the display.
Briefly, when selecting the 8 × 8 font
size, the organization of the graphics
bitmap divides the horizontal pixels into
byte-sized groups of eight. However, in
the case of the 6 × 8 font, the horizontal
pixels are grouped by six. In practice,
this means that if you use the 8 × 8 font
exclusively, the graphics routines
could be streamlined by taking advan-
tage of simple shift instructions rather
than multiply/divide instructions.
For versatility, my graphics library rou-
tines handle both font sizes. Thus, they
must use the multiply/divide instruc-
tions rather than streamlined routines
using the shift instructions. However,
you must choose one specific font size
for a given project. Given that, you then
set the font size jumper properly and also
set the variables
Fontsize, LcdRows,
and
LcdCols accordingly (in statements
found at the start of the Main procedure).
The bitmap image routine, however, is
implemented for the 6 × 8 font size only,
as I mentioned earlier.
All graphic LCD panels require a
negative voltage (V
EE
) in addition to
the 5-V logic power supply. Although
some models include this V
EE
supply on
the controller PCB itself, the more com-
mon ones, including the one I use, don’t.
The V
EE
supply must be approxi-
mately –7 (for a 64 × 240) to –16 V (for
a 128 × 240). Check your panel’s
datasheet for its exact requirements.
The current requirement for this
supply is extremely modest—only
about 2 mA. However, this voltage
must be stable and adjustable (using a
potentiometer and software) in order
to change the contrast of the display.
The voltage needed to provide opti-
mum contrast is temperature-depend-
ent. In an environment where the
LCD panel is exposed to wide temper-
ature variations, the controller must
adjust the V
EE
supply automatically in
response to temperature changes, or
the display will be hard, if not impos-
sible, to read. I didn’t make
allowances for automatic contrast
control over wide temperature swings.
I’ve handled the V
EE
power supply in
several different ways in past projects.
I started by using Maxim’s MAX749
digitally adjustable LCD bias supply
chip, which works well but is expen-
sive. I later designed my own clone of
this with a Tiny AVR microcontroller
(“Build a Graphics LCD Bias Supply,”
Circuit Cellar
, issue 144, 2002). For
this project, I retained the analog
charge-pump components of the afore-
mentioned circuit, but let the Z8
Encore! provide the PWM signal that
the Tiny AVR microcontroller had pro-
vided. This design is beneficial because
it allows for software control of con-
trast, requiring only a single Z8 Encore!
I/O pin (a PWM output). In contrast,
both the MAX749 and my Tiny AVR
design required two I/O lines from the
host micrcontroller to control contrast.
In operation, the Z8 Encore! generates
an active-low PWM signal with a period
of 100 µs and a pulse width that varies
from about 35% to 55% depending on
the 8-bit contrast parameter. This active-
low signal drives a 2N3906 PNP transis-
tor with its emitter connected to V
CC
and forces current to flow through a 100-
µH inductor to ground. During the time
that this transistor is shut off, the mag-
netic field in the inductor collapses and
generates a negative potential across C8
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(which must be a tantalum capacitor).
An 18-V Zener diode connected across
this supply limits the maximum voltage
developed to a safe value. A 50-k
Ω
trim-
ming potentiometer and an PNP emit-
ter-follower provide for the coarse con-
trast control. The software adjustment
of the PWM duty-cycle provides fine
contrast adjustment.
Depending on your LCD panel’s V
EE
voltage and current requirements, you
might have to adjust the PWM’s peri-
od and width range by changing some
constants in the firmware itself. My
code is optimized for the AZ Displays
AGM2412B 128 × 240 panel, which
requires about –14 V. The software
adjustment of contrast is performed by
the
VeeGen(period,width) proce-
dure in the library.
I used the AGM2412B-NC-BTD
LCD panel for this project. It’s a 128 ×
240 panel with a CCFL backlight. For
any panel with a CCFL backlight, you
need a high-voltage supply. I’ve seen
surplus modules advertised for approx-
imately $5, but I used a standard Digi-
Key JKL BXA-501/KIT module, which
takes a 5-V input and produces approx-
imately 600 V to fire the cold cathode
fluorescent tube used as the backlight.
The module is extremely efficient,
requiring only a few hundred milliamps
of current at 5 V to light the tube.
Figure 1 shows both the CCFL back-
light power supply and R7 connected up
to pin 9 of JP1 plus a ground on pin 20.
R7 is a current-limit resistor for pow-
ering an LED backlight like those
found on some displays. You need only
one of these two circuits, depending
on the backlight used in your display.
I used a Microchip 24LC256 I
2
C
flash memory device (U3) to take
advantage of the Z8 Encore!’s built-in
I
2
C port. This stores bitmap images.
Downloading required bitmap images
from the PC, which is where they are
generated, is done via an RS-232 serial
data link operating at 19,200 bps. The
level-shifting between the Z8 Encore!’s
3.3-V and RS-232 levels is done by U2,
which is a Maxim MAX232. Although
Maxim makes level-shifters specifical-
ly designed for 3.3-V logic microcon-
trollers, I had only MAX232 devices
on hand. They interfaced well with
the Z8 Encore! without causing any
problems, but I had to power them
from the 5-V logic supply. Because I
used both 3.3- and 5-V devices in this
project, I had to use two regulators—a
7805 and an LM1086-3.3—to provide
regulated power.
I built my circuit on a vector board
(see Figure 1). As you can see in Photo 1,
there’s plenty of space left for other
circuitry. Just before I finished writing
this article, I found an inexpensive Z8
Encore! trainer board from Futurlec,
which includes all of the components
in my circuit except the few passive
components that make up the LCD
V
EE
generator. If you want to try out a
graphics display on a Z8 Encore!, take
a look at this option.
UTILITY PROGRAM
To use bitmap images with this proj-
ect, you must generate them on a PC
in Windows bitmap format and then
use the T6963Z8 utility application
that I wrote. This program allows you
to choose a PC bitmap image, which
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Brian Millier is an instrumentation
engineer in the Chemistry Department
at Dalhousie University in Halifax,
PROJECT FILES
To download the code, go to ftp.circuit
cellar.com/pub/Circuit_Cellar/2004/168.
SOURCES
AGM2412B-NC-BTD LCD panel
AZ Displays, Inc.
www.azdisplays.com
JKL BXA-501/KIT Module
Digi-Key Corp.
www.digikey.com
Z8F6401 Microcontroller
Zilog, Inc.
www.zilog.com
24LC256 Flash EEPROM
Microchip Technology, Inc
www.microchip.com
Z8 Encore! trainer board
Futurlec, Inc.
www.futurlec.com
T6963 LCD Controller
Toshiba
www.semicon.toshiba.co.jp
must be black and white and small
enough to fit the display, and down-
load it to the 24LC256 I
2
C flash mem-
ory device.
When using a brand-new 24LC256 I
2
C
flash memory device, you first must use
the initialize flash memory function.
This puts a known signature pattern in
the flash memory and stores a user-
selected, eight-character name. It also
zeroes out the entire flash memory chip,
which takes approximately 5 min. If you
attempt to initialize a flash memory
device that already has image data
stored in it, the program will notify you
and give the flash ID name. Then, you
will have the option to abort or proceed,
thereby erasing the existing image data
stored in the flash memory device.
After this, you can select bitmap
images that you’ve prepared, and then
download them to the flash memory.
After each image is downloaded, a
window indicates the number of the
next free image. The utility program
generates a map file, which has the
same name as the one you gave the
flash memory chip during the initial-
ization procedure, and a .MAP exten-
sion. This text file contains the PC’s
bitmap file name and image number
associated with each image stored in
flash memory.
The utility program also contains a
matrix where you can draw custom-
defined characters for downloading to
the T6963’s CGRam. This is only
temporary, of course, because they
will disappear when the LCD panel’s
power is removed. However, the utili-
ty program also displays the list of 8
bytes needed to produce that custom
character, and those values can be
hard-coded into your program for per-
manent access to that character.
A NEW OPTION
Although I still intend to use the
Atmel AVR chips for most of my proj-
ects, I do like many aspects of the Z8
Encore! family. Now that I can con-
nect it up to a graphics LCD panel, I’ll
probably find a place for it in some
future projects. I hope that you can
make use of the graphics library in
your own projects.
I
Figure 1—
This is the complete schematic of my LCD panel controller. The RS-232 link is needed only for initially downloading bitmap images, if used.
Canada. He also runs Computer
Interface Consultants. You may reach
him at brian.millier@dal.ca.
