PROJECTS MICROCONTROLLERS
ATM18 = RFID Savvy
EM4095, ATmega and Bascom
Wolfgang Rudolph and Gerhard Günzel (Germany)
RFID appears to be well on its way to becoming a technology of the future. Many people mistrust this
technology, while some see it as the answer to every problem. One thing is certain: these tiny devices
will be everywhere around us in the future, whether or not we notice them. Our ATM18 board provides
the ideal basis for experimenting with RFID devices and implementing your own ideas.
One of the potential applications of two windings as they were, they are
RFID operating principle
the Bascom program for this project still coupled by the magnetic field and
is using RFID to control a door opener. Let s start by considering a mains the same effect still occurs  although
Naturally, you can also use the relay transformer. If you connect the pri- the secondary cannot supply as much
card to connect the board to many mary to the mains and the secondary power with this arrangement. If you
other types of equipment and switch to a load, a current flows in the pri- connect an ammeter in the primary cir-
a specific function on or off, with the mary winding as well as the second- cuit, you can clearly see that the pri-
process being triggered by an RFID ary winding. After all, how else could mary current changes when the load
device. But first let s take a brief look a current flow in the secondary? If you on the secondary winding is connected
at the theory. now remove the iron core but leave the or disconnected. Even if the two wind-
46 elektor - 6/2009
VDD to all blocks
VSS to all blocks
RDY/CLK
BIAS & AGND
BIAS & SHORT
SHD to all blocks
to all blocks
AGND DETECTION
AGND & READY
DC2
VSS
DVDD
LOCK
RDY/CLK
FCAP FCAP
ANT1
ANT1 SHD
LOOP VCO & ANTENNA
EM4095
FILTER SEQUENCER DRIVERS
ANT2 DVDD DEMOD_OUT
DVSS MOD
HOLD
MOD SYNCHRO
ANT2 AGND
VDD CDEC_IN
DEMOD_IN CDEC_OUT
SO16
DMOD_IN SAMPLER FILTER COMPARATOR DMOD_OUT 080910 - 14B
080910 - 14A
CDEC_OUT CDEC_IN DC2
Figure 1. Block diagram and pin assignments of the EM4095 RFID reader IC.
ings are separated by a few centime- there are now a variety of semiconduc- chip and a coil. The transponder chips
tres, the effect of the load on the sec- tor devices that you can use to build a that are compatible with the EM4095
ondary winding is still measurable in very simple RFID receiver. For our RFID include the EM4100, EM4102, EM4150,
the primary circuit. reader we selected the EM4095, a EM4170, and EM4069. We used a
transceiver IC made by the Swiss com- transponder with the EM4012 chip for
This is the operating princi- our experiments. Although
ple of a passive RFID trans- this chip is still available,
ponder, in which an HF gen- according to the manufac-
Main specifications
erator drives an antenna to turer it has been replaced
RFID Reader:
generate an electromagnetic by the EM4200, which can
" Connects to Elektor ATM18 test board
field. This field induces a also replace the EM4005 and
" Compatible with RFID tags using EM4102 and EM4200
voltage in the receiver cir- EM4105.
" 5 V supply voltage
cuit, and the energy trans- Unlike many other RFID
ferred in this manner powers Bascom RFID software for ATM18: devices, such as the Mifare
" Reads RFID data
the receiver chip. The soft- cards already used in some
" Outputs RFID data via RS232
ware in this chip controls a Elektor projects, these EM
" Shows RFID data on LC display
load in the receiver circuit. chips are not ISO standard-
" Relay control by RFID
Just as with a mains trans- ised, but they are neverthe-
former, the different current less widely used and readily
levels in the receiver can be available.
