It’s 23:00, the 30th of May. For me, this marks the end of the Retro Challenge 2017/04. I wish I had something working by now, but I must admit defeat — for now.
I did manage to get the HD6309 computer working once when it produced a continuous string of the letter E via the UART. This convinces me that all the connections are correct but that there are timing or glitching issues. The CPU seems to be running some program, just not the one I wrote 🙂 .
To eliminate a partially dead CPU, I replaced it with another one. No change. The CPUs came from EBAY, all from the same seller. Maybe they’re all factory rejects, who knows?
The TL866A programmer has served me well, but I’ve done the “EEPROM dance” more times than I care to remember.
I went over the circuit several times using the oscilloscope. Apart from ringing on the READ, WRITE and E lines, nothing appeared to be overly wrong.
As a last-ditch effort, I added 100pF capacitors to ground on the READ and WRITE lines, which removed much of the ringing. That didn’t make it work any better however. The ringing is most likely a consequence of using “modern” parts, such as the two ATF16V8B GALs. They are pretty fast devices and it shows.
The 150ns access time of the ROM should short enough to supply a HD63C09 running at just over 1.8 MHz with data. The RAM is definitely fast enough, having an access time of 55ns. I might try reducing the clock speed in the future, just to make sure.
I’m glad I participated in the Retro Challenge and it was a challenge indeed! Part of the fun was helping Alan with his 68000 project and seeing others building and developing things: recreations of Andy Warhol classics, a COSMAC system, a TMS9900 CPU implemented in an FPGA and many others.
I’ll probably partake in the Retro Challenge again in the future, although I’m not sure I’d take on a completely self-designed computer with a PCB anytime soon. Speaking of which, I will upload all the design files and software to my GITHUB account sometime this week.
I hope you enjoyed this journey as much as I did. That’s it for now!