48K ULA CPU clock signal on pin 32 - waveform incorrect?
Posted: Tue Jan 30, 2024 3:01 am
I've been having issues with my 16K (upgraded to 48K) Speccy.
I did an S-video mod on it and as my TV doesn't accept S-Video via SCART i bought a S-Video/Composite to HDMI convertor.
But i've had issues with it not syncing properly, it would sync for a second at most and then the picture would jump around and then sync again.
I thought it might have been the S-Video mod board, so i removed it and went back to the normal composite mod but i was still getting the same thing happening with the sync problems.
After much investigation i've found that the crystal oscillator input into the ULA pin 39 is fine (a nice 14MHz signal).
But the CPU clock signal coming out on pin 32 keeps dropping cycles, i've attached the waveform i'm getting.
Now i'm assuming that the CPU clock should be constant, or does the ULA stop and start the clock ..... i wouldn't have thought so but i'm not at a level where i fully understand the workings of these lovely little machines yet.
I did an S-video mod on it and as my TV doesn't accept S-Video via SCART i bought a S-Video/Composite to HDMI convertor.
But i've had issues with it not syncing properly, it would sync for a second at most and then the picture would jump around and then sync again.
I thought it might have been the S-Video mod board, so i removed it and went back to the normal composite mod but i was still getting the same thing happening with the sync problems.
After much investigation i've found that the crystal oscillator input into the ULA pin 39 is fine (a nice 14MHz signal).
But the CPU clock signal coming out on pin 32 keeps dropping cycles, i've attached the waveform i'm getting.
Now i'm assuming that the CPU clock should be constant, or does the ULA stop and start the clock ..... i wouldn't have thought so but i'm not at a level where i fully understand the workings of these lovely little machines yet.