Electronics > Beginners

Troubleshooting Microprocessors

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james_s:
Ok so it looks like you have a clock signal there? I don't have the schematic in front of me so I'm not entirely sure what each scope capture is showing.

What's the reset line on the CPU doing?

atmfjstc:
The clocks seem to check out. And the /RESET and RDY lines seem to be high, so there is nothing keeping the processor from working. I am seeing some activity on the A0 address line though. Previously you said that "Currently the address bus is not clocking and basically all the signals are stagnant - I assume this is a good starting point." - are you sure this is the case?

Maybe try sampling A0,A1,A2,A3 with a longer timebase. If there is continuous activity here, it could be the CPU is working after all, in which case you can focus on the VIC-II chip output and modulator.

You could also try (blindly) typing:

POKE54296,15

as soon as the computer boots up. This should cause a 'pop' in the audio.

bostonman:

--- Quote ---What's the reset line on the CPU doing?
--- End quote ---

Sitting at +5V DC (and appears a bit noisy). It's labeled 'U7 Pin 40' in my scope pictures.


--- Quote ---Maybe try sampling A0,A1,A2,A3 with a longer timebase. If there is continuous activity here, it could be the CPU is working after all, in which case you can focus on the VIC-II chip output and modulator.
--- End quote ---


I already tried a good VIC chip since that has a socket. As for the address lines, the waveform I included (Pin 7 - Address A0) should have more activity on it (when compared to a good one). Some address lines either sit at 5V DC while some look like the wave I included whereas on a good one, these signals have lots of activity on them. Also, I tried typing "load"*",8,1 blindly, three times, with a power reset after each time, and the disk drive didn't load. When turning on the C64, it does cause the drive to spin for the two or three seconds it normally does, but the C64 doesn't acknowledge what I typed.

james_s:
What happens if you momentarily force the reset line to ground? Do you get a spurt of activity on the address lines? It's possible the CPU is starting up then due to some fault the code ends up off in the weeds and halts.

bostonman:
Is touching a ground wire to the reset pin safe for the chip?

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