TOPIC: A System to Try Out PIC Microcontrollers
===============================================

David Tait

Electrical Engineering Labs
School of Engineering
The University
Manchester M13 9PL
UK

[e-mail]
http://www.man.ac.uk/~mbhstdj/piclinks.html


Copyright (C) 1996 David Tait
8th September 1996 (preliminary)


The TOPIC system is a very convenient way to learn about the Microchip
PIC16C84 microcontroller. TOPIC is a combination of a small circuit
board that connects to the printer port of an IBM compatible PC and a
program to control the board. The hardware includes both a PIC16C84
microcontroller development circuit and an on-board programmer. The
associated software downloads programs to the PIC16C84 in the form of
Microchip hex files (either INHX8M or INHX16 format) and controls the
execution of the PIC. Alternatively the TOPIC board can be used as a
standalone PIC16C84 programmer or, by not including the programmer
components, a standalone PIC development circuit.

The programmer reserves two of the PIC's I/O pins (RB6 and RB7) and
these are unavailable for general purpose use. However, if the TOPIC
board is disconnected from the target circuitry during programming, and
disconnected from the PC while running, all I/O pins are available.
Despite the loss of two I/O pins, running programs while the board is
still attached to the PC offers the most convenient mode of operation.
The I/O pins are not wasted because when the board is connected to the
printer port it is possible to use RB6 and RB7 to communicate with the
PC as an aid to debugging.

The schematic for the TOPIC hardware is provided in the Windows
Paintbrush viewable file TOPIC.PCX. If you don't use Windows and need a
DOS-based utility to view the files then you might consider the PICEM
package by John Bridges - look for PICEM27.ZIP on any SimTel site. The
files COPPER.PCX and LAYOUT.PCX give details of a printed circuit board
design for the TOPIC hardware (The schematic shows a crystal oscillator
setup but the board can accommodate crystal, ceramic resonator or RC
oscillator components). All the PCX files have a resolution of 300 dots
per inch (dpi) and should give good quality artwork on most printers.
All the components to populate the board are available in the UK from
Maplin Electronics.

The software is supplied as a pre-compiled executable file (TOPIC.EXE)
and as C source (in SRC.ZIP). The TOPIC program expects a string of
switches (introduced by - or /) and single filename. TOPIC is
compatible with the __CONFIG, __IDLOCS and DE directives of Microchip's
MPASM assembler; this means that using TOPIC to download a program can
be as simple as:

TOPIC PROGRAM.HEX

where PROGRAM.HEX includes both the program and the configuration
information. It is possible to override (or specify) configuration
information on the command line using one or more of the characters from
the set (upper-case and lower-case letters are equivalent):

c Enable code protection
p Enable power-up timer
w Enable watchdog
l LP oscillator
x XT oscillator
h HS oscillator
r RC oscillator

For example:

TOPIC -PRX PROGRAM.HEX

programs the PIC with power-up timer, watchdog and XT (i.e. crystal)
oscillator enabled. Other command line switches include:

g Go. Lets the PIC run (no file argument needed).
d Dump. Dumps the PIC contents to the specified file.
e Erase. Erases the PIC (no file argument needed).
v Verify-only. Verifies the PIC against the specified
file contents.

Running TOPIC with no arguments pulls the PIC reset line low. The
Following batch file can be used to apply a short reset pulse:

@ECHO OFF
TOPIC > NUL
TOPIC -G > NUL

You can erase, program and run with a single command such as:

TOPIC -EG PROGRAM.HEX

The behaviour of the TOPIC system can be changed by some environment
variables:

PPLPT=nn The LPT port to use (default nn=1). Can be 1, 2 or 3.
PPDELAY=nn Adds a delay of nn*0.83 microseconds to printer
port accesses (default nn=6). Useful for fast PCs.
The valid range of nn is 0 to 127.
PPDUMP=nn If nn=16 files are dumped in INHX16 format (default
is INHX8M).

It's probably best if the appropriate commands are included in
AUTOEXEC.BAT. For example to eliminate the printer port delays and tell
TOPIC to use LPT2 use:

SET PPLPT=2
SET PPDELAY=0

The Files WALK.ASM and WALK.HEX are the source code and associated hex
file for a simple test program you can use to exercise the TOPIC board.
See WALK.ASM for details.