I've been wondering how I could add a backup battery to my Game Doctor, since its RAM is wiped every time you unplug the device. In this thread (
http://www.famicomworld.com/forum/index ... pic=7955.0) we've already established that the GD uses DRAM, which will need to be replaced with SRAM. I come to you guys for a little extra technical advice on how to do so. Thanks!
What??? You know that's what the AC adapter input is for?
Insert it and the DRAM & SRAM is retained... That's actually the GD's most famous selling point!
Or get a 9V battery pack if for some reason you require it to be battery backed. Get a big one because it must keep the oscillator, ASIC and DRAM powered, a rather large current draw.
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The idea to replace the DRAM with 32M of SRAM is ridiculous, the RAM will cost more than a GDSFIII is worth and honestly doesn't make any sense. Don't waste your money or capable parts building this.
If you are just curious about adapting DRAM to SRAM in your design I don't understand why you need two CR2032s. LP SRAM as typical in battery backup applications retains state at well under 3V since their point is to be powered by a 3V battery and holdout to the knee voltage. Also using a linear regulator in a battery circuit is a quick way to drain your battery. Diodes are only needed if you're going to do an OR-wired supply where current is diverted from the battery to the console's rail when active. In that case just put a 1K resistor in series with the battery, use fast switching diodes and decouple VDD well.
As for the logic, you don't need "demultiplexers" (usually referred to as decoders) to demultiplex the address bus*, you would need negative-edge-triggered registers to hold row and column address bits for the SRAM. Latches as mentioned will not work since in DRAM systems /RAS is asserted *through* the /CAS strobe and thus the row latch will be corrupted by the column address.
*you would need a '138 or similar because you must decode the most significant address lines to select your 4M or 16M chips