I had a slightly frustrating weekend on the Z80 project. Since I switched to using VHDL instead of schematics, everything stopped working.

Of course, I had finished wiring up the RAM chip as well, so I wasn't sure if that was the problem, or if I had written bad VHDL code. But I switched back to the old schematic version of the CPLD code, and it still didn't work. Everything was very strange. And the power supply seemed to be running hotter than previously.

Eventually, while I was checking all the wires were plugged into the breadboard properly, I suddenly noticed that the RAM chip was plugged in in reverse. Which meant it was getting 5v on its ground connection and was grounded on it's Vcc input. And when I touched it, it was burning hot. Which probably also explained the hot electronics smell.

I'm assuming I've blown up the chip, but it shouldn't have affected anything else on the board (hopefully!), so I've replaced it with another one, and things have started looking better. I can send bytes down through the FTDI and the LEDs flash. Except that the LEDs aren't counting in binary correctly, and after a number of flashes (sometimes one or two, sometimes hundreds), the WAIT line goes high and it stops reading from the FTDI.

Then I realised that I was still powering the computer through the USB JTAG board, and now that the RAM is also connected the meagre amount of power from the USB connector might not be enough to make the system work. So I switched back to my 1A power supply and it all started working perfectly again.