HOW TO Battery backup on-chip RAM of a microprocessor?

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

I have a circuit board with a NEC 78C10 single-chip microcomputer
(roughly 15 year old!). 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
JUST the on-chip RAM (not the whole board). Using a big battery pack
(9VDC) outside the current small case is not a feasible solution.

I found the pins' name but not the specification for this NEC 78C10. I
think this chip has no sleep mode.

Is there any way to mimic the sleep mode to keep only the RAM since
there is no need to run the board when there is no power at the 9VDC

Please help.

Re: HOW TO Battery backup on-chip RAM of a microprocessor? wrote:
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Go to the bottom of this page. Use the "site search" and enter 78C10.

The closest I could get to a datasheet, is for the 78C10A. Page 38 mentions
a "Stop mode" and there is an actual "STOP" pin on the chip (pin 63). It is
an active low signal, and apparently disables the oscillator. The datasheet
also mentions the data retention voltage minimum value is 2.5V.

The chip will still be drawing microamps of current. It is unclear whether
any pullups on the chip will still be working or not (input leakage may be
counted separately).

Also, when a voltage rail has an "operating" value and a "standby" value,
sometimes there are rules as to how fast the voltage can ramp up or down.
So you may still need some application information, as to how best to prepare
for the Stop mode.

Since this is a datasheet for the 78C10A and not the 78C10, the
rules for that chip might be different.

The first part of your project, would be testing the STOP pin. Does
it behave like an input or an output (requires testing with resistors
connected to the rail) ? Next, assert the STOP signal (it is active low).
Measure the current consumption of the system. Does the current draw
drop at all ? You may have to change settings on your ammeter, to get
a reading when the thing is stopped. Since the current will vary by orders
of magnitude, be careful not to damage your meter (if it is an analog

The second part of your project, might be investigating reducing the
holding voltage. If you use three 1.5V dry cells, and some diodes to
implement current steering, you might be able to have the battery
take over automatically, when the normal power source is removed.
If the voltage coming from the battery pack is reasonably close to
the normal 5V operating voltage, you might not even have to worry
about ramp effects.

Getting the proper datasheet for the project would really help, as
it would take the uncertainty out of your project. I cannot tell how
much of a difference there is between 78C10A and 78C10.


Re: HOW TO Battery backup on-chip RAM of a microprocessor?

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Data sheet is online at

Quoted text here. Click to load it

On the basis of a quick reading of the datasheet there does appear
to be one.  The /STOP pin will undoubtedly be connected to Vcc in
normal operation.  Driving it low will stop the processor.  That takes
the power down to 10uA typical.  You can get it down to 1uA with
a lower standby voltage but I'd be tempted not to mess around: 10uA
will take around 20 months to drain even a 150mA battery pack,
assuming you lose nothing in getting the supply voltage right.

Reading a little further, it appears that you can't simply re-raise
/STOP to awaken the processor: /RESET needs asserting too.  A little
more study is needed to see if that can provide a workable solution.
I'd recommend asking on comp.arch.embedded: you may find someone
with past experience of the procesors.

Andrew Smallshaw

Re: HOW TO Battery backup on-chip RAM of a microprocessor?

Many Many Thanks to both Paul and Andrew!
I will try to make a battery backup this week-end.
Happy Easter!

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