Prototyping with pulled parts has pitfalls. I needed one more 555 to test a GAL16V8^[1] based counter. Why GAL based? My preferred sources didn't have any 74x469 or '491s in stock, and I've been meaning to fool around with some GALs. Considering the parts I tend to favor, it's odd that I don't have any sort of ROM burner laying around. No matter, that's what the Z8 dev board is for. Between GALasm and ATFBlast docs, I built a working programmer. I cheated a bit, though. First, I didn't bother with proper logic level interfacing, and I used the SPI port for the serial data transfers (reverse the bit order). After a bit of learning, problem solving, debugging, and DeMorgan-izing, I had a counter ready for testing. The output was gibberish. Back one step, more code, debugging, more de-coupling capacitors, WinCUPL, test cases, Re-Morgan-izing, and now hair-pulling. About one month of this passes, but a simplified counter has just one error in the count. It is jumping from 255 to 1, despite the LSB term being:

Q0.D=/Q0;

How can this be? I hooked up flip-flop, an XOR gate, and a few LEDs, just to watch this last clock cycle. The problem disappeared. The flip-flop output should match the output of the Q0 output of the GAL with the XOR lighting a LED when it doesn't. It never lit. Check again. Still working. The other LED? It is on the high-side of the 555's output. Take it out, and the bug returns! I've never seen a 555 with an open-collector output before. My parts shortage has an open. Just goes to show that one can never have enough 555s.

I'll have to keep this 555 handy, could be helpful should I ever need to reverse-engineer a locked GAL16.

Just a few changes to the down-counting logic, and now I have an 8 bit up/down counter that mostly fits in a single GAL16V8. All it needs is an external pair of Wired-OR and a pair of Wired-AND gates to fit in a breadboard friendly 12 pin wide footprint.