PCM

From NeoGeo Development Wiki
Revision as of 18:38, 15 January 2016 by Furrtek (talk | contribs)
Jump to navigation Jump to search
NEO-PCM chip found in a MVS cartridge. Picture courtesy of [MVS-Scans].

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 reduce cost.

PCM (not "NEO-PCM" for some reason) is a V ROM banking and demultiplexing chip, found in AES and MVS cartridges, and on ROM-only arcade boards. Used to demultiplex V ROM (ADPCM-A and ADPCM-B) data from mixed-content ROM chips.

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

On some Cartridge boards, VA20~VA22 can be 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 4-bit counter on 68KCLKB for synchronisation.

Internal logic

Takes advantage of the asynchronous reading from the YM2610. See YM2610 bus timing.

Verilog: todo.

Pinout

File:Pcm pinout.png

OpenOffice Draw file: File:Pcm.odg

Has an inverting gate 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