A 6809 Single Board Computer

As well as the 6502, 6800, and 68000 chips, another CPU I used early in my career was the Motorola 6809, the more powerful successor to the 6800. Having completed some retrocomputing projects with the other chips, I start looking for a project where I could play with this CPU for nostalgia purposes.

I soon came across a Single Board Computer design by Grant Searle. A five or six chip design, it has serial i/o and can run a version of Microsoft BASIC that he adapted from the version in the Radio Shack Color Computer.

The basic specs are a 6809 processor running at just under 2 MHz, 32K of static RAM, 16K of EPROM, and a 6850 ACIA-based serial port.

The design is simple, potentially buildable on a breadboard, and had been reproduced by other people, so I decided to give it a try.

I entered my own schematic using the EasyEDA web-based CAD software. This would make it easier to make design changes and potentially a printed circuit board (PCB).

I made a few small changes in my version of the board:

• Used a 6-pin connector for a standard FTDI USB to serial breakout board and omitted any RS-232 line driver/receiver circuitry. I also omitted the optional hardware handshaking circuit.
• Added a simple power on reset circuit to the pushbutton reset.
• Added a power on LED.
• Provided a jumper to select USB or external power source.
• Added a 32-pin header with access to most signals, for future expansion and to help with debug.

The system can plug into a computer's USB port where it shows up as a serial port. It is powered by USB (taking about 230 mA of current) and you communicate with it using a terminal emulator.

The provided BASIC is a port of Microsoft Extended BASIC for the 6809-based Radio Shack Color Computer. A disassembly of it was published in two books. In Grant Searle's port, all code and commands that were not applicable (e.g. graphics, sound, cassette tape i/o) were removed. It is just under 10KB in size. Pretty typical of Microsoft BASIC of the era, it has a few quirks, e.g. available memory i available from the variable MEM rather than a function FRE(). Extended Color Computer BASIC has some additional commands that are useful like RENUMber, additional math functions, PRINT USING, TRON/TROFF and even EDIT to support editing program lines. Apparently this was the last version of Microsoft BASIC that Bill Gates personally worked on. You can find online copies of books on the Colour Computer and its version of BASIC.

I took the BASIC firmware and got it to build under Linux using the as09 cross-assembler with no warnings. I found and fixed a small typo that might have affected the PRINT USING command.

I breadboarded the CPU, clock, and reset circuit on a solderless breadboard. Then I got the 6809 to free run by forcing all the data lines all low. I decided not to breadboard the entire circuit as it would be quite tedious to do so, wouldn't fit on my small breadboard, and the design looked stable and had been verified by others, so I opted to move directly to a PCB layout.

I made a PCB layout from EasyEDA (using the autorouter). Once it looked good, I ordered some PCBs from JLPCB, a partner of EasyEDA which offers PCBS for as little as $2 per board.

There are high quality double-sided boards with plated-through holes, silkscreened, solder masked and even electrically tested. They are built in about one day and arrive in about 5 business days. With the availability of suppliers like this, it really doesn't make sense to etch your own boards at home.

I had found and orders the remaining parts I needed on Ebay. Some, like the Motorola 68B50 and 68B09, are readily available as new old stock (NOS).

I programmed the firmware onto a 27C128 EPROM using my UV eraser and programmer.

Once the boards arrived, I build one up, starting with the power, then clock and reset circuits. Everything checked out and before long BASIC was up and running!

I read some books on Color Computer BASIC to review the commands, played with the commands and tested some old programs. Here is an example, a little horoscope program I wrote in BASIC some time again, which ran with some minor changes:

           YOUR HOROSCOPE
           --------------

THE PROGRAM GENERATES A PERSONAL
HOROSCOPE. I NEED SOME INFORMATION
ABOUT YOUR DATE AND LOCATION OF BIRTH.
I WILL THEN GENERATE AN ANALYSIS BASED
ON YOUR ASTROLOGICAL DATA.

WHAT IS YOUR FIRST NAME? Fred
YOUR YEAR OF BIRTH (E.G. 1980)? 1980
YOUR MONTH OF BIRTH (1=JAN)? 6
DAY OF THE MONTH (1-31)? 21
COUNTRY OF BIRTH? Canada
IN WHAT CITY? Ottawa
CALCULATING HOROSCOPE...PLEASE WAIT.

Fred, YOUR ASTROLOGICAL SIGN IS:
CANCER.

HERE IS MY ANALYSIS:

AT TIMES YOU ARE EXTROVERTED, AFFABLE,
SOCIABLE, WHILE AT OTHER TIMES YOU ARE
INTROVERTED, WARY, RESERVED.

YOU WORRY ABOUT YOUR HEALTH AS YOU GET
OLDER.

YOU ARE CONCERNED ABOUT THE HEALTH OF
AN AGING RELATIVE.

GENERATE ANOTHER HOROSCOPE (Y/N)? 

Looking for machine language monitors, I found source code for the ASSIST09 program that Motorola offers for their 6809-based development ports. I got it to cross-assemble under Linux, and modified it to work with the 6850 ACIA instead of what the Motorola hardware used. With a little debugging, I got it running. It works well, and provides most feature you want for development and debug, including memory display and change, register display and change, running and breakpoints, and generating and loading Motorola S record files. here is a sample session:

ASSIST09
>D 1000 20

      0  1  2  3  4  5  6  7  8  9  A  B  C  D  E  F  
1000 45 41 4C 49 53 54 49 43 2E 22 20 3A 20 90 00 10  EALISTIC." : ...
1010 56 2F 26 87 20 22 54 48 45 20 42 49 52 54 48 44  V/&. "THE BIRTHD
>R
PC-F842 A-00 B-00 X-20FE Y-F002 U-60C2 S-6051 CC-F4 DP-00 
PC-
>M 1000
45-55
>P 1000 100F
S13100055414C49535449432E22203A2090001014
S9030000FC
>

I am able to cross-assemble code on a Linux computer and then load the S record file onto the board via the serial port. This is much more efficient than erasing and burning EPROMs.

Note that for uploading you need to add delays due to no hardware handshaking. I use the ascii-xfr program on Linux to do this, as well as the minicom terminal emulator.

I wrote a couple of example programs that run with the ASSIST09 monitor, using it's SWI functions for i/o.

Next, I combined BASIC and ASSIST09 into one EPPROM. It comes up in ASSIST09 but you can get to BASIC using the "G D000" command. This offers the ability to play with BASIC or machine language without swapping EPROMs.

ASSIST09
>G D000
6809 EXTENDED BASIC
(C) 1982 BY MICROSOFT

OK
10 FOR I = 1 TO 10
20 PRINT I,
30 NEXT I
RUN
 1               2               3               4               5
 6               7               8               9               10
OK

I have built up a second board and am just waiting to receive a second 68B09 chip to complete it. I have a few programming projects in mind to work on, and some code (like a small C compiler) that I want to look at.

If you want to give this a design a try, I encourage you to do so. Feel free to use my PCB design files if you wish, or just breadboard it up.

References

1. Grant's 6-chip 6809 Computer: http://searle.hostei.com/grant/6809/Simple6809.html
2. My git code with firmware and other software: https://github.com/jefftranter/6809/tree/master/sbc
3. EasyEDA project: https://easyeda.com/tranter/6809-Single-Board-Computer
4. AS9 assembler: http://home.hccnet.nl/a.w.m.van.der.horst/m6809.html