How to access 27C322 with 2 74xx257 and only one 74LS139 ?

[Starfox]

Hi,

I am trying to build a SNES card based on 1 27C322, 2 74xx257 and only one 74LS139.

Here's a picture:
http://imageshack.us/photo/my-images/11/61748144.jpg/

I have an 1J3B-01 4MB Hirom board with an 74ls139 as Sram decoder and 64k Sram.
This is how its connected by Nintendo.

Code: Select all

A15	 1    16   VCC	   
A13	 2    15   Pin 7 of 74LS139	   
A14	 3    14   A21	   
NC	  4    13   A22	   
NC     5    12    NC	   
NC 	 6    11   SRAM Pin 20=/CE	   
Pin15  7    10    NC	   
GND 	8    09   GND  

How do I have to connect the 74LS139 when using 27C322 with 2 74xx257?
I really don't want to use an Mad-1 or a second 74ls139 as decoder.

Regards,
Michael

[magno]

I suppose you saw my HiROM and LoROM cards on the web; I designed the cartridges so you can use both MAD-1 and 74LS139/74LS138 as ROM and RAM decoders. These are the photos for those who haven't seen them yet

HiROM with MAD-1:


HiROM with 74LS139:


LoROM with MAD-1:


LoROM with 74LS139


I used 74LS139 for both ROM and RAM decoder, so maybe my solution is not what you need, but anyway I'll explain how I did:

For HiROM, you must map SRAM from $3X:6000 to $3X:7FFF for 64Kbit games (it also works for 16Kbit games). In this part of SNES' memory map, /ROMSEL (a.k.a. /CART) is always '1' (not active) and the condition to enable the SRAM is A23 = '0', A22 = '0', A14 = '1' and A13 = '1'. You don't need both A23 and A22 at the same time, so you may choose to use one of them.

For LoROM, you must map SRAM from $7X:0000 to $7X:1FFF for 64Kbit games (it also works for 16Kbit games). In this part of SNES' memory map, /ROMSEL is always '0' (active) and the condition to enable the SRAM is A21 = '1', A20 = '1', A15 = '0' and /ROMSEL = '0'. You must use all those signals to enable the SRAM.

Hope this helps you.