How to keep CPU's on-chip RAM's content with a small battery

This question is relevant to most microprocessors with on-chip RAM.

I have a circuit board with a NEC 78C10 single-chip microcomputer (roughly 20 year old!).

Data sheet is online at

formatting link

It run with an 9VDC adapter (0.5A). The only RAM in this board the on-chip (CPU) 256-byte!

This RAM keeps the last board settings (enter manually via 10 buttons). There is no facility to input or output these data (total

256) by any other methods! There is no EPROM or static RAM on board!

I wish to retain this 256-byte RAM contents for weeks even when there is no power. Is there a way to use a 4.5 (to 5v) battery-pack (AA or 9v) to keep the on-chip RAM (not the whole board). Using a big battery pack (9VDC) outside the current small case is not a feasible solution.

There is a /STOP and a /RESET pin. I can understand electronic schematic a bit but not know how to "invent" on, in this case to activate and deactivate the addon circuit to keep the RAM when the CPU is in STOP mode.

Please help.

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To quote the Princess Bride, "This word... I do not think it means what you think it means".

Assuming the design is fully static, it should be possible to build a circuit that detects when main power has failed, and then pushes the CPU into reset and switches over to battery power.

But it will be fairly complicated - you'll have to isolate the CPU from the rest of the circuit. And how do you know that the code doesn't just initialize all those variables when it restarts?

If you stop the CPU without also freezing the state of the external hardware to which it is connected, how do you know the code will be able to recover from going into hibernation and waking up in a possibly different environment?

All in all, not a problem you're likely to solve. It is probably less effort, and definitely more certain, to disassemble the code and patch in a different set of defaults!

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