74
Issue 168 July 2004
CIRCUIT CELLAR
®
www.circuitcellar.com
O
ne of my more memorable excur-
sions back in my college days was to
Herkimer, New York. My buddies and I
had been traveling for a number of hours
and needed to stretch. Pulling off the
road next to an abandoned railroad tres-
tle, one of the guys got excited when he
saw the rock layers exposed in the exca-
vated underpass. We spent hours looking
through the sedimentary layers for fos-
sils until we were asked to leave by the
landowner. I still treasure a trilobite—a
worthless ugly hunk of rock from the
Paleozoic era (543–248 mya)—some-
where amongst my cache of things.
Herkimer is known for its diamonds,
not fossils. Yep, you can still prospect for
Herkimer diamonds. Seeing dollar signs?
Well, hold on, a Herkimer diamond is
actually quartz. If you choose your claim
wisely, you can still find some sizable
hunks of quartz crystals buried in the
ground (see Photo 1). As you can see,
these are considerably more attractive
than what you would see if you popped
the can off of a microprocessor’s crystal.
But, where would we be today if it was-
n’t for the crystal? Don’t look now, but
soon the crystal might be just a
thing of the past.
Linear Technology has
released a programmable oscil-
lator that covers a frequency
range of 1 kHz to 68 MHz using
no external components. It is
available with either an SPI
(LTC6903) or an I
2
C (LTC6904)
interface, and needs only a sin-
gle supply from 3 to 5.5 V. The
clock output is available as nor-
mal and/or inverted output, and
it can be externally controlled
using the output enable (OE)
for the feedback loop to stabilize. (You
can prevent frequency overshoot when
changing frequencies by always chang-
ing the DAC values first.)
SERIAL DATA CONNECTION
The device powers up using a default
of OCT=0 and DAC=0 (1.039 kHz). To
change values in the device, write a
16-bit data word to its serial interface.
The LTC6903 uses SPI, and the LTC6904
uses I
2
C. Both devices are write-only
because neither have the circuitry for
returning data. The SPI device requires
only the data word to be transmitted
(maximum 20-MHz clock). The I
2
C
device, which requires an addi-
tional address byte (maximum
100-kHz clock), would be slow-
er even if the clocks were equal.
But, in this case, you can see
that the SPI device is orders of
magnitude faster to begin with.
The data word comprises
the 4-bit OCT value, the
10-bit DAC value, and the
2-bit CFG value. The config-
uration value determines the
state of an output enabled by
the OE input pin. Table 2
defines the choices. Figure 2
Lose the Crystal
FROM THE BENCH by Jeff
Bachiochi
control input. Output selection also can
be controlled via the 2 least significant
bits of the 16-bit serial data word,
where the upper 14 bits are for fre-
quency selection.
Refer to the LTC6903/4 innards in
Figure 1, and you can see a master VCO
(34 to 68 MHz), which receives its drive
from a 10-bit DAC. The DAC is part of
the closed-loop feedback to keep the
VCO stable. The VCO and DAC are
designed to run over a single octave fre-
quency range, 1–2x (frequency mini-
mum, x, to frequency maximum, 2x).
This gives a fine frequency control of
better than 0.1% between the minimum
and maximum frequency (approximately
34 kHz steps between 34 and 68 MHz).
The VCO output goes through a pro-
grammable divider. The 4-bit octave
(OCT) data controls the divider. This is
the coarse frequency control; it doesn’t
affect the VCO frequency.
Table 1 shows ranges selected using
the 4-bit OCT data. Changes in the
OCT data provide instantaneous CLK
output changes, whereas changes in
the DAC data may take up to 100 µs
There are many different silicon oscillators out there, but Jeff thinks the LTC6903 is a stand-
out. Read on to learn how he used it with a microcontroller to control a four-digit LED.
Linear’s LTC6903/4 Programmable Oscillator
Photo 1—
This exotic Herkimer diamond shows off its
crystalline structure thanks to Mother Nature.
8
+
–
Master
oscillator
f
M0
= 68 MHz × k
Ω
I
SET
V
+
– V
SET
I
SET
+
–
A1
V
SET
Serial port
1
2
3
4
GND
SDI
SCK
*SEN (LTC6903)
ADR (LTC6904)
DAC
V
+
7
OE
CLK
*CLK
Programmable
divider
6
5
OCT
6903 BD
Figure 1—
This eight-pin device has circuitry to create a master VCO. The output
can be divided for a wide range of user-controlled frequencies via an SPI
(LTC6903) or I
2
C (LTC6904) interface.
www.circuitcellar.com
CIRCUIT CELLAR
®
Issue 168 July 2004
75
a good alternative where reduced accura-
cy can be tolerated. Besides its small
size, the digital programmable oscillator
(DPO) has other advantages when used
as an input clock for a processor or
microcontroller. At power-up, it defaults
to its lowest frequency 1.039 kHz. This
allows a processor to begin executing
code. The processor can reprogram the
DPO on the fly, thus speeding up its
execution for normal operation. When
the circuit needs to save energy, it can
throttle back to a comfortable speed
to reduce the required circuit current.
It can even turn off the oscillator com-
pletely, although this is a death sentence
until power is recycled.
Frequency error is approximately
0.5%, while maximum error over the
entire output range at 5 V is approxi-
mately
±
2.25%. The VCO used in the
actual frequency generation has some
jitter associated with it, which can con-
tribute approximately 0.5% (at mini-
mum undivided frequency, 34 MHz) to
3% (at maximum undivided frequency,
64 MHz). Other lower ranges reduce jit-
ter because of averaging inherent
in dividing the frequency.
PROJECT TIME
I’m always looking for ways
to create test equipment. The
LTC6903 seemed like it has great
potential: it is no problem to get
a microcontroller to set any
desired output frequency using
this device. However, without a
user interface inputting the
appropriate value for the desired
frequency, it is next to impossible.
Oh sure, you can watch the output
with a oscilloscope and have
Up/Down buttons increment
through the 2
14
codes, if you can
wait all day. So, to make the circuit
portable, a user-friendly interface is
important. And, because the device
doesn’t have infinite resolution,
you also need feedback on what fre-
quency is actually obtainable.
This project uses a microprocessor
to control a four-digit LED using
three push buttons to set a frequency
value. The value is converted to the
closest OCT (4-bit coarse) and DAC
(10-bit fine) control values. The
LTC6903 is updated with these, and
the LED is updated with the frequency
produced using the OCT and DAC val-
ues. This value can be different from
the input values. Figure 3 shows the cir-
cuit I used to support the LED, push
buttons, and LTC6903 DPO.
WHO’S GOT THE BUTTON?
I wanted to give you an easy way to
change frequency values without sim-
ply incrementing or decrementing the
14-bit DPO value with two push but-
tons. I added a third button as a mode
control. I created seven modes:
change digit1, digit2, digit3, digit4,
decimal point, multiplier, and
LTC6903 (Run mode). You can tell
which mode you are in via a flashing
display. Depending on the mode, the
Up and Down buttons will increment
and decrement a digit, move the deci-
mal point (around to one of the four
digits), or change the multiplier (alter-
nate between megahertz and kilo-
hertz, one of the two colon dots).
In LTC6903 mode, the display val-
ues are read and a frequency value is
defines the bit placement of the
OCT, DAC, and CFG values.
SPI communication begins when
the device is enabled (*SEN). The
LTC6903 is then ready for the most
significant bit of the 16-bit data
word. Data bits are clocked into
the device on the rising edge of the
clock line. After the first 8 bits of
data are received, they are trans-
ferred into an internal register. The
second 8 bits received are trans-
ferred internally as well after com-
pletion. You can choose to end the
transmission by raising *SEN after
only 8 bits if the lower 8 bits of
the DAC and the 2 bits of configu-
ration do not change.
I
2
C communication does not
require a chip enable (*SEN). The
LTC6904 defines this pin as an address
selection input. Thus, you can have two
I
2
C devices connected to the I
2
C bus and
control them independently. An I
2
C
transmission begins with a start bit.
(It’s defined as a lowering of the data
line, SDA, while the clock line, SCL, is
in a high state.) Data is considered good
during the high period of the clock line.
A transmission consists of 3 data bytes,
the device address, the most significant
data byte, and the least significant data
byte. After each of the 8 bits, the
device responds with an acknowledge
bit (ninth bit time) as an assurance that
the data was received. Failure to
acknowledge aborts the transmission.
As with the SPI device, the I
2
C com-
munication may end after the most sig-
nificant data byte has been sent if the
lower 8-bits of the DAC and the 2 bits
of configuration do not change.
