PCM

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Crt pcm.jpg
Package QFP80R
Manufacturer Logo fujitsu.jpg
First use 1992 ?
Used on Cartridges

PCM (not "NEO-PCM" probably because it was used in some SNK claw cranes) is a V ROM banking and demultiplexing chip, found in AES and MVS cartridges, and on ROM-only arcade boards.

Early cartridges had separate ADPCM-A and ADPCM-B sound ROMs. Since one was often emptier than the other, SNK devised a way to mix both types in the same ROM chip to save space and reduce costs.

NEO-PCM2 can also be found in late cartridges, and with NEO-YSA2.

On some Cartridge boards, VA20~VA22 are used to select which of the 4 possible V ROMs to use (decoded by a LS139).

Uses several latches for address and data, as well as a counter clocked by 68KCLKB for synchronisation.

Internal logic

The trick takes advantage of the asynchronous and predictable access to the ADPCM-A and APDCM-B data by the YM2610. See YM2610 bus timing.

[Verilog definition].

Pinout

PCM pinout.png
Edit this pinout

Has an inverter on pins 26 and 27.

  • 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