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[Mystic_Merlin]

Ok these are 2 different questions but rather than opening 2 threads...

1) I noticed some DSP clones had some of their pin cut or bent off the socket on some copiers.

[kyuusaku]

None of those pins cut makes any sense unless they have jumper wires to new signals. Maybe someone tried to disable the internal DSP in a very bad way.


The 24 MHz clock clock was divided by 3 to get 8 MHz for the floppy controller and also served as timing for the DRAM controller. 1/24000000 = 41ns DRAM cycles.

The new board must divide 16 Mhz by 2 for the floppy, and use 62ns DRAM cycles, which sounds worse but must be OK.

You can't add crystal frequencies like that, the 7.xx MHz oscillator could be for DSP if the system clock isn't internally divided by 3 on that chip. You can check by tracing which chip(s) it goes to, depending on the system it drives it might be OK to replace with 8 MHz.

[Mystic_Merlin]

Thanks kyuusaku!

[kyuusaku]

I just noticed you have data lines 0-8, that's 9 data lines! There's only an 8-bit bus so I looked up the uPD77C25 datasheet and pin 14 is GND and pin 28 is VCC.

It's slightly possible that the cloned DSP has a different pinout, but I think someone strangely disconnected the power rails, which of course need to be connected for proper operation. Have you tested a DSP game on it? All I can think of is that someone did that so that plugin DSP games wouldn't conflict (I thought Pro Fighters could detect that..), though it's not a real solution at all.

Also looking at the datasheet, DSP operates at ~8 MHz so that crystal most certainly is for DSP. My guess though is that if games didn't need it to run at ~7 MHz, they would have used the 8 MHz floppy clock. To me though it doesn't seem critical since the DSP isn't synchronized to the CPU.

[Mystic_Merlin]