I've revised and updated GAL replacement logic and redid SMD layout from scratch. 😵
I've managed to remove a couple of ICs from the GAL replacement section and re-structure logic utilization between different parts of the board.
Also, at the moment I beleave that no software was using "4-bit volume control" feature, so I've removed diode-resistor DAC from AY's port A and corresponding LM13600 opamp.
I've left in place "mono switch" on output, but reconstructed analog output section a bit.
AY8930 (and YM2149) have internal frequency divider, that was used on the original board, giving 14.318M (from ISA) /4 (on D-triggers) = 3,58M /2 (on internal divider) = 1.78975M for AY.
But "the original" AY-3-8910 doesn't have that. So, I've decided to switch off internal divider and place an external one on another D-trigger.
Next, I've thought "what the heck, AY-chips of all kinds were used in many different machines, what frequency it was run in those?".
I've did some research and came up with the list:
- 2.000 MHz = Atari ST
- 1.7898 MHz = CSM
- 1.7734 MHz = ZX Spectrum
- 1.75 MHz = Pentagon 128k
- 1.000 MHz = Amstrad CPC
Feel free to correct me if I'm wrong.
So... Keeping in mind FMonster project and its clock generation block I've managed to implement all those frequencies in new board. 🤣
Also I'm thinking about removing joystick ports from SMD-variant of the board. But I'll keep those in "proper" TH replica. 😕
On the screenshot below there are no:
- PCM circuitry
- analog mixing/filtering stage
- clock generation unit
White thick line denotes fully completed "AY-only" block (no PCM, no DMA, no joysticks) ready to be put into FMonster.