Ricoh RF5C68 (Mega-CD PCM chip) question

[jregel]

I’ve been looking at the data sheet for the RF5C68/RF5C164 PCM sound chip as used in the Mega-CD (as well as the FM Towns and Sega System 18 and 32). It states that “Waveform memory space” is “64 K-Bytes max.”, which I guess aligns with the 64K of sound RAM in the Mega-CD. But it also states that it can “Interface with waveform memories” and “Can be directly coupled with two 256K (32K x 8) pseudo SRAMs, mark ROMs or SRAMs. Presumably this would have been used in other uses such as the System 18 and System 32 boards.

Can anyone explain how these two statements relate to each other? If the chip could only address (?) 64K of memory, how does it interface with 2 x 256K PSRAM/ROM/SRAM? Would this extra memory be used for samples?

Thanks.

[axel]

Because the 64K limit is just for the waveforms, not the total address space of the chip. It can probably address at least a megabyte.

[roce]

Perhaps it has banking with a 64kb window?

[jregel]

Thanks for the replies.

I thought it was probably related to banking as well, but the documentation seems to suggest that it treats a 64K memory space as lower 32K and higher 32K external waveform memories, selected by registers RAMC1B (lower) and RAMC2B (higher). The interface from the RF5C68 to the waveform memories has a 15bit address bus, so only capable of addressing 32K (banked with the RAMC1B and RAMC2B registers to get 64K).

I can't see anything to suggest how banks of 256K would be addressed. If anyone has any other ideas, I'd be interested in hearing them as apart from the datasheet, I can't find much more at this low level.

Thanks

[roce]

It could be just terminology then, 256K chip meaning Kbits = 32kb. The 32Kx8 probably means stuff is read at 8 bits per read.

[jregel]

It could be just terminology then, 256K chip meaning Kbits = 32kb. The 32Kx8 probably means stuff is read at 8 bits per read.

I think you're probably right with this suggestion. It's the only thing that makes sense and re-reading the datasheet, it does explicitly state Waveform Memory is "65 K-bytes" whereas the other references are to "256K" and "32 x 8" (not mentioning that this is in bytes, so presumably only in bits!). Thanks!

[TmEE]

Yeah, that 256K is Kbits, which is 32KBytes. The chip can use pair of 8bit wide 256Kbit memories and get the 64KByte max memory space. None of the registers in the chip allow going beyond 64KBytes. If one wants more some external method becomes necessary.