Breadboarding a 68000 Computer in Under a Week

We’ve been lurking over at Big Mess ‘o Wires as [Steve] geared up for his 68000 computer build. One of his previous posts mentioned a working breadboard version but we figured it would be a ways off. Surprise, he’s got it working and what you see above took just 6 days of “occasional work” to get running.

The chip in use is actually a 68008 but we remember reading that he does plan to migrate to a 68000 because this one lacks the memory pins to address more than 1 MB of RAM. The trick here was just to get the thing running and he made some common choices to get there. For instance, he grounded the /DTACK in much the same way [Brian Benchoff] explained in his own 68k build.

We’re not sure if his address decoding was a time saver or not. If you study [Steve’s] original planning post you’ll learn that he’s going to use programmable logic to handle the address decoding. But above he wired up 74-series logic chips to perform these functions. On the one hand you know your Hardware Description Language isn’t the problem, but did you terminate one of those wires where you ought not?

Additional tripping points include a bouncing reset pin. Looking at that we’d tell [Steve] there’s a problem with his chip, except that this was his first thought as well. He went the extra mile by building and testing a replica of the reset system. This makes our brain spin… shouldn’t the reset be among the most reliable parts of a processor?

At any rate, great work so far. We can’t wait to see where this goes and we hope that it unfolds in a way that is as exciting as watching [Quinn Dunki’s] Veronica project take shape.

16 thoughts on “Breadboarding a 68000 Computer in Under a Week

      1. Indeed this is a nice build, the 68K was very easy to design with you can do a 68K or 68020 design in under a week no problem. The point here is as you say it’s a very nice build and I for one am happy to see the venerable 68K back in service

  1. For the RESET, I suggest using a 555 plus an inverter. Take a look at the schematics of the Commodore C64 to see how it’s done. Some people might consider it overkill, but it will be reliable.

  2. Yes, I built one of these (wire wrap) in under an hour using only my toes (when i was 7)
    My actual hands where tied up building a fusion reactor at the time. /sarchasm.

  3. “This makes our brain spin… shouldn’t the reset be among the most reliable parts of a processor?”

    You would think so, but no. Assume reset is well-behaved at your peril – especially on older chips, but even on newer ones.

    I just spent two days troubleshooting my system after adding a W5200 Ethernet chip. Turns out it didn’t like the power-on reset signal I was giving it, even though it worked fine with its predecessor, the W5100. And so it was starting up in a state where it drove MISO active low continuously regardless of chip enable, preventing SPI communications not only with it, but every other device on the same bus too.

    That odd behavior had me very close to concluding I had a faulty chip and ordering another, when a bit of an accident gave me the clue I needed to figure it out.

    1. Indeed, reset systems cause all sorts of fun, intermittent problems. Especially with really old circuit that use a RC element directly on the reset pin. At least nowadays we have 3-pin TO-92/SOT-23 power-on-reset devices!

  4. Thanks for the link! Yes, I also built a similar machine on a protoboard at university, but it had a custom CPU made out of individual 74LS chips instead of using a commercial processor like the 68000. Now get off my lawn! :-)

    I’m now working on getting a very minimal Linux to run on this protoboard machine. But the protoboard is really just a testing ground for ideas, and I hope to eventually build a larger-scale system with a custom PCB, 68EC000, more RAM, ethernet, and other goodies. It should be fun!

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