detected in the primary circuit. In this pany EM Microelectronic-Marin SA,
way, a RFID chip can send data to the which can be used with various trans-
The EM4095
transmitter without itself transmitting ponder chips made by the same com-
a signal. pany. The transponder, which is also The block diagram and pin assign-
called a  tag , simply consists of a ments of the EM4095 (Figure 1) show a
Incidentally, utilisation of this handful of pins for the exter-
principle was explicitly pro- nal circuitry. For our project,
hibited in the early days of we use the application cir-
RDY/CLK
radio broadcasting. After the cuit shown in Figure 2. It has
first German broadcasting separate power rails for the
CDC2
station started operating on antenna portion and the chip
1 16
CFCAP
29 October 1923, it didn t take (DVDD and VDD). However, a
2 15
RSER
long for a few hobby garden- single power supply is ade-
SHD
3 14
LA
ers in the surrounding area quate for our simple appli-
DEMOD_OUT
+5V
4 EM4095 13
to discover that with a suit- cation. The resonant circuit
µP
MOD
5 12
able antenna, they could not of the antenna coil has two
CRES
CAGND
only listen to the radio pro- 6 11
tasks: it must generate the
+5V
gramme but also power their transmit signal for the trans-
10
7 CDEC
CDV1
jury-rigged lighting system. ponder, and it must receive
8 9
This significantly reduced the amplitude-modulated sig-
CDV2
the effective range of  Radio nal from the transponder.
080910 - 15
Berlin Welle 400 , and it was
consequently prohibited. The resonant circuit consists
Getting back to the present, Figure 2. EM4095 application circuit diagram. of the antennal coil LA and
6/2009 - elektor 47
PROJECTS MICROCONTROLLERS
CLOCK
EXTRACTOR
Logic
COIL1
Clock
Signal on
VDD
Transponder coil
AC1
Cress
Csup
FULL WAVE MEMORY
SEQUENCER
RECTIFIER ARRAY
AC2
Signal on
Transceiver coil
VSS
COIL2
Serial
Data Out
DATA
080910 - 17
RF Carrier Data
MODULATOR
DATA
ENCODER
Modulation
080910 - 16
Control
Figure 3. Block diagram of the EM4102 transponder chip. Figure 4. The 125-kHz signals on the reader and transponder .
the capacitor CRES. These components (pin 6). This input is used to feed in controller. The MOD signal is used to
determine the resonant frequency, the amplitude-modulated signal gener- write data to the transponder. The
which should be approximately ated by the transponder. The maximum microcontroller holds the MOD input
125 kHz. The EM4095 is designed for rated voltage on this input is 4 V. Low during read operations. SHD is the
operation in the frequency range of 100 Capacitor CDEC decouples the DC sup- enable input of the EM4095. The reader
to 150 kHz and uses a phase-locked ply voltage between the sampler out- IC enters sleep mode when this input
loop (PLL) to control the frequency. put and the filter input. Capacitor CDC2 is set High, and in this mode its cur-
Naturally, the transponders also oper- works with an internal filter circuit to rent consumption is only a few micro-
ate in this range. The series resistor form a bandpass filter. amperes. When SHD is pulled Low,
RSER keeps the current in the antenna the EM4095 wakes up and executes
driver below the maximum rated value. Four lines are normally necessary for a start-up process that takes 41 clock
As backup protection, the EM4095 also the connection to the microcontrol- cycles. DEMOD_OUT feeds the demod-
has a short-circuit detection circuit that ler: SHD, MOD, DEMOD_OUT, and ulated Manchester-coded data stream
disables the output stage in case of an Ready/Clock. to the microcontroller.
overload.
The RDY/CLOCK line initially transmits The manufacturer provides several
CDV1 and CDV2 form a capacitive voltage the status signals, and later on, after formulas in the data sheet and Appli-
divider that feeds part of the antenna the EM4095 has started operating, it cation Note 404 for calculating the
voltage back to the DMOD_IN input transmits the clock signal to the micro- values of the external components. If
RFID
In addition to security and logistics, animal identification is an impor-
Good things last long
tant application area for RFID. In November 2002, the US Food and
Drug Administration (FDA) approved a controversial use of RFID tech-
Radio-frequency identification (RFID ) technology is actually old hat.
nology: chips embedded in people. The  Verichip RFID chip produced
The first commercial predecessors of the current technology were
by the US company Applied Digital Solutions is designed to be embed-
launched in the 1960s. However, only now is the world ready for com-
ded subcutaneously (beneath the skin). In an emergency, doctors can
prehensive data collection.
immediately read out vital patient information, such as the person s
blood type and any current allergies or illnesses. However, there are
RFID is a refinement of a technology that originated during the Sec-
now access control systems for companies and even discos that are im-
ond World War. A system called  Identification Friend or Foe (IFF) was
plemented using embedded RFID tags. Everyone should carefully con-
developed at that time, with the objective of enabling the American
sider whether he or she is willing to have a system of this sort  installed
armed forces to distinguish allied aircraft from enemy aircraft. The
in their body.
original equipment was a large as a suitcase and very expensive.
