8085 SBC

In which I attempt to use up my retro 8085 CPUs

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I found that I had a dozen 8085 CPUs in my parts box. I don't remember where I got them from, certainly not old PCs as those have 8088s. Perhaps I dismantled many embedded controllers. Anyway I've decided to put them to use.

A short history of the 8080 and Z80 8-bit CPUs

First came the 8080. This required 3 power supplies and had a cumbersome set of clock pins, so much so that there was a separate chip available, the 8224, to drive it. Intel realised that this was putting designers off so they produced the 8085 which only required a +5V power supply and simpler clock circuitry. However the Z80 with its far more capable instruction set eclipsed the 8080/8085 in personal computers. Although software like CP/M and apps mostly stuck to the 8080 subset of instructions for complete coverage. The 8085 was used in many embedded controllers. It did have the advantages of a serial interface and prioritised interrupts.

I have quite a few 8085 chips in my collection so I decided to put them to use. This time though I didn't design the circuit but adopted the Minimax 8085 design by Sergey Kiselev. (I have linked to the Retrobrew Computers wiki page, which has working download links, but Sergey's original page has code samples.)

Goals of this project

  1. Get acquainted with ordering PCB fabrication over the Internet
  2. Learn about using GALs, as this SBC uses one to implement the glue logic
  3. Attempt to port the SDCC to generate the 8080 instruction set

  • Programmng the GAL

    Ken Yap03/17/2019 at 10:56 0 comments

    No no, not your wife/partner/girlfriend, you can't program those, they have minds of their own. Actually this is not even my joke, it's in the GALasm documentation, see further down.

    The GAL used is a 16V8 which is way way obsolete but still readily obtainable for a dollar or two. Here's an old tutorial on how to use them. Really I don't know why people don't use them more to replace a whole bunch of TTL or CMOS logic. For one thing if you make a logic error you only need to fix the equations and reprogram, not rework a whole bunch of ICs and connects. For more complicated logic there are CPLDs and FPGAs which are used by many projects on Hackaday.

    One thing though, to program these GALs you need a hardware programmer. The popular TL866 programmer, besides doing (E)EPROMs, and many MCUs, also handles the 16V8.

    A compiler takes logic equations and generates JEDEC files which are written into the GAL. You don't need a compiler if the project supplies precompiled JEDEC files. But I wanted to learn to use such a tool for future reference. The compiler I used is the open source GALasm at Github. I found and fixed some issues with the code that provoked warnings from recent gcc versions and submitted a pull request, which the current maintainer gracefully accepted.

    I have successfully programmed and verified a GAL. So that's the second goal accomplished. But bear in mind, I have not soldered a single joint or powered up anything yet. The true test is yet to come.

  • Getting the PCB fabricated

    Ken Yap03/17/2019 at 10:35 0 comments

    This was quite easy because as I mentioned, I didn't design the circuit but took an existing design. But this step took about 3 weeks mainly because of the shipping time. You can see pictures of the product on this page; it's quite good. So that's the first goal accomplished.

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