Need a little help with the 139

[klonoa]

none of those correctly deal with the A15 low = backup SRAM, A15 high = cart ROM
this is especially important for that huge 4 MB flash size.

i'm sure the original wiring for the 74139 could have gotten lazy and omitted A15, because the 1 MB ROM could be made to not overlap into the SRAM space, and the SRAM in ROM space would not have mattered.

without accounting for A15, the backup SRAM and the uppermost quarter of ROM overlaps.

Ah i see.

so it should be something more like:

Code: Select all

                            __  __
                  SNES #49 |01\/16| +5V
                       A15 |02  15| NC
                      GND |03  14| NC
                   RAM /CE |04  13| NC
                   ROM /CE |05  12| NC
                        NC |06  11| NC
                        NC |07  10| NC
                       GND |08  09| NC
                            ------

Is there a page or document explaining the logic of various snes cartridges?

[whicker]

That mapping is more in line with how the memory map for mode 20 (LoRom) is laid out.
I believe it will work for your situation (other games might not like that SRAM located over so many banks)

Please remember to also have pin #23 /CPURD going to an /OE on your flash chip, in case there is an attempt to write to ROM.


As for documents, might as well use the official:

Search for "SNES Developer Manual" on Romhacking.net
Book 1, Page 2-21-2 shows the memory map. The shaded area is where /ROMSEL (/CART) is active.
Book 1, Page 2-21-3 shows the LoRom memory map (mode 20)
Book 1, Page 2-21-4 shows the HiRom memory map (mode 21)

It's better to look at what was the intended mapping, than get bogged down on how individual games laid out their memory.
Starting at Book 1, 1-2-24 shows memory maps of a lot of development or final PCB's.

It's lasted this far, so please don't post screenshots of copyrighted material or quote paragraphs verbatim from the manual, otherwise the black helicopters in whisper mode might arrive...