RFID technology was also used in electronic article surveillance systems
as early as the 1960s. At that time, the memory capacity of an RFID tag
Lending libraries, such as the new Vienna Main Library, use RFID tags
was only 1 bit. The 1980s saw the introduction of RFID technology in
for inventory control. Special types of RFID readers can read a group of
the automobile industry. RFID chips were initially used in anti-start sys-
RFID tags in a single operation. This technique is called  bulk reading .
tems, fuelling cards, and remote vehicle entry systems.
Another application for RFID technology, which is already extensively
As in all other fields of electronics, miniaturisation proceed rapidly. This exploited, is electronic transport cards that can be read without physical
opened up more and more application areas, such as credit cards, ac- contact and reloaded. They are used successfully in numerous Asian
cess control systems, and personal identification systems. metropolises, such as Hong Kong and Singapore.
48 elektor - 6/2009
ulated voltage is tapped off, and the
reader can filter the transponder data
out of its own  attenuated signal.
1 1 1 1 1 1 1 1 1 9 header bits
The read data can be coded in various
8 version bits or D00 D01 D02 D03 P0
customer ID D10 D11 D12 D13 P1
manners. Protocols such as Manches-
D20 D21 D22 D23 P2
ter, bi-phase, PSK and FSK are widely
32 data bits D30 D31 D32 D33 P3
used. In our EM4102, the data (which
D40 D41 D42 D43 P4
is hard-coded in the memory) is Man-
D50 D51 D52 D53 P5
chester coded for output. Manches-
D60 D61 D62 D63 P6
D70 D71 D72 D73 P7 ter coding is commonly used for serial
D80 D81 D82 D83 P8
data transmission on a data line. The
D90 D91 D92 D93 P9 10 line parity
payload data is XORed with a signal at
PC0 PC1 PC2 PC3 S0 bits
twice the data clock rate, which elim-
4 column parity bits
080910 - 18 inates the need for a separate clock
line. The clock and payload data are
first synchronised at the start of the
coding process, and the clock and data
Figure 5. Configuration of the 64 bits stored in the EM4102.
are XORed on each edge of the clock
signal. This produces the transmitted
data stream, which is used to modu-
late the 125-kHz carrier signal.
you find this too complicated, you can The voltage induced in the coil is fed
use a spreadsheet available from the to a full-wave rectifier and used as the In our case, the data stream consists of
manufacturer s website or the project operating voltage. The clock frequency 64 bits. This is broken down as follows:
page on the Elektor website (www. is generated in the Clock Extrac- header (9 bits), data (40 bits), row parity
elektor.com/080910). This makes tor. The Sequencer uses this clock to (10 bits), column parity (4 bits), and stop
the calculations relatively easy and shift data out of the memory into the bit (1 bit). This is illustrated in graphic
semi-automatic. data encoder. There the data is proc- form in Figure 5. The data is read out
essed according to the protocol that is row by row from the top left to the bot-
used and fed to the Modulator, which tom right as 64-bit data string. The nine
The transponder
drives the antenna coil. The reader  1 bits are the header. Each of the next
RFID devices are actually modules that and transponder frequencies are syn- ten rows consists of four data bits and
are available in a variety of forms, such chronised. The modulator produces an one parity bit. The last row consists of
as smart cards, small glass tubes, and amplitude-modulated HF signal in the the four column-parity bits and the stop
key fobs. transponder coil, with the data in the bit, which is always  0 . The first eight
Figure 3 shows block diagram of the sidebands. bits of the data field are customer-spe-
EM4102 transponder chip and its indi- cific. The purchaser can store a fixed
vidual functional blocks. The only The signal envelope at the reader is company ID or internal company code
external component is the coil con- exactly the opposite (see Figure 4). As here, assuming that a sufficiently large
nected to the Coil1 and Coil2 pins. previously mentioned, part of the mod- quantity of chips is ordered.
