I've send those commands to the chip to generate a square wave for channel A of period 256 (contant M2 clocking with 2 MHz is turned on)
Code: Select all
CPU W 00 C000
CPU W 00 E000
CPU W 01 C000
CPU W 01 E000
CPU W 08 C000
CPU W 0F E000
CPU W 09 C000
CPU W 00 E000
CPU W 0A C000
CPU W 00 E000
CPU W 07 C000
CPU W FE E000
When I tried to send next commands when this pin is low, they were completely ignored.
And when this pin is held low from the very beginning and then I ran the above code, I don't get square wave at all.
And when I run Gimmick game with this pin held low, there is a weird cracking noise instead of external audio.
So my guess is that this pin is somewhat related to the /SEL pin in YM2149.
Changing values of this pin does not alter the functionality of pins 1/2 - they're still behaveing like AMP-IN and AMP-OUT.