More Debugging

A project log for CPU4

This is my fourth home brew CPU

agp.cooperagp.cooper 09/18/2017 at 01:550 Comments

More Debugging

The first issue was that the high PROM has some other data on it. Argh!!

So a new PROM was burnt.

The next issue was that the PCB schematic was not identical to the design, so I cut a track and jumped some pins to fix. Later I realised I had made the modification on the fly and had not documented it. I may have to undo the fix (later). Argh!!

After two days of debugging I realised I had the PROMs swapped! Argh!!

It now seems to be stuck in the keyboard wait loop (address 0x0F to 0x19).

Checking the RD/WR sockets I can see signals but they are !WR for !RD, and !RD for !WR.

Checking the schematic I have miss-labelled sockets. Argh!!

Another bad board, but at least most of the board is working.


New board arrived Tuesday, Assembled it today (I was busy with my Midi project).

Powered up, partially working. Very erratic. Checked the clock, all over the place.

Removed the Register Board to get at the Clock chip and everything was good.

Yes, I know this problem a bad power supply. Swap out the power supply and everything works as it should.


Okay, I have a CPU that can display and modify memory. Next is to emulate a CPU such as TTA so it can execute programs. This is purely microcode that has to be written.

So I have a working CPU (until I find something that does not work!).