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
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 (total256) 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.