YM2610 bus timing

ADPCM-A bus

Access to the ADPCM-A ROMs for each channel is done sequentialy (in the channels order ?). A ~667kHz clock is derived from the YM2610's 8MHz clock by dividing it by 12.

• Period 0: The address LSBs are set
• Period 1: SDRMPX goes high
• Period 2: The address MSBs are set
• Period 3: SDRMPX goes low
• Period 4: SDROE goes low, RAD0~7 becomes hi-z
• Period 5: ROM outputs data (when is it latched ?)

Each period is 1.5us, a complete read cycle is 6 * 1.5 = 9us.

ADPCM-B bus

The access to the ADPCM-B ROMs is done synchronously with a 4MHz clock, derived from the 8MHz clock.

• Period 0: The address LSBs are set
• Period 1: SDPMPX goes high
• Period 2: The address MSBs are set
• Period 3: SDPMPX goes low
• Period 4: SDPOE goes low, PAD0~7 becomes hi-z
• Period 5: ROM outputs data (when is it latched ?)

Each period is 250ns, each complete read cycle is 6 * 250 = 1.5us. Reads can happen every 2 ADPCM-A read at most (gives a 55.5kHz samplerate).

Cycle relation

SDROE and SDPOE go low at the same time.