However, rapid technological progress in recent years has led to a con- transponders contain fixed, non-alterable ID codes These tags are very
fusing proliferation of incompatible systems, in part due to the failure small and maintenance-free. Transponders are often attached to a
to define uniform standards in a timely manner. The future widespread plastic film along with their antennas. This form of transponder can be
printed with visible information and handled similarly to paper. These
use of RFID technology will doubtless depend in part on the extent to
 smart labels are available in several versions. Depending on the type
which agreement on uniform standards can be achieved. To enable
RFID technology to be used across company boundaries and nation- and the frequency band, they can be read out at distances ranging
from a few centimetres to 100 metres.
al borders, interested parties in the industrial and mercantile sectors
acting under the guidance of European Article Numbering (EAN) In-
RFID tags with writable memory offer higher performance and in-
ternational and EPCglobal have founded the Uniform Code Council
creased versatility. Depending on the type, the memory capacity ranges
(UCC). EPCglobal has developed the  Electronic Product Code (EPC),
from a few bits to several hundred kilobytes. RFID tags with built-in
which can be used to describe manufacturers and products uniquely.
encryption mechanisms are used in applications with high security re-
This code consists of 96 bits, divided into four groups: an 8-bit header,
quirements RFID tags with a microprocessor and an internal operating
a 28-bit manufacturer code, a 24-bit object class code, and a 36-bit
system are also available now. These tags are usually produced in the
serial number. This is sufficient to identify 6.87 1010 items in each
form of smart cards (dual-interface cards).
of 1.67 1010 object classes for each of 2.68 108 manufacturers.
RFID devices are basically classified as passive, active or semi-active,
This standard is currently supported by Wal-Mart, the Metro Group,
depending on how they are powered. In contrast to active RFID de-
Carrefour, and Tesco. With the combination of the product code and
vices, passive RFID devices do not have an internal source of power.
a database, it should be possible to fully describe every merchandise
Semi-active devices switch automatically into sleep mode, and they can
item in the world.
attain a very long battery life of up to six years. Active tags have the
Several manufacturers, such as NXP, Infineon and Texas Instruments, advantage that they can be used together with integrated sensors for
produce various types of RFID tags for a wide range of applications. temperature monitoring or precise localisation (in combination with a
In the simplest and most economical versions (read-only tags), the GPS receiver).
6/2009 - elektor 49
PROJECTS MICROCONTROLLERS
to solder the coil leads and the con-
necting links to the ATM18 board.
+5V
The antenna coil is a DIY construction.
The circular coil, which can be seen
in the photo, consists of 160 turns of
C1
0.6-mm enamelled copper wire. The
100n
coil diameter is approximately 23 mm,
RY/CL
which means that you need 12 metres
of wire. If you can find a suitable form
K1
7 4 K1
in your junk box and chuck it in a drill,
VDD DVDD
2 14
winding the coil is very easy.
RY/CL SHD
ANT1 R1
3 13
ANT1 DEMOD OUT
68
10 12
CEDEC IN MOD
K2
The inductance of the coil is approxi-
C10
C3 IC1
mately 780 µH. If you have an induct-
EM4095
100n
1n 9 ance meter or a multimeter with an
CEDEC OUT
K3 C4
6 16
inductance range, you can check the
ANT2 DC2
1n
ANT2
C5
value of the finished coil. However,
8 15
DEMOD IN FCAP
the exact value is not especially criti-
47p
VSS AGND DVSS
C6 cal. In our tests with this antenna coil,
1 11 5
the transponder with the EM4102 was
C8 C7
C2 C9
*
recognised immediately every time.
1n 100n
10n 10n
We obtained the same result with the
rectangular coil shown in the photo at
080910 - 11
the head of this article. It comes from
an EM4095 reference design kit that is
available from MCS Electronics (which
Figure 6. RFID reader circuit for connection to the CC2 AVR board.
also produces Bascom). The circuit dia-
gram is described at a site referenced
by one of the links at the end of the
The transponder sends this 64-bit article.
The RFID reader board
sequence repeatedly as long as oper-
ating power is available, which means The ATM18 board needs an extension In practice, the EM4095 reader board
as long as it is within range of the to enable it to recognise and read RFID works OK without any alignment or
reader. The coded signal starts and devices. This extension is the RFID tuning, but for maximum sensitivity
ends with the same phase state. As reader board. For this purpose, we use the oscillator frequency should be as
a result, the start sequence of nine the previously described EM4095 in close as possible to 125 kHz. To avoid
 1 bits can be detected unambigu- the circuit shown in Figure 6. The IC accidentally shifting the oscillator
ously when the received signal is is wired the same as in the data-sheet frequency while measuring it, do not
processed. application circuit diagram shown in make the measurement directly on the
Figure 2. The capacitor values are: CRES coil, but instead use a small  sniffer
Transponders using the EM4102 are = 2 nF (C3 + C4), CDV1 = 47 pF (C5), coil brought within range of the
among the simplest type of RFID and CDV2 = 1 nF (C2). Series resistor antenna coil. A circular or rectangular
devices. They emit only the data stored RSER has a value of 68 (R1). The sec- coil with a diameter or edge length of
in the chip during manufacture, with ond capacitor position C6 in parallel around 60 mm, made from insulated
no possibility of storing user-gener- with C5 allows the value of CDV1 to be or enamelled wire with a diameter of
ated data in the RFID device. Their sole adjusted if necessary. 0.5 to 1 mm, is suitable for use as a
utility arises from the fact that each sniffer coil.
