Also, what's the 'best case' configuration when it does work, and the smallest change to make it "not work"? ie: what's installed on the board etc
(Topic ID: 239784)
Perth
tried with new and old cpu. not sure when it stopped working, it passes every test(with no driver board). However will not run game code, as when the driver board is hooked up, the data buss is pulled up to 5v then becomes corrupted. since i removed everything on the driver board connected to data buss. i figured it was the pullups(resistances were fine)(headers were redone), and manually pulled them up to 5v and watched them corrupt as they were pulled up.
the only things that have changed since it original working was. new sockets. cpu pia ram and roms. continuity between pads and pins checked and no neighboring shorts
So I did a little testing on my MPU and driver board. The only time I get all logic highs on the data bus is when the reset signal is low AND the driver board is connected. With the driver board disconnected and reset is low, the data bus is also low.
This would make sense since the pull-up resistors at the 40 pin connector are on the driver board side of the connector and doing their job correctly. I'm not sure why they're even required.
So it seems you have a problem with the MPU chip not having enough current to override the pull-up resistors and actually pull a line low. Since you have tried a different CPU chip and gotten the same results perhaps it comes down to a power supply issue. you may have the correct voltage, but I wonder about having enough current.
I'll try to find some time to see about measuring how much current my MPU is drawing from the power supply.
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