Well, for something like that, I wouldn't trust a seperate power supply.  I'd
attach a flash chip to the pic also, and if it did experience an outage, it
could easily replace the contents of the DP Ram.   If the ram changes from the
EFI side, write it to the flash.

-Adam

Eric Aos wrote:
>
> Does anyone have a circuit already, or any clues on where to find an
> answer? I'm really not a Hardware person (Just struggling to be a
> software person).
>
> I have a 16F84 managing a DP Ram chip, that replaces the EPROM in my
> car's EFI computer. I would like to have a backup battery supply, so
> when I start it, or if I should unhook the battery, I won't loose the
> contents in Ram, and so it keeps working (Still need to fire the plugs,
> and add fuel, when it's cranking). I've found a circuit for a battery
> backup (Using a couple of Transistors, to get the regular supply to drop
> out if it's too low, with a diode to isolate the battery). But I would
> like to recharge the battery as well. Did I mention that there is very
> little room left on the board, and in the box that this goes in?