30 July 2018[This page was recovered from the TheStuffWeBuild WordPress Archive and adapted for this site in 2025.]
Animation showing the bus LEDs loading Basic at a very low clock speed
I recently came across a Motorola 6809 and 6802 CPU along with many other 80s-vintage 7400-series ICs in a dumpster-dive tear down of some antiquated analytical equipment. The 6800 series of Motorola CPUs from the 70s and 80s hold a special status in computer history for their wide-spread use in many personal computers of the time.
The MC6809 (first produced in 1978) was one of the first 8-bit processors to exhibit 16-bit features. The one used in this project was stamped in 1987. These were commonly used in the familiar Commodore SuperPET and TRS-80 (RadioShack/Tandy) Color Computers among many others of the time.
The bulk of this project was based of Grant's DIY "Simple 6809" PC, and its custom ROM, all of which which can be found here:
http://zx80.netai.net/grant/6809/Simple6809.html (Wayback Machine archive linked HERE)
Besides shrinking it to a single 7cm by 9cm piece of perfboard, I also added a few extra features (reads: feature creeping). The main bonus feature being the three 74LS244N line driver ICs and 24 red LEDs. Together, these devices display exactly what the 6809 is "thinking". The right 16 LEDs indicate the current binary address, and the left 8 LEDS indicate the eight data bits on that address.
In order to make the most use of these LEDs, I also added a jumper-selectable breakout header for the clock input. By default, the 7.3728MHz crystal (1.8432MHz clock) is used. This however only yields a faint fading/twinkle effect on the LEDs. An external TTL level clock can be generated from an external 555 timer to create any arbitrary clock speed. Serial communication unfortunately will cease to work at non-standard frequencies. I found that a 10Hz clock speed creates a pleasing effect similar to the W.O.P.R. (War Operation Plan Response) from the 1983 WarGames film. At lower clock speeds, you can start to see and differentiate the different tasks the 6809 is capable doing.
Rats nest of wires and a cheat sheet for the custom version of MS Basic that lacks some moot features.
