27C322 to SNES bus?

[Starfox]

Hi,

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

Here's a picture:
[/url]http://imageshack.us/photo/my-images/11 ... 44.jpg[url]

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

[nintendo2600]

OK so which one of these schematics is correct?
I see tons of different info in this thread and
alot of pictures. I just want to make sure I'm
gonna follow one that when I print it out it
will be correct. Thanks in advance.

[Markfrizb]

The ROM is 16-bit wide but file data is addressed sequentially in 8-bit bytes, so the "endianness" determines which byte is placed in the upper and lower 8-bits of the 16-bit word. The circuit is configured for big-endian data (even bytes are D15-8), but there's a good chance the Willem software chose little-endian. But if this is the case, you don't need to reburn the ROM (normally you would), you just need to exchange 139 pins 4 and 5 which in effect swaps bytes.

Digging up an older post here.....

Kyuusaku, is this the benefit of using the 245+245+139 over the 257+257+32 design?

Or is there a way to easily swap the endian-ness of the data lines that I'm not seeing?

Is it very common for programmers to swap endian's or is that a quirk and nothing to be alarmed about?

[Markfrizb]

OK so which one of these schematics is correct?
I see tons of different info in this thread and
alot of pictures. I just want to make sure I'm
gonna follow one that when I print it out it
will be correct. Thanks in advance.

This works for me.
THANK YOU to ALL who advised me on this. I learned a lot!

The jumper shown on the pic is for smaller than 32mbit roms. The jumper is necessary for the smaller roms (i.e. 27c160) to keep the rom in 16bit mode. IF you never plan to use a smaller than 32mbit rom, then you can hardwire that line to the rom or use the SL (solder lug) as shown.

[lunaris]