REPLACING CRYSTAL
We all have been content with using
crystals for a long time. Although
they’ve been reduced in size, they still
are susceptible to shock. Resonators are
Minimum frequency (
≥≥
) Maximum frequency (
<<
)
Octave
34.05 MHz
68.03 MHz
15
17.02 MHz
34.01 MHz
14
8.511 MHz
17.01 MHz
13
4.256 MHz
8.503 MHz
12
2.128 MHz
4.252 MHz
11
1.064 MHz
2.126 MHz
10
532 kHz
1063 kHz
9
266 kHz
531.4 kHz
8
133 kHz
265.7 kHz
7
66.5 kHz
132.9 kHz
6
33.25 kHz
66.43 kHz
5
16.62 kHz
33.22 kHz
4
8.312 kHz
16.61 kHz
3
4.156 kHz
8.304 kHz
2
2.078 kHz
4.152 kHz
1
1.039 kHz
2.076 kHz
0
Table 1—
The frequency resolution is approximately 0.001 × f. The
minimum frequency of an OCT is with the DAC=0x00, and the
maximum frequency is with the DAC=0x3FF.
Figure 2—
The 16-bit data word value comprises the 4-bit OCT
value, the 10-bit DAC value, and the 2-bit CFG value. The data
is sent most significant bit first. All three values can be changed
with a single transmission.
D15
D14
OCT.3
D13
D12
D11
D10
D9
D8
OCT.2
OCT.1
OCT.0
DAC.9
DAC.8
DAC.7
DAC.6
D7
D6
DAC.5
D5
D4
D3
D2
D1
D0
DAC.4
DAC.3
DAC.2
DAC.1
DAC.0
CFG.1
CFG.0
CFG1
CFG0
CLK
*CLK
0
0
On
CLK + 180°
0
1
Off
On
1
0
On
Off
1
1
Powered down
Table 2—
The 2-bit configuration value determines how
the outputs CLK and *CLK are treated as well as the
power-saving circuitry.
76
Issue 168 July 2004
CIRCUIT CELLAR
®
www.circuitcellar.com
calculated. OCT and DAC values are
determined and sent to the LTC6903.
Finally, the OCT and DAC values are
used to calculate a frequency and the
LED display is updated with the actu-
al frequency produced.
OCT & DAC
Although the LTC6903 uses a 14-bit
value to set a frequency, the OCT and
DAC variables differ in function. The
basic VCO circuit is designed to oper-
ate at the 34- to 68-MHz octave. An
octave, as in music, is a doubling of fre-
quency (i.e., the range of the VCO). The
VCO operates within an octave’s range
based on a 10-bit resistor DAC within
the loop. A constant current feeds the
DAC resistor, creating a voltage used to
set the VCO frequency. Think of the
DAC as a control that sets a frequency
between 0.5 and full scale, or 68 MHz.
Or, you can think of the 10-bit DAC
value as always adding an offset to the
VCO’s lower frequency (34 MHz).
Using the latter line of thought, the
output frequency is made up of two
components: the DAC (offset or fine
tuning) and the OCT (base frequency
tines—one for multiplication and one
for division—cut down on the code
space required for the math routines.
The code is based on the flow chart in
Figure 4. The execution spends most of
its time in the main loop; periodically, it’s
in the interrupt routine. There are two
interrupt routines. The first is a CCP1IF
interrupt. The CCP1 module takes input
from the LTC6903 and counts edges.
When the count matches 50, the CCP1
interrupt is generated and the count is
cleared. By using this interrupt to toggle
an output pin, a frequency of one one-
hundredth of the LTC6903 is created.
I plan to add a MAX7400 eighth-order,
low-pass filter to the circuit using this
output as the filter input and the
LTC6903 output as a clock into the fil-
ter. The filter’s output will be a sine
wave equal in frequency to the input.
This will give both sine and square wave
outputs of one one-hundredth of the
LTC6903’s frequency (10 Hz to 680 kHz).
Note that this interrupt is unnecessary
for this stage of the project.
The second interrupt, which is from
timer2, is set for approximately 3 ms.
The LED display is in the form of four
or coarse tuning):
where
and
The DAC fraction normally would be
a number between zero and one. I avoid-
ed using the large floating-point routines
by carefully rearranging the equation to
keep the numbers large but less than
4,294,967,295, which is the maximum
limit of a 32-bit integer. This keeps
rounding errors mostly insignificant.
I started using a number of multipli-
cation and division routines (8 × 8, 8 ×
16, 16 ×16, 8 × 24, etc.) and rapidly ran
out of programming space. So, I decid-
ed to use 32-bit math for almost every
situation. Although it was not the
fastest execution when I only needed
the product of two 8-bit numbers, speed
was not critical. Having only two rou-
fine setting = coarse setting DAC fraction
or
fine setting
×
= 2
1,039
OCT
×
×
DAC
1024
coarse setting = 2
1,039
OCT
×
f = coarse setting + fine setting
Figure 3—
This project uses drive transistors to provide adequate column current for a port to sink a single digit (seven segments and a DP). The frequency displayed is trans-
lated into commands to set the appropriate frequency on the LTC6903 output.
www.circuitcellar.com
CIRCUIT CELLAR
®
Issue 168 July 2004
77
multiplexed digits. Each digit plus the
colon has a column drive transistor to
supply approximately 100 mA for each
row of LED segments. The seven seg-
ments plus DP of each digit share the
eight rows of the display. A digit’s seg-
ment is enabled whenever data placed
on a display row is low and its associat-
ed column has been enabled by the col-
umn-drive transistor. Each time a
TMR2IF interrupt occurs, the last LED
column driver is disabled, data for the
next digit is placed on the display rows,
and the new digit’s column is enabled.
The five columns get updated every
15 ms (5 × 3 ms), which is about 60
Hz. Indication of which mode you’re in
is handled by the interrupt routine as
well. After you press the Mode button,
the mode variable is incremented (0–6).
Mode 0 is the Run mode in which the
display does not flash. All of the other
modes flash various parts of the display.
Modes 1 through 4 flash one of the four
digits. Mode 5 flashes the DP. Mode 6
flashes one of the two colon dots (upper
equals megahertz, lower equals kilohertz).
A variable blink is incremented each time
through the interrupt. During a column
update, the mode is checked. If there is
a match, the column is enabled only if
blink is greater than 128. This flashes the
proper segments at approximately 1 Hz.
SETTING THE LTC6903
When the mode is less or greater than
zero (editing the display), none of the
calculation routines are being executed.
This happens only after you have incre-
mented through the editing modes (1–6).
After leaving mode 6, you are forced
back into mode 0. As you can see in
Figure 4, a number of calls
are made to determine
which value to send to the
LCT6903 based on what you
have set on the LED display
(using modes 1 through 6).
The microcontroller has
synchronous serial port
hardware and can communi-
cate with the LTC6903 by
simply placing bytes in the
SSPBUF register. (Note that
this microcontroller also has
support for I
2
C, and this
could be used to talk with
an LTC6904.) After the
LTC6903 has been updated,
additional calculations are
made to determine the actu-
al frequency based on the
OCT and DAC. This is done
so the display can be updat-
ed with new values that are
closer to what is being pro-
duced as opposed to what
has been asked for.
MSOP8
The LTC6903 is only
available in the SMT ver-
sion. Obviously, Linear
wants the footprint to be
less than that of a crystal so
you can eliminate your
crystal without changing
the PCB space requirement.
I often mount these devices
in a DIP header so I can easily plug them
into a DIP socket (see Photo 2). After
bending the tiny leads up and down, I
was able to tack on short lengths of
bare wire wrap wire to each leg using
a regular soldering iron. It takes more
patience than skill to do this. If you get
this far, use a small clip as a heatsink
on each wire as you solder the eight
wires to a DIP header. This allows the
initial joint to remain cool.
Many companies have introduced sili-
con oscillators, but the LTC6903/4 has
innovations that make it stand out from
the crowd. Linear might intend to replace
the crystal in many microprocessor appli-
cations, but I think this device will find
its way into a number of unusual designs.
Are those little wheels in your head
spinning yet? Got any use for a large
quartz crystal?
I
Initialization
Main
Up button
pushed?
Down button
pushed?
Mode button
pushed?
N
N
N
Y
Y
Y
Increment digit,
DP, or multiplier
Decrement digit,
DP, or multiplier
Increment mode
(0-1-2-3-4-5-6-0)
Go to
display
Interrupts
Exit
interrupts
CCP1 Count
reached?
Timer2
overflow?
Toggle DIV100
output bit
Turn off last column.
Update next column’s row data.
Turn on next column or skip based
on mode (flashing display).
N
N
Y
Y
Display
Mode = 0?
