#vcfsw2024

I was looking for a simpler project. My recent 68030 work has been challenging and really pushing the limits of what I can do. I wanted something I could work on, but perhaps where someone else has already worked out the hardest parts.

I find laying out PCBs to be rather relaxing. It's one of those repetitive, almost meditative tasks, like needlepoint or whittling. The kind of hobby where I can turn on some music or a comfortable old TV show, zone out for a few hours, and wake up to this new thing that I created.

Debugging however is very mentally taxing, and the design work required to have a functional schematic to create a PCB for is an active whole-mind prices. So what I really needed was an existing project I could design a board for.

Enter [Grant Searle]. If you're not familiar with [Grant Searle], he has excellent designs for breadboard computers with a very minimal parts count. I studied his minimal Z80 design when I was first starting to build my own computers and learned a lot from it. I highly recommend his work for anyone who is interested in learning how to build their own computer but doesn't know where to start.

I was recently given a Rockwell 6502 CPU pulled from a dead LED marquee. I've never actually worked with 6502, so this seemed like a good time to try building Grant's 8-chip (or 7-chip) 6502 computer.

A few hours later, I had a PCB design completed, gerbers generated, and an order placed. Less than $5 for 5 boards, including shipping. A couple weeks later they arrived in the mail.

I did end up making a few modifications to [Grant]'s design. Instead of a clock circuit made from a discrete crystal and a couple inverter gates, I used a TTL oscillator because I've always found them to be more reliable. I also added support for an FTDI USB Serial adapter chip so that the board can be used with a modern computer as a terminal. And finally, since a PCB is much harder to add new components to relative to a solderless breadboard, I added an expansion header. All of it wrapped up in a compact PCB with lots of helpful silkscreen marking.

I realized after I had ordered the PCBs that the 16kB ROM chips [Grant] used are no longer manufactured or readily available. I have plenty of 8kB EEPROM chips on hand however. Thankfully the OSI BASIC interpreter [Grant] ported to this design fits within 8kB, so I was able to make a few adjustments and re-assemble it to work with the ROM chips I have on hand.

After a small glitch with my EEPROM programmer, it works!

It's quite a change going from my 33MHz+ 68030 to this tiny 6502 running at just under 2MHz. The BASIC text-based Mandelbrot renderer that completes in seconds on my 68030 takes four and a half minutes on the 6502. Not bad at all, considering my bus-impaired 68000 build takes 9 minutes to do the same.

This was a fun little project. It was a nice little break from some of the more difficult projects I've been working on. I have shared the project on GitHub for anyone who might want to take a look.