I do have a logic probe. What do I probe on the 68k?
Great thread, but asayuki lost me lol, I need to learn this stuff!Assuming the worst possible case (data or address pin stuck or shorted), you might want to ensure:
- RESET pulses low at boot, then gets stuck high
- HALT is high
- BR is not stuck low
- Address, R/W and data bits are all changing state
- What is the state of the FCx bits
One important thing to check is to see if you get activity at boot just to stop moments later.
AFAIR, part of the addressing logic for the 68k is in the custom, so you can check that the BIOS actually does get read (data, address, CS, OE are moving at the BIOS).
By the way, as somebody already pointed out, you could check if you see the sync pin at the jamma changing state.
EDIT: Also, replacing the xtal / oscillator does not guarantee that all parts of the board get the required clocking signals, including the CPU.