Call
Digits2Freq
Call
Freq20CTDAC
Call
Set6903
Call
OCTDAC2Freq
Call
Freq2Digits
Call
Digits2Display
Go to
Main
N
Y
Figure 4—
The main loop consists of the UI (buttons) and calculations. The
interrupt routine displays the frequency using a four-digit LED display.
Photo 2—
The LTC6903 is hand-wired to an eight-pin
DIP header for easy removal. Three of the four push
buttons are used as a user interface for editing the dis-
play and ultimately the LTC6903.
Jeff Bachiochi (pronounced BAH-key-
AH-key) has been writing for
Circuit
Cellar since 1988. His background
includes product design and manu-
facturing. He may be reached at
jeff.bachiochi@circuitcellar.com.
SOURCES
LTC6903/4 Digital programmable
oscillator
Linear Technology Corp.
www.linear.com
PIC16F873 Microcontroller
Microchip Technology, Inc.
www.microchip.com
PROJECT FILES
To download the code, go to ftp.circuit
cellar.com/pub/Circuit_Cellar/2004/168.
tricity and magnetism as discovered
centuries ago by household names like
Oerstad, Ampere, and Faraday.
Keeping things real simple, here are
some electromotive force (EMF) basics.
A current passing through a conductor
generates a magnetic field surround-
ing the conductor. That’s what Hans
Oersted (1777-1851) discovered almost
200 years ago when he noticed that a
compass needle would move under
the influence of the magnetic field sur-
rounding a current-carrying wire. In
essence, Oersted’s moving compass
needle was the first electric motor.
Nature tends to balance things out,
so the opposite is true, as Michael
Faraday (1791-1867) discovered.
78
Issue 168 July 2004
CIRCUIT CELLAR
®
www.circuitcellar.com
A
ccording to the market statistics,
something on the order of 5 billion
electric motors are shipped each year.
It’s enough to make your head—and a
lot of other stuff—spin (see Photo 1).
The fact that the number of motors
shipped is roughly equal to the num-
ber of MCUs inspires contemplation
of an underlying convergence of both
technical and societal factors.
As you’ll see, the inexorable march
of MCUs finds our little friends push-
ing their way into ever-lower cost and
higher-volume motor control applica-
tions. One driving force is a desire to
conserve precious energy resources.
That’s more than an abstract concern
here in California, where some resi-
dents are wondering when the lights
will go out again, and others are wor-
rying about the new power plant being
built in their backyard.
Of course, the more that motors do
the heavy lifting, the less we the
couch potato-inclined populace have
to. Whether it’s an escalator, Segway, or
even the power windows in your car,
motors make life easier, although some
decry the fact that we seemingly bank
the calories saved on our waistlines.
Whether you’re going green or getting
lazy, motor control is increasingly a
topic of interest to engineers across a
broad range of fields. Let’s see what’s up.
I’M AMPED
When it comes to selecting an electric
motor for a particular application, there
are myriad of choices (see Figure 1).
Despite the various bells and whistles,
they all rely on basic concepts of elec-
Change the magnetic field around a
conductor (i.e., pass a wire near a mag-
net), and a current is induced. That’s
how a transformer works, and it also
explains why a motor can be a genera-
tor and vice versa.
As enunciated by Andre Ampere
(1775-1836), the magnitude of the
effect (i.e., power of a motor) depends
on the length of the wire within that
field and the amount of current pass-
ing through the wire. His original defi-
nition (although subsequently
replaced with a slightly different inter-
national standard) defined an ampere
as follows: the amount of current in
two straight parallel
conductors
of infi-
nite length a meter apart that gener-
ates a magnetic attraction between
the
conductors
with a force of 2 × 10
–7
newtons per meter.
Another key factor is the orienta-
tion relative to the magnetic field,
namely the force (torque) is maximized
when the wire is perpendicular to the
magnetic field and minimized (zero)
when aligned. This is simple enough to
see with a compass by forcing the nee-
dle due east or west and releasing it.
The needle accelerates quickly (maxi-
mum torque) toward north, overshoots
(mechanical inertia), and slowly (mini-
mum torque) settles back to proper ori-
entation. The exact amount of force at
any particular point is related to the
sine of the angle (i.e., 1 at 90° and 0 at
0°). Dust off your old math book
because such trig relationships are, as
you’ll see, at the heart of motor control.
Thus, motor control basically boils
down to the interaction between two
Motoring (Part 1)
SILICON UPDATE
by Tom Cantrell
Intelligent motor control should be the only kind of motor control you design into your proj-
ects. But before you start sketching your next design, take some time to learn about the elec-
tric motors at your disposal. Tom covers them all, from brush to brushless.
a)
Photo 1—
Motors come in all sizes, from 1000-plus-hp
behemoths (Photo 1a courtesy of www.hampton-
power.com/products.html) to tiny MEMS motors built
using semiconductor manufacturing techniques
(Photo 1b courtesy of www.sarcos.com/memsspec_
wobble.html).
b)
Motor Basics
www.circuitcellar.com
CIRCUIT CELLAR
®
Issue 168 July 2004
79
magnetic fields, one in the sta-
tionary part of the motor (sta-
tor) and one the rotating part
(rotor). By rotating one of the
magnetic fields relative to the
other, work is performed as the
rotor moves in a continual effort
to restore magnetic alignment.
The analogy that comes to mind
is a dog chasing its tail, with a
big dish of dog food serving as
the power supply.
With the previous discussion
in mind, a few observations
become apparent. First, one of
the magnetic fields can be cre-
ated by permanent magnets;
but at least one must (and both
can) be created by electromag-
nets in order to allow electrical cir-
cuits to generate and control the rotat-
ing magnetic field.
Going further, it’s apparent that the
performance characteristics of the
motor (speed, torque, smoothness,
efficiency, etc.) are vitally dependent
on the ability to precisely control the
rotating magnetic field in the opti-
mum manner for the task at hand.
But, the task at hand varies greatly
(i.e., there’s no single right way when
it comes to choosing or controlling a
motor). Some applications are laugh-
ably simple, such as a fixed-speed fan.
Others, such as the motor in an elec-
tric vehicle, require infinitely variable
speed control. An application such as
a conveyor belt that carries different
size boxes (i.e., varying load) may
require constant speed, but variable
torque. Other applications call for just
the opposite—constant torque at vari-
able speed (e.g., a stepper motor).
Many applications have start-up
issues. Often, more torque is initially
needed to overcome the inertia of the
load (including the motor and related
mechanical apparatus itself) than is
required after steady-state operation is
achieved (e.g., disk drive spindle
motor). But sometimes it’s desirable
to limit torque at start-up to
minimize mechanical shock
(or whiplash for passengers in
an elevator).
Another factor to consider is
duty cycle, which encompasses
a spectrum of time- and load-
varying scenarios (see Table 1).
It’s just this variability in appli-
cations that explains why there
are so many different kinds of
motors, and why the ability to
exploit MCUs for intelligent
motor control is such a big deal.
KEEP IT SIMPLE
To find out how we got
where we are, let’s take a brief
look at where we came from by
starting with the brushed DC motor.
The common permanent magnet (PM)
version relies on magnets mounted on
the stator with coils of wire wrapped
around poles on the rotor.
Operation is conceptually simple: pass
current through the rotor coils, which
creates an electromagnet whose field
interacts with that of the permanent
magnets, generating torque that causes
the rotor to turn. Conductive brushes
rubbing against a rotor-mounted slip
ring provide the connection between
the power supply and the coils.
Great, so now you’ve got a motor
that will turn a bit and then, like a
Electrical
motor
Brush
motor
DC motor
Universal motor
Brushless
motor
Synchronous motor
(permanent magnets)
Asynchronous motor
(induction)
DC motor
Switched
reluctance motor
Stepper motor
Figure 1—
Designers have a number of options when it comes to choosing
the appropriate motor for a particular application.
Duty cycle type
Description
Continuous running
Operation at constant load of sufficient duration to reach thermal equilibrium.
Short-time duty
Operation at constant load during a given time, less than required to reach thermal equilibrium, followed by a rest enabling the machine
to reach a temperature similar to that of the coolant (2°K tolerance).
Intermittent periodic duty
A sequence of identical duty cycles, each including a period of operation at constant load and a rest (without connection to the mains). For
this type of duty, the starting current does not significantly affect the temperature rise.
Intermittent periodic duty
A sequence of identical duty cycles, each consisting of a significant period of starting, a period under constant load, and a rest period.
Intermittent periodic duty
A sequence of identical cycles, each consisting of a period of starting, a period of operation at constant load, followed by rapid electric
with electric braking
braking and a rest period.