EM4102 contains a code that is issued A small PCB (Figure 7) has been
only once, which means that each tag designed for assembling this circuit. If the frequency measured using the
can be identified uniquely. This is fully It connects to the ATM18 board. The sniffer coil differs significantly from
sufficient for applications such as door extension board is available from the 125 kHz, you can correct the situation
openers, anti-start systems, or identi- Elektor Shop with the SMD compo- by changing the number of turns of
fying cattle. nents pre-assembled, so you only have the coil. Reducing the number of turns
C1,C9,C10 = 100nF
COMPONENT LIST
C6 = not fitted (see text)
Resistor
Semiconductors
(SMD 0805)
IC1 = EM4095 (SMD SO16)
R1 = 68
Miscellaneous
Capacitors
K1 = solder pin
(SMD 0805)
K2 = 2-way pinheader, 0.1 lead pitch
C5 = 47pF
K3 = 5-way pinheader, 0.1 lead pitch
C2,C3,C4 = 1nF
Figure 7. RFID reader PCB for assembling the reader circuit
PCB with SMD parts prefitted, incl. pinhead-
C7,C8 = 10nF
shown in Figure 6.
ers, order # 080910-91
50 elektor - 6/2009
increases the frequency, and of course,
increasing the number of turns reduces
the frequency.
Testing and connection
For the first functional test of the
EM4095 reader card, you need a 5-V
power source and an RFID tag that
can be detected by the EM4095. This
can for example be a transponder with
an EM4102 in the form of a key fob, as
shown in the large photo.
Figure 8 shows an example of the
acquisition of an RFID tag. The upper
waveform in the oscillogram is the
modulated 125-kHz signal, and the
Figure 8. Oscillogram showing the modulated 125-kHz signal (top)
lower waveform is the data signal at
and the data signal at the DEMOD_OUT output of the EM4095 (bottom).
the DEMOD_OUT output (pin 13 of the
IC or pin 2 of connector K2 on the PCB).
As you can see, the transponder sends
its data as it is supposed to  this is
what you should see when everything
is in order.
In order to evaluate and display the
ANT.
data, the reader board must be con-
nected to the CC2 system as shown
in Figure 9. Here SHD is connected to
PB3, MOD is connected to PB4, and
DEMOD_OUT is connected to PD2,
along with the connections for +5 V
and ground. The connections between
the CC2 board and the LC display mod-
ule and the relay board are also shown
in the wiring diagram. The only thing
you still need at this point is the Bas-
com program, which is our next topic.
RFID and BASCOM
The program code is very simple
because almost all of the tasks are
handled by a library ( lib ) function.
The lib function formats the data
read from the RFID tag into bytes and
+5V
GND
places the data on the stack. If you
read this data as a normal array using
the example Bascom code, the byte
sequence is reversed. You should bear
this in mind and take it into account
in the further processing of the data.
DATA
CLK
Familiar subroutines are used to dis-
LCD 20 x 4
play the data on the LCD module and
to control the relays on the relay board.
All of these routines have been used
DATA
CLK
before in related articles in this series,
+5V
where they are described in detail. The GND
program outputs the data to the serial
080910 - 12
interface in addition to displaying it
on the LCD module, so the data can
be saved or further processed on a PC
and displayed using a terminal emula-
Figure 9. Wiring diagram of the RFID project with the RFID reader board, ATM18 board, LC display module,
tor program. and relay board with port expander.
6/2009 - elektor 51
+5V
GND
MOD
SHD
DEMOD OUT
PROJECTS MICROCONTROLLERS
input pin for the Manchester-coded
signal.  Int is the expected interrupt.
