PCM
Jump to navigation
Jump to search
| |
| Package | QFP80R |
| Manufacturer | 
|
| First use | 1990 |
| Used on | Cartridges |
PCM (not "NEO-PCM" for some reason) is a V ROM address demultiplexing and data multiplexing chip, found in AES and MVS cartridges, and on SNK crane game boards.
Early games had separate ADPCM-A and ADPCM-B V ROMs. With the PCM chip, SNK devised a way to mix both types in the same ROM chip to save space and reduce costs.
The NEO-PCM2 evolution can be found in late cartridges and ROM-only arcade boards.
Internal logic
The data multiplexing trick takes advantage of the predictable access relation between the ADPCM-A and APDCM-B accesses by the YM2610 (see YM2610 ADPCM bus), and a an internal timing counter clocked by 68KCLKB.
Pinout
- INVIN, INVOUT: Simple inverter
- SDPOE, SDROE, SDPMPX, SDRMPX: from the YM2610
- SDRAD0~SDRAD7: ADPCM-A multiplexed bus part
- SDPAD0~SDPAD7: ADPCM-B multiplexed bus part
- SDRA8, SDRA9, SDRA20~SDRA23: ADPCM-A address bus
- SDPA8~SDPA11: ADPCM-B address bus
- V D0~V D7: V ROM(s) data bus
- V A-1~V A22: V ROM(s) address bus
- When the MODE pin is low, V A19~V A22 become 4 active-low decoded outputs based on value from the internal V A19 and V A20.