Continuous operation
A sequence of identical duty cycles, each consisting of a period of operation at constant load and a period of operation at no load. There is
periodic duty
no rest period.
Continuous operation
A sequence of identical duty cycles, each consisting of a period of starting, a period of operation at constant load, followed by an electric
periodic duty with
braking. There is no rest period.
electric braking
Continuous operation
A sequence of identical duty cycles, each consisting of a period of operation at constant load corresponding to a predetermined speed of
periodic duty with related
rotation, followed by one or more periods of operation at another constant load corresponding to the different speeds of rotation (e.g., duty).
load and speed changes
There is no rest period. The period of duty is too short to reach thermal equilibrium.
Duty with nonperiodic
Duty in which the load and the speed generally vary nonperiodically within the permissible range. This duty includes frequent overloads
load and speed variations
that may exceed the full loads.
Table 1—
International standards bodies establish motor specifications based on a number of factors, including duty cycle, as defined here by the International Electrotechnical
Commission (IEC).
80
Issue 168 July 2004
CIRCUIT CELLAR
®
www.circuitcellar.com
compass, come to rest. The clever
solution is to split the brush connec-
tion and coil wiring so that, just as the
rotor nears alignment with the perma-
nent magnets, the current flow is
reversed—a process known as commu-
tation. Like our easily entertained dog,
just when he’s almost got his tail, it
moves out of reach and round and
round he goes.
The PM DC motor is pretty cool.
The torque is constant across the
speed range, making for reliable start-
ing and smooth operation. A brushed
motor doesn’t need an MCU because
the control (i.e., pole sequencing) is
built into the mechanics of the motor
itself. However, the clever mechanics
are also the downside because brushes
have wear-out and maintenance
issues. And don’t forget that sparks
tend to fly when the current is
reversed at the commutator, which
not only creates electrical noise but,
in fact, can be a showstopper in safety-
critical applications (e.g., a fuel pump).
Variations on the DC brush motor
theme include those that replace the
permanent magnets on the stator with
electromagnets using either a single
power supply for both stator and rotor
(series wound) or dual supplies (sepa-
rately wound). The latter is well suit-
ed for high-starting torque applica-
tions (e.g., scooters, golf carts, etc.). A
“universal” brushed motor is so-
named because it’s wired in such a
way that it will work on either AC or
DC. It isn’t easily reversible, but that’s
not a problem for a lot of applications
(e.g., fans and kitchen appliances).
BRUSH OFF
Because of its simplicity, and thus
low cost and reliability, the AC induc-
tion motor is probably the single most
widely used type of motor. Alternating
current passing through coils on the
stator induces a magnetic field on the
rotor (either wound or the affection-
ately named “squirrel cage”), thereby
eliminating the need for brushes and
their maintenance, electrical noise,
and safety issues.
The good news is that a single-phase
AC induction motor is not only
mechanically simpler than a brushed
DC motor, but it’s also conceptually
simpler. By virtue of the fact that the
current is alternating, the power sup-
ply itself performs the commutation
(i.e., the pole polarity switches along
with the phase of the supply).
The bad news is that there are a few
possible gotchas to watch out for. The
biggest one is that the starting torque
is low. In fact, the motor may not
start at all, but just sit there vibrating.
There are a number of jump-start
schemes that all generally boil down
to mixing and matching three differ-
ent add-ons. First is the addition of a
separate start winding that’s designed
so that it generates a field at a differ-
ent angle than the main winding,
thereby giving the motor a directional
bias at power-up. A start capacitor is
usually thrown in to boost the current
available for starting to many times
the rated continuous current. After
the motor reaches a speed at which
the main winding torque is sufficient
(e.g., 75%), a centrifugal switch cuts
the start winding out of the circuit.
Variations include using permanent
capacitors (eliminating the need for
the centrifugal switch) or separate
start and run capacitors.
A so-called shaded pole single-phase
AC induction motor gets rid of the
starting add-ons altogether by shield-
ing poles to induce relative lags in the
magnetic fields in such a way that the
motor starts turning. It’s simple and
cheap, but the downside is low start-
ing torque and low efficiency. That’s
OK for something like a household
fan where the starting torque required
is low and the motor doesn’t go
through a lot of start/stop cycles, but
that’s about it.
Stepping up to a multiphase motor
addresses the starting problem direct-
ly because the geometry of three sepa-
rate coils in a triangle arrangement
allows for the establishment of a clear
directional bias from the get-go. This
is helpful for applications that need
more starting torque or go through a
lot of start/stop cycles. Of course, the
downside is the higher cost of both
the motor itself and the control elec-
tronics needed to juggle a three-phase
power supply.
Slip is an inherent factor for all AC
induction motors (see Figure 2).
Normally, you would expect the speed
of the motor simply to be determined
by the frequency of the alternating
current (i.e., rate of rotation of the
magnetic field in the stator), but
because the field in the rotor must be
induced, there’s always some lag. Put
another way, if the rotor catches up to
the rotating magnetic field, there is
zero torque. But because the motor
has inherent electrical and mechanical
losses that require some torque to
overcome, the rotor can never catch
up. As a result, the actual motor speed
is slightly (e.g., 5%) less than the base
speed of the rotating magnetic field,
and the amount of slip varies with the
load changes.
10% 20%
30%
40%
50%
60%
70% 80% 90% 100%
Rotor speed (percentage of full speed)
Sample load torque curve
Pull-up torque
Full-voltage start torque
Full-voltage stator current
Pull-out torque
7 × FLC
LRC
6 × FLC
5 × FLC
4 × FLC
3 × FLC
LRT
2 × FLC
1 × FLC
Current (percentage of
motor full-load current)
2 × FLT
1 × FLT
Torque (percentage of
motor full-load torque)
Figure 2—
At start-up, a three-phase AC induction motor draws a huge amount of locked rotor current (LRC) until
the motor reaches 80% of full speed. At that point both current and torque drop quickly as the motor approaches
95% of full speed, where it delivers full load torque (FLT) with high efficiency (i.e., low current).
www.circuitcellar.com
CIRCUIT CELLAR
®
Issue 168 July 2004
81
POWERFULLY CORRECT
Any talk about AC and induc-
tive loads has to include discus-
sion of power factor correction
(PFC). The topic goes beyond
motors (it relates to all AC pow-
ered devices), and the effect is
felt mostly by large generators
(utilities) and users (e.g., com-
mercial and industrial plants).
Nevertheless, here goes.
Let’s start with the simple
example of an AC supply driv-
ing a resistive load (e.g., a light
bulb). If you hook up an oscillo-
scope, you’d see that the voltage
and current remain in phase
(i.e., the sine waves are aligned),
as you can see in Figure 3. In
this case, the familiar watts =
volts × amps equation holds.
Notice that the power consumed
(watts) is always positive (i.e., the
voltage and current are always either
both positive or both negative).
But if you replace the resistor with
an inductor that draws the same
amount of current, look what hap-
pens. Now the voltage and current are
no longer in phase: the current flow
lags by 90°, and there are regions of
both positive power (positive voltage
and current) and negative power (posi-
tive voltage and negative current or
negative voltage and positive cur-
rent). This seems odd. There is volt-
age present and current flowing, but
there is no power!
Put more formally, the power factor
of a circuit is defined as the cosine of
the phase angle (one for perfectly
aligned voltage and current wave-
forms, and zero for waveforms 90° out
of phase). Because real-world circuits
comprise a mix of resistive and induc-
tive loads, the line current is a com-
posite of the two. For example, a cir-
cuit containing equal resistive and
inductive loads would have a phase
angle of 45° and, thus, a power factor
of 0.71 (cosine of 45°).
This is more than a numbers game.
A lower power factor translates into a
direct loss of efficiency (i.e., watts in
versus work out), so a motor’s actual
work output is derated, depending on
its power factor. It also means generat-
ing and distribution facilities must be
upsized to overcome the difference.
With the emphasis on energy conser-
vation (and the fact that nobody likes
paying higher energy bills), it’s no
wonder there’s been a big push to
improve power factors with regula-
tions (and utility imposed price penal-
ties) pushing for at least 0.9 and
beyond. A classic solution is to add
capacitors because capacitance causes
a leading (versus an inductor’s lagging)
phase angle.
Another often overlooked power
factor, harmonics, comes into play
any time the voltage and current
waveforms deviate from a pure sine
wave. For instance, a square wave
approximation generates many higher-
frequency harmonics that cause an
increase in true versus apparent power
consumption. Harmonics not only
represent an additional power factor
loss, but also can damage other equip-
ment sharing the same line or even
the power source itself. The push to
improve power factors includes the
explicit consideration of harmonics
(see IEEE, Recommended Practices
and Requirements for Harmonic
Control in Electrical Power Systems
,
519, 1992).