Start
 Demod and  Int are physically linked
main program
together. As you can see from the data
sheet, interrupt 0 is associated with
PIND.2.
Config Hitag = 64 , Type = Em4095 , Demod = Pind.2 , Int = @ int0
Config Int0 = Change
On Int0 Checkints
Config Int0 = Change
Enable Interrupts
DIM Array (5) as Byte
The Manchester-coded signal is
applied to interrupt pin D.2. The inter-
rupt routine is triggered by the rising
DO
and falling edges of the input signal.
lib routine
readhitag :
On Int0 Checkints
If Data
 On Int0 declares the interrupt routine
Is
from
LCD no
call readhitag transponder
call lib function
 Checkints .
no transponder
true
= OK
data in array
Enable Interrupts
yes
return = 1
This sets the Interrupt bit in the sta-
tus register.
read 5 byte
from array
Checkints:
Call _checkhitag
Return
LCD transponder
relay 1 sec on
 Checkints is the interrupt routine
that was previously declared with  On
Int0 .
LOOP
DIM Array(5) as Byte
 DIM Array(5) reserves memory space
TAG interrupt routine for five bytes.
The main routine runs in a DO loop, and
initially all it does is to indicate via the
input
Lib Routine
transponder
LC display that nothing is happening.
_
checkhitag
The Checkints interrupt routine is only
triggered when a transponder enters
INT 0
the range of the EM4095. The interrupt
routine in turn calls the library function
 _checkhitag . This assembly-language
Checkints
routine reads in and decodes the Man-
Call
_
checkhitag
chester-coded bit string. It calculates
Return
and checks the parity and then stores
the data bytes in an array. If a time-
080910 - 13
out or any other sort of error occurs,
the function terminates without writ-
ing new data to the array. The main
Figure 10. Flow chart of the Bascom software for the RFID project.
routine then continues from the point
where it was interrupted.
The flow chart in Figure 10 depicts The IF statement evaluates the return
The details
the logical structure of the RFID reader value of the  Readhitag(Array(1) )
program. The actions shown in red and A number of entries in the main rou- function. The  Readhitag function,
green are not handled by normal Basic tine are necessary in order to link in which is integrated in Bascom, also
commands, but instead call assembly- the library. The entries shown in red calls EM4095.lib. It passes the start
language routines in em4095lib. The relate to interrupt handling, while the address of the previously defined byte
RFID library for the EM4095 reader entries shown in green are for calls to array (with  1 as the initial address)
IC is integrated into Bascom version the library function. as a call parameter. The library rou-
1.11.9.0 and later. tine stores the data at the address
Config Hitag = 64 , Type = Em4095 ,
passed in the call and returns a  1
Demod = Pind.2 , Int = @int0
When a transponder is recognised, the if the result is valid (data present
data is decoded using an interrupt rou-  Hitag is the division factor for the in memory). The Interrupt bit in the
tine. The data is saved in memory and clock frequency.  Typ is the reader status register is then cleared to pre-
can re read out as a byte array. IC type.  Demod is the number of the vent the data from being changed by
52 elektor - 6/2009
another interrupt. Now the data can
be read from the array and displayed
in a FOR& NEXT loop.
The example program also actuates a
relay for one second at this point. After
this, the Interrupt bit is set again in
the status register and the entire proc-
ess starts again from the beginning.
A wealth of possibilities
As usual, this project is intended to
provide a sound basis for developing
your own applications. The hardware
and software described here are suit-
able for recognising and reading RFID
devices fitted with the very economi-
Figure 11. When a RFID tag is recognised, the data read from the tag is shown on the LC display.
cal EM4095 chip (see Figure 11). A
relay on the CC2 relay board closes
and opens at one-second intervals as
long as a RFID device is acquired by battery identification, childproofing a
Internet Links
the reader. The relay contact can be PC or television set, a DIY anti-start
http://avrhelp.mcselec.com/index.html
used to control another device, such unit for a motor boat, and so on.
(EM4095 reader under: Bascom Hardware >
as a door opener. We re sure that you can come up with
Reference Designs)
many other potential applications.
www.elektor.com/080910 (Elektor project
However, the potential applications are (080910-I)
page for this project, with download fi les for
practically unlimited. Some examples
the software and PCB layout)
include a cat door controlled by a  cat
RFID tag, a battery charger with RFID
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6/2009 - elektor 53