View Full Version : Force hard reset of z80 cpu on neogeo

04-19-2020, 11:19 PM
Assuming the z80 is defunct and not being responsive to REG_SOUND commands (0x01/0x03) is they any way to force a hard reset of it?

I would have thought the 68k's RESET instruction would do it, since it suppose to reset external devices, but testing it on hardware it resets the whole board.


04-20-2020, 07:13 PM
The Z80 reset line is connected to the common reset line used by the 68k and other devices. If you want to reset the Z80 only, you'll have to disconnect it from the main reset, build a dedicated reset circuit w/ button, and then logically AND them together.

04-20-2020, 07:23 PM
That sucks, thanks for the info.

04-21-2020, 12:22 AM
What are you trying to do exactly? If it's for a noble cause, I can put some thought into it when I have some spare time.

04-21-2020, 11:55 AM
I'm working on making the diag bios handle z80 issues better. In doing failure testing with the ram it usually results in the sm1 code having issues handling the 0x01/0x03 slot switch requests as would be expected. It seems pretty rare that the z80 actually crashes, but the PC could be anything when the cut over to the diag m1 happens.

Since the 68k reset instruction doesn't help, I've been looking at / experimenting with other ways to recover.

- Hope NMI's are still enabled from sm1, trigger one and have nmi code that recovers
- Hope IRQ's are still enabled from sm1 and the ym2610 triggers one and recover that way
- Have code very early on in the bios that the user can trigger that will hard cut over to the diag m1 and hopefully never give the sm1 a chance to run

04-21-2020, 01:14 PM
Would using the /reset line be suitable for your needs? The process I detailed above isn't super involved. You just need to remove the z80 and install a socket. I could design a daughterboard that sits between the two and provides the dual reset functionality.

04-21-2020, 03:01 PM
I appreciate the offer. Having to desolder the z80 and put it in a custom board just to run a test to tell you if the z80's ram is bad is a bit overkill :)

04-29-2020, 08:37 PM
I may combine this with a z80 keyboard reset for the msx so you don't have to feel bad about having a single purpose board designed to test one or two things.