BEST OF BOTH WORLDS
Wouldn’t it be nice to have a motor
with the performance advantages of a
brushed DC motor and the mechani-
cal simplicity (no brushes) of an AC
Power
Average
0
Average
Power
0
Voltage
Current
0
0
Voltage
Current
Figure 3—
As shown here, a purely inductive load exhibits a
power factor of zero as measured by the sine of the phase
angle between voltage and current. Fortunately, this isn’t the
case for real-world circuits, but so-called wattless power (i.e.,
power factor less than one) represents a loss of efficiency that
calls for a power factor correction (PFC).
Hot Chips 16 brings together designers and architects of high-performance chips, software, and systems. Presentations focus
on up-to-the-minute real developments. This symposium is the primary forum for engineers and researchers to highlight
their leading-edge designs. Three full days of tutorials and technical sessions will keep you on top of the industry.
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w System-On-Chip Bus
w System-On-Chip Bus
w System-On-Chip Bus
w System-On-Chip Bus Ar
Ar
Ar
Ar
Arch.
ch.
ch.
ch.
ch. Optimized f
Optimized f
Optimized f
Optimized f
Optimized for 1GHz MIPS Pr
or 1GHz MIPS Pr
or 1GHz MIPS Pr
or 1GHz MIPS Pr
or 1GHz MIPS Processors
ocessors
ocessors
ocessors
ocessors PMC-Sier
PMC-Sier
PMC-Sier
PMC-Sier
PMC-Sierra
ra
ra
ra
ra
Panel
Moderated b
Moderated b
Moderated b
Moderated b
Moderated by J
y J
y J
y J
y John Nick
ohn Nick
ohn Nick
ohn Nick
ohn Nickolls
olls
olls
olls
olls NVIDIA
NVIDIA
NVIDIA
NVIDIA
NVIDIA
Outsour
Outsour
Outsour
Outsour
Outsourcing Engineering De
cing Engineering De
cing Engineering De
cing Engineering De
cing Engineering Dev
vv
vvelopment Offshor
elopment Offshor
elopment Offshor
elopment Offshor
elopment Offshore
ee
e
e
Potpourri
• MDGRAPE-3 chip:
MDGRAPE-3 chip:
MDGRAPE-3 chip:
MDGRAPE-3 chip:
MDGRAPE-3 chip: A 165-Gflops
A 165-Gflops
A 165-Gflops
A 165-Gflops
A 165-Gflops ASIC f
ASIC f
ASIC f
ASIC f
ASIC for molecular dynamics sim
or molecular dynamics sim
or molecular dynamics sim
or molecular dynamics sim
or molecular dynamics simulations
ulations
ulations
ulations
ulations Genomic
Genomic
Genomic
Genomic
Genomic
•
•
•
•
• Accelerating Next-Generation Public-k
Accelerating Next-Generation Public-k
Accelerating Next-Generation Public-k
Accelerating Next-Generation Public-k
Accelerating Next-Generation Public-ke
ee
e
ey Cr
y Cr
y Cr
y Cr
y Cryptogra
yptogra
yptogra
yptogra
yptograph
ph
ph
ph
phy on
y on
y on
y on
y on
General-Purpose CPUs
General-Purpose CPUs
General-Purpose CPUs
General-Purpose CPUs
General-Purpose CPUs
Sun
Sun
Sun
Sun
Sun
• Ho
• Ho
• Ho
• Ho
• How SolarFlar
w SolarFlar
w SolarFlar
w SolarFlar
w SolarFlare Comm
e Comm
e Comm
e Comm
e Communications br
unications br
unications br
unications br
unications brok
ok
ok
ok
oke the 10Gbps on UTP bar
e the 10Gbps on UTP bar
e the 10Gbps on UTP bar
e the 10Gbps on UTP bar
e the 10Gbps on UTP barrier
rier
rier
rier
rier
SolarFlar
SolarFlar
SolarFlar
SolarFlar
SolarFlare
ee
e
e
Keynote
Ste
Ste
Ste
Ste
Stev
vv
vve J
e J
e J
e J
e Jur
ur
ur
ur
urv
vv
vvetson
etson
etson
etson
etson
DFJ
DFJ
DFJ
DFJ
DFJ
Nanotech and the Futur
Nanotech and the Futur
Nanotech and the Futur
Nanotech and the Futur
Nanotech and the Future of Moor
e of Moor
e of Moor
e of Moor
e of Moore’
e’
e’
e’
e’s La
s La
s La
s La
s Law
w
w
w
w
Embedded Systems
•
•
•
•
• A Fast P
A Fast P
A Fast P
A Fast P
A Fast Po
o
o
o
ow
w
w
w
wer
er
er
er
ertrain Micr
train Micr
train Micr
train Micr
train Microcontr
ocontr
ocontr
ocontr
ocontroller
oller
oller
oller
oller
Infineon
Infineon
Infineon
Infineon
Infineon
•
•
•
•
• The Mote Re
The Mote Re
The Mote Re
The Mote Re
The Mote Rev
vv
vvolution:
olution:
olution:
olution:
olution: Lo
Lo
Lo
Lo
Low P
w P
w P
w P
w Po
o
o
o
ow
w
w
w
wer
er
er
er
er Wir
Wir
Wir
Wir
Wireless Sensor Netw
eless Sensor Netw
eless Sensor Netw
eless Sensor Netw
eless Sensor Network De
ork De
ork De
ork De
ork Devices
vices
vices
vices
vices
UC Berk
UC Berk
UC Berk
UC Berk
UC Berkele
ele
ele
ele
eley
yy
yy
Low-Power Processors
•
•
•
•
• A 90nm embed
A 90nm embed
A 90nm embed
A 90nm embed
A 90nm embedded DRAM single chip LSI with a 3D gra
ded DRAM single chip LSI with a 3D gra
ded DRAM single chip LSI with a 3D gra
ded DRAM single chip LSI with a 3D gra
ded DRAM single chip LSI with a 3D graphics,
phics,
phics,
phics,
phics,
H.264 codec engine
H.264 codec engine
H.264 codec engine
H.264 codec engine
H.264 codec engine,,,,, and a r
and a r
and a r
and a r
and a reconfigurable pr
econfigurable pr
econfigurable pr
econfigurable pr
econfigurable processor
ocessor
ocessor
ocessor
ocessor
Son
Son
Son
Son
Sony
yy
yy
•
•
•
•
• A Lo
A Lo
A Lo
A Lo
A Low-P
w-P
w-P
w-P
w-Po
o
o
o
ow
w
w
w
wer Opter
er Opter
er Opter
er Opter
er Opteron Pr
on Pr
on Pr
on Pr
on Processor
ocessor
ocessor
ocessor
ocessor
AMD
AMD
AMD
AMD
AMD
Instruction Set Automation
•
•
•
•
• The End of ISA Design:
The End of ISA Design:
The End of ISA Design:
The End of ISA Design:
The End of ISA Design: P
P
P
P
Po
o
o
o
ow
w
w
w
wer
er
er
er
er T
T
T
T
Tools f
ools f
ools f
ools f
ools for Optimal Pr
or Optimal Pr
or Optimal Pr
or Optimal Pr
or Optimal Processor Generation
ocessor Generation
ocessor Generation
ocessor Generation
ocessor Generation T
T
T
T
Tensilica
ensilica
ensilica
ensilica
ensilica
• Long
• Long
• Long
• Long
• Long W
W
W
W
Wor
or
or
or
ords and
ds and
ds and
ds and
ds and Wide P
Wide P
Wide P
Wide P
Wide Por
or
or
or
orts:
ts:
ts:
ts:
ts: Rein
Rein
Rein
Rein
Reinv
vv
vventing the Configurable Pr
enting the Configurable Pr
enting the Configurable Pr
enting the Configurable Pr
enting the Configurable Processor
ocessor
ocessor
ocessor
ocessor
T
T
T
T
Tensilica
ensilica
ensilica
ensilica
ensilica
• OptimoDE
• OptimoDE
• OptimoDE
• OptimoDE
• OptimoDE
ARM
ARM
ARM
ARM
ARM
High-End Processors
•
•
•
•
• The Montecito Pr
The Montecito Pr
The Montecito Pr
The Montecito Pr
The Montecito Processor
ocessor
ocessor
ocessor
ocessor
Intel
Intel
Intel
Intel
Intel
•
•
•
•
• A 32-wa
A 32-wa
A 32-wa
A 32-wa
A 32-way Multithr
y Multithr
y Multithr
y Multithr
y Multithreaded SP
eaded SP
eaded SP
eaded SP
eaded SPARC Pr
ARC Pr
ARC Pr
ARC Pr
ARC Processor
ocessor
ocessor
ocessor
ocessor
Sun
Sun
Sun
Sun
Sun
• Intel P
• Intel P
• Intel P
• Intel P
• Intel Pentium 4 Pr
entium 4 Pr
entium 4 Pr
entium 4 Pr
entium 4 Processor on 90nm
ocessor on 90nm
ocessor on 90nm
ocessor on 90nm
ocessor on 90nm T
T
T
T
Technolog
echnolog
echnolog
echnolog
echnology
yy
yy
Intel
Intel
Intel
Intel
Intel
www.circuitcellar.com
CIRCUIT CELLAR
®
Issue 168 July 2004
83
induction motor? Yes, it would, which
is why brushless DC (BLDC) motors
are such hot items, especially under
the hood of the latest-and-greatest
hybrid vehicles (see Photo 2).
