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A Minimalist 6802 Microcontroller

2016/04/28

Back in the mid-90's a collegue of mine developed a number of very cool little single-purpose microntrollers using Motorola 68HC05 processors. This set me to wondering what I could do along similar lines for a general-purpose device.

I happened to have a couple of sticks of Motorola 6802 CPUs and 6820 I/O chips on hand and decided to see how few ICs it would take to implement a worthwhile microcontroller.

This little blue beastie was the result. It doesn't look like much but it was one of my favourite prototype builds.

The device consists of a Motorola 6802 CPU, a 6820 PIO, an EPROM, and not a heck of a lot else.

The processor is clocked at 3.58 MHz with a surplus color TV crystal and has 128 bytes of RAM memory built-in. The 6820 provides two 8 bit bidirectional I/O ports there's 2 KB of program memory in EPROM. The only logic needed was a single inverter as a chip select for the EPROM. I used a transistor for this and avoided having another logic chip on the board.

The back side (below) shows a sort of hybrid wiring technique using 22 gauge wire-wrap wire soldered to the pins of standard PC board chip sockets.

It worked quite well but my capability for programming EPROMS with 6802 code was limited, to say the least. Eventually I got hold of a 6800-series cross-assembler that would generate code for the thing, but it ran on MS-DOS and my EPROM burning software ran on CP/M. Interesting times.

Something like this could control a simple security system, an automobile ignition system or, with the addition of some solenoids and transistors to drive them, a hot water solar heating system.

The 6800-series processor is an eight-bit computing relic whose descendants are still widely used in the 21st century.

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