The Three Chip Retrocomputer

Where homebrew computers are usually complex bundles of wires and chips, [Mike]’s own single board computer is not. It’s a three-chip computer with only a CPU, RAM, and a microcontroller that is able to emulate the retrocomputers of yore.

Normally, a homebrew computer project requires some amount of ‘glue’ logic – a few NAND, OR, or inverters to combine signals and send them where they’re needed for address decoding. This tiny pocket computer doesn’t need any of that; all the address decoding is done on a 40-pin PIC microcontroller.

With 64kB on the PIC 18F46K22, there’s enough space for all the address decoding logic, space for a pseudo ACIA mapped onto the $DF page, and a ROM image that provides a monitor program and a copy of BASIC. Basically, with the addition of a USB to serial adapter, this is a three chip 6502 single board computer, and with the right ROM monitor can emulate an Apple I, Woz monitor included.

Yes, 6502 projects are a dime a dozen, but [Mike]’s work with the address decoding logic on the microcontroller is top-notch. There are a few remaining chip select lines in his schematic, and with another microcontroller it would be easy to add VGA out, a compact flash adapter, or some other really cool peripherals. Good thing there’s an expansion port on this thing.

17 thoughts on “The Three Chip Retrocomputer

  1. That is a slick piece of work. It’s only relatively recently that microcontrollers have become fast enough to be able to decode the control signals of a another process in real-time. I had casually looked at doing something similar with a Z80 and an ARM-7 core a few years back, but the LPC-2148 just isn’t fast enough to keep up with a 2MHz Z80.

    This is seriously cool. If has PCBs available, I may have to build one. I spent thousands of hours on 6502-based systems (KIM-1, Apples, and some other more obscure ones).

    Nice work, Mike!

    1. I have a several of the PDP-8 boards (they actually use the Intersil 6100 chip, which was used in the Decmate II). I’ve also got a couple of the IO boards another fellow did, and one of the front panels. It’s fun to pull them out once in a while and fire up OS/8.

      There are some FPGA implementations of the PDP-8. I haven’t looked at those recently, but a pocket PDP-8 isn’t out of the question.

  2. “Retrocomputers of yore” strikes me as either redundant or incorrect. The computers of yore weren’t, at the time, retrocomputers; they were just computers. A “retrocomputer of yore” would be, say, a 1975 ENIAC emulator.

        1. Noe step this up in speed and processing power, a lot. I mean *a lot*. Then write a custom OS just to mine scrypt based cryptocoins. A 3 chip Dogecoin miner… Take that, $1400 mining boxes.

  3. interesting choice. you might be able to make it a two chip SBC by using a flash based FPGA in the place of the RAM and the PIC. the FPGA would be a more optimal solution to implement the glue logic, and most have some SRAM blocks tied into the routing fabric. Would be a bit more complex, though.

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