Conceptually, a BLDC motor is a
permanent magnet brushed DC motor
turned inside out: the magnets are
mounted on the rotor, and the coils
are on the stator. Because the coils no
longer rotate, there’s no need for
brushes to make the power connec-
tion (see Figure 4).
Of course, without brushes, another
way must be found to switch the pole
polarity at the proper time. The solu-
tion is to determine the rotor’s posi-
tion in some way and to control the
coil drive in real time, a process
known as “electronic commutation.”
Traditionally, BLDC motors include
sensors, such as Hall effect, to deliver
rotor position information to the
motor controller. More recently, how-
ever, sensorless BLDC motors have
become all the rage.
As the name implies, these motors
dispense with the need for separate
rotor position sensors. Instead, they
exploit a phenomenon known as back
EMF. As I described earlier, current
passing through the stator coil gener-
ates a field that interacts with the
rotor’s permanent magnet field to
cause the motor to turn. But also
recall from EMF 101 that a conductor
passing through a magnetic field gen-
erates current. So, as the rotor (mag-
net) is flying past the coil (conductor),
a small amount of EMF is generated
counter to the EMF that’s causing the
motor to turn in the first place.
By monitoring the back EMF, you
can determine the speed and position
of the rotor without needing any extra
sensors, wires, or I/O ports. However,
the relative magnitude of back EMF
can be small relative to forward EMF,
and it also varies with the speed. That
makes low-speed back EMF detection
(and thus precise control) especially
difficult. Of course, in general, there’s
plenty of electrical noise to cut
through as well. Fortunately, today’s
MCUs and motor-control chips are
increasingly up to the task, allowing
sensorless BLDC motors to make sig-
nificant inroads against both brushed
DC and AC induction motors.
Fulfillment of the promise offered
by intelligent MCU-based motor con-
trols is found in the switched-reluc-
tance (SR) motor, arguably the sim-
plest of all. It’s somewhat similar to a
BLDC, except the rotor has formed
poles (looks like a gear) and is made of
Rotor magnet N
Hall effect sensors
Accessory shaft
Hall effect sensor magnets
Stator windings
Stator windings
Rotor magnet S
Driving end of the shaft
Figure 4—
Brushless DC motors are popular because they combine the performance (e.g., flat torque curve) of
brushed DC motors with the mechanical simplicity and reliability of an induction motor. In order to perform electron-
ic commutation, the motor controller monitors the rotor position using traditional sensors (e.g., Hall effect) and,
more recently, sensorless (e.g., back EMF) techniques.
Photo 2—
Advanced electric motors and their controls
are sparking the emergence of next-generation hybrid
gas and electric engines such as this one from Honda.
Only 60-mm thick, the 144-V BLDC motor boosts
horsepower by 10% and torque by 20% (photo cour-
tesy of Green Car Group, www.greencars.com).
positioning errors (e.g., stalls, external
disturbances, loss of power, or con-
troller errors). Using a stepper motor
open loop (i.e., no external position
detection) versus a conventional DC
motor with position sensors running
closed loop remains an interesting
choice for designers.
The fine positioning resolution and
smoothness of rotation can be further
enhanced with half-stepping, as
shown in Figure 5. Instead of fully
activating a set of coils (as in full-
stepping), partial coil activation caus-
es the rotor to move to an intermedi-
ate position between steps. However,
because partial activation reduces the
overall current, the holding torque at
the half-step position is reduced. To
compensate, a technique known as
current profiling increases the current
through the active coil so that the
total current at all (full- and half-step)
positions is the same.
Microstepping takes the current-
profiling trick further. Instead of just
full- or half-activation, the coils are
driven with much finer granularity
capable of achieving fractional degree
accuracy with tens or even hundreds
of microsteps per full step. Although
it’s a DC motor, the coil drive wave-
84
Issue 168 July 2004
CIRCUIT CELLAR
®
www.circuitcellar.com
forms for microstepping start
to look like overlapping sine
waves, little different from
those that drive an AC motor.
CONTROL FREQ
Whew. I have to confess
that even this brief examina-
tion of the various types of
electric motors went on
longer than expected.
Originally, I’d intended to
combine the motor and con-
trols discussion in one arti-
cle, but as I got into it, I real-
ized each topic has more than
enough meat to justify its
own article. This month I
talked about iron, copper, and
magnets. Next month I’ll talk
about silicon and software.
Everyone knows motors are
here to stay and new applica-
tions continue to emerge.
The real story is that even for
traditional applications, the
desire to go green and cut our energy
bills along with the march of silicon
means the subject of motor controls is
far from old hat. As they say in poli-
tics, the spinning is just beginning!
I
a magnetically permeable
material (something a mag-
net will attract, like iron)
rather than having it’s own
(permanent or induced) mag-
netic field.
When opposing pairs of
coils on the stator are ener-
gized, the nearby poles on
the rotor attempt to align. If
the number of coils and
poles are equal (or integral
multiple), the rotor moves to
a stable position and then
stops. But by using fraction-
ally less poles on the rotor
(say, six) compared to the
number of coils (say, eight),
a coil/pole pair that’s
aligned during one phase is
slightly out of alignment for
the next phase.
Precise control is required
to overcome the main chal-
lenge posed by SR motors,
torque ripple. In essence, the
motor wants to move in steps equal to
360° divided by the number of coils
(e.g., 45° steps for an eight-coil
motor). This can cause vibration,
mechanical stress on the bearings,
and audible noise.
STEP LIGHTLY
Speaking of steps, let’s finish up this
month with the aptly named stepper
motor. It’s kind of like a BLDC on
steroids, one that divides a single rev-
olution of the rotor into, for example,
200 discrete steps (1.8° per step).
Furthermore, as long as the coils
remain energized, the motor will
exhibit strong holding torque at each
step position. Torque is further
enhanced for bipolar stepper motors in
which the coils are driven with cur-
rent in both directions.
With precise stepping angles and
strong holding torque, the stepping
motor is uniquely well suited for posi-
tioning applications. As a practical
matter, all other motors require
closed-loop control for anything
approaching precision positioning. By
contrast, a stepper motor can be, and
usually is, run open loop with perhaps
a limit switch or intermittent calibra-
tion as the only concession to possible
RESOURCES
DS00887A, www.microchip.com/1010/
suppdoc/appnote/all/an887/index.htm.
Motor Fundamentals,” Microchip
Technology, DS00885A, 2003,
www.microchip.com/1010/suppdoc/ap
pnote/all/an885/index.htm.
Tom Cantrell has been working on
chip, board, and systems design and
marketing for several years. You may
reach him by e-mail at tom.cantrell@
circuitcellar.com.
1
2
3
4
Full-step current
two-phase “on”
Phase 1
Phase 2
+I
1
–I
1
–I
2
+I
2
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
Half-step current
Phase 1
Phase 2
+I
1
–I
1
–I
2
+I
2
Half-step current
profiled
Phase 1
Phase 2
+I
1
–I
1
–I
2
+I
2
Figure 5—
Controlling a stepper motor’s winding currents with finer granularity can
boost the positioning resolution to hundreds (half-stepping) and even thousands
(micro-stepping) of steps per revolution.
www.circuitcellar.com
CIRCUIT CELLAR
®
Issue 168 July 2004
85
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SERVICES
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turn serial data into video text
Insert-ready Single Board Computer modules in sub-miniature
dimensions (as small as 47x55 mm) populated by:
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Interface Keypads, Switches, or RS-232 to your PC Keyboard Input
Up to 12 x 12 matrix Programmable RS-232 Port Macro Capability
The KE24 is the ultimate in flexibility. Inputs or serial data can
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93
Understanding Embedded Security
Closed-Loop Motion Control for Mobile Robots
PICs, DRAMs, and Graphics Displays: Build a Graphics LCD Driver
E-Field Sensor-Based Monitoring System
Ham Radio Repeater Locator
ABOVE THE GROUND PLANE
Stepper Drive (Part 1): Analog
APPLIED PCs
PSoC 101
FROM THE BENCH
Is There a Robot in Your Future?: Emerging Robot Technologies
SILICON UPDATE
Motoring (Part 2): Motor Control Chips and Software
94
Issue 168 July 2004
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89
Abacom Technologies
89
ActiveWire, Inc.
