PCM: Difference between revisions

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[[File:crt_pcm.jpg|right|thumb|NEO-PCM chip found in a MVS cartridge. Picture courtesy of [[http://www.mvs-scans.com MVS-Scans]].]]
[[File:crt_pcm.jpg|right|thumb|NEO-PCM chip found in a MVS cartridge. Picture courtesy of [[http://www.mvs-scans.com 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.
Early cartridges had separate [[YM2610|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 cost.


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


{{Chipname|NEO-PCM2}} can also be found in late cartridges and with {{Chipname|NEO-YSA2}}.
{{Chipname|NEO-PCM2}} can also be found in late cartridges and with {{Chipname|NEO-YSA2}}.
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On some [[:Category:cartridge boards|Cartridge boards]], VA20~VA22 can be used to select which of the 4 possible V ROMs to use (decoded by a LS139).
On some [[:Category:cartridge boards|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 {{Sig|68KCLKB|68KCLKB}} for synchronisation.
Uses several latches for address and data, as well as a 4-bit counter clocked by {{Sig|68KCLKB|68KCLKB}} for synchronisation.


=Internal logic=
=Internal logic=


Takes advantage of the asynchronous reading from the {{Chipname|YM2610}}. See [[YM2610 bus timing]].
The trick takes advantage of the asynchronous and predictable access to the ADPCM-A and APDCM-B ROMs by the {{Chipname|YM2610}}. See [[YM2610 bus timing]].


Verilog: to do.
Verilog: to do.

Revision as of 15:06, 15 August 2016

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 save space and 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.

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 clocked by 68KCLKB for synchronisation.

Internal logic

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

Verilog: to do.

Pinout

File:Pcm pinout.png

OpenOffice Draw file: File:Pcm.odg

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