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PostPosted: Mon Jun 25, 2018 12:50 pm 
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Joined: Sun Jun 12, 2011 12:06 pm
Posts: 350
Location: Poland
I want to discuss best way for RAM battery backup in SNES cartridges (without using specialised chips or MAD1).

From those of SNES cartridges with battery that has I have examined, they:
* Use MAD chip to protect WRAM
or
* Use transistor circuit to control positive chip enable line
Image

I want to make programmable cartridge but I have only 62256 memories (with one, negative chip enable line).
I created circuit like shown below. I put R3=100k but then for example Final Fanasy VI hangs after title movie.

So I changed R3 to 1k and it seems to work, but is that proper way? Should also VCC be taked into account so when it drops below certain level, /CE goes high?
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PostPosted: Mon Jun 25, 2018 1:04 pm 
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Joined: Sun Apr 13, 2008 11:12 am
Posts: 7313
Location: Seattle
RacerMate Challenge uses the "PNP transistor as voltage threshold detector" with some random 74HC logic to combine +CE into /CE.

There's a circuit I've seen bouncing around the forum involving a common-base amplifier to combine the two signals, also.


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