93
All Electronics Corp.
93
Amazon Electronics
81
AP Circuits
7
Atmel
91
Bagotronix, Inc.
81
Bellin Dynamic Systems, Inc.
39
Belsoft
18
Beta Layout Ltd.
49
Bitscope Designs
14
CadSoft Computer, Inc.
87
Carl’s Electronics
88
CCS-Custom Computer Services
92
Conitec
85
Cyberpak Co.
1
Cypress MicroSystems
65
CWAV
91
DataRescue
85
Decade Engineering
85
Digital Creation Labs Inc.
91
Digital Products
85
DLP Design
25
Earth Computer Technologies
90
EE Tools
(Electronic Engineering Tools)
26
EMAC, Inc.
35
Entrelogic Corporation
The Index of Advertisers with links to their web sites is located at www.circuitcellar.com under the current issue.
Page
71
ESC Boston
24
ExpressPCB
85
FDI-Future Designs, Inc.
87
Fitzhugh & Waggoner, Inc.
92
Front Panel Express
86
Hagstrom Electronics
21
HI-TECH Software, LLC
82
HOTChips 16 Symposium
48
ICOP Technology, Inc.
89
IMAGEcraft
31
Imagine Tools
86
Intec Automation, Inc
93
Integrated Knowledge Systems
91
Intrepid Control Systems
90
Intronics, Inc.
17
Jameco
64, 86
JK microsystems, Inc.
81
JR Kerr Automation & Engineering
9
Kg Systems, Inc.
83
LabJack Corp.
83
Lakeview Research
89
Lawicel HB
53
Lemos International
2
Link Instruments
69
Linx Technologies
34
MaxStream
88
MCC (Micro Computer Control)
11
Microchip
92
MicroControls
90
Microcontroller Services
95
Micromint
93
Micro Digital
92
microEngineering Labs, Inc.
88
MJS Consulting
90
Mosaic Industries, Inc.
41
Mouser Electronics
66
MVS
86
Mylydia, Inc.
C2
NetBurner
88
OKW Electronics, Inc.
92
Ontrak Control Systems
26
PCBpro
10
PCBexpress
87
PCB Fab Express
C4 Parallax, Inc.
85
Phytec America LLC
87
Phyton, Inc.
90
Picofab, Inc.
93
Pulsar, Inc.
88
Quality Kits & Devices
86
Quantum Composers, Inc.
87
R2 Controls
55
R4 Systems, Inc.
27
Rabbit Semiconductor
Page
Page
Page
11
Reach Technology, Inc.
25
Remote Processing
58
RoboNexus Conf. & Expo
93
Rogue Robotics Corp.
3
Scott Edwards Electronics, Inc.
88
Sealevel Systems
5
Sierra Proto Express
91
Softools
46
Syspec, Inc.
83
Systronix
89
TareTronics, Inc.
90
TAL Technologies
C3
Tech Tools
72, 73
Technologic Systems
87
Technological Arts
90
Tern, Inc.
86
Trace Systems, Inc.
91
Triangle Research Int’l, Inc.
46
Trilogy Design
47
Velocity Semiconductor
93
Weeder Technologies
91
Zanthic Technologies, Inc.
89
Z-World
September Issue 170
Deadlines
Space Close: July 9
Material Due Date: July 20
Theme:
Signal Processing
Bonus Distribution:
Embedded Systems Conference
Boston 2004
Wescon North America
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Preview of August Issue 169
Theme: Embedded Programming
W
hen something works so well for business it is only natural to assume that it can apply to vacations too. After all, if you take your laptop and stay
in touch with the office, you arent really running away on vacation. Youre merely changing the initiation point of your office e-mails. Of course, this pre-
sumes that the Internet exists where you are going.
Last month I spent a week vacationing near Manchester, Vermont. To make a long story short, I knew I was in trouble when the innkeeper said,
Wideband what? when I asked about connecting to the Internet. It was either make a long-distance telephone call to New York for the nearest IP, or
hunt for one of the few local businesses with a cable modem connection. None of these included the hotels. Evidently the leaf peepers who overwhelm
Vermont every year prefer low-tech entertainment like beer and brat. According to the innkeeper, the first communication priority, if any, would be to
add some cell phone service. Welcome to a black hole.
The search wasnt going to be easy and it called for desperate measures. From everything I could tell, the only pastimes in Manchester are either
shopping in one of the local factory outlets or hanging out at one of the many restaurants and cafes while waiting for the next factory outlet to open. If
there is a hip cafe with Internet service, no one seemed to know where it is. I decided the only way to find an Internet connection in this town was to
take PC in hand and go war driving.
War driving is when you go driving through a neighborhood with a wireless-enabled notebook computer and map houses and businesses that have
wireless access points. War driving comes from a play on the older term war dialing. Film buffs should recognize war dialing from the 1983 movie
War Games with Matthew Broderick, in which his character uses an IMSAI computer to automatically call telephone numbers looking for any modem-
connected games and bulletin-board systems. He inadvertently ends up with a direct connection to a high-level military computer that gives him con-
trol over the U.S. nuclear arsenal. Obviously, everything hits the fan after that.
If there was a coffee house with Wi-Fi, I could simply tell by turning on my laptop and driving down the street. So, with my trusty Sony TR3A (3
lbs, 10.6² screen) on the dashboard, I drove through Manchesters two main streets and looked for traces of 802.11b. To my amazement, I picked up
seven different signals in 2 miles. None were encrypted!
I got a signal outside one cafe in the center of town. As I walked in with my computer, the screen popped up with a little notice saying that Internet
service was $6.95 per day and I should sign up now. I noted the location as a backup in case I couldnt find free service someplace else. About 10¢
out the cafe door, I picked up a much stronger Wi-Fi signal with a random-looking alphanumeric ID. There wasnt any commercial business that looked
like it could be the source. Curious, I clicked on Internet Explorer and instantly my home page appeared. I had no idea whose Wi-Fi it was, but what
the heck, it worked. I got back in my car and downloaded my e-mail.
After my quick unofficial access, I decided to see whether or not the other five signals I had picked up could be accessed as easily. It turned out
that three allowed me to connect with no trouble. The other two did not register as encrypted, but they did not allow me access for some reason, which
I did not pursue. Certainly, if I had made a directional antenna out of a coffee can like most war drivers, I might have picked up many more signals,
but my exercise was to get a quick e-mail fix, not hijack all the Wi-Fi hot spots in Vermont.
The best news was that I did indeed find a free Internet cafe among the seven signals I trapped. One of the great attractions in Manchester is the Northshire
Bookstore. Recently they added a large cafe that includes free Wi-Fi along with a computer for public use. Thanks to Northshire, now its cappuccino with Wi-Fi.
Of course, this discussion cant help but bring up the question of Wi-Fi connection legality. Unfortunately, every government entity has an opinion
and an equally ridiculous unenforceable consequence to go along with it. The all too typical pattern of legal thought these days is to presume that the
guy who installs the unencrypted, publicly placed so everyone can trip over it, omnidirectional broadband transceiver has no responsibility. Its the
unfortunate sop with a laptop who happens to walk within range of this transmission who is the bad guy. Come on now.
The age of passing the buck has to stop. If we are going to have laws governing Wi-Fi and similar wireless technologies, it should be presumed
that anything with public access is fair game if it isnt encrypted or otherwise encoded to prohibit access. Instead of blaming the guy sitting in a coffee
shop picking up a signal with his laptop, people should be forced to take responsibility for their Wi-Fi transmission when they install it. If they dont want
it to be public, they should turn on the safeguards. Accessing a public signal should not trigger legal consequences. But, hacking a signal when some-
one has taken pains to tell you that it isnt public should.
War Driving
steve.ciarcia@circuitcellar.com
96
Issue 168 July 2004
CIRCUIT CELLAR
®
www.circuitcellar.com
by Steve Ciarcia, Founder and Editorial Director
PRIORITY INTERRUPT
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