I'm working on a battery operated system that I would like
to shut down gracefully when someone yanks its battery.

I'm using a TPS6021x charge pump regulator from TI, which
has a low battery indicator to notify the system if the power
ever falls below ~1.18V, which of course will be the case if
the battery is pulled.  What I'd like to do is hook up the LBI
output to the INTx pin, so upon power failure, the PIC (in
this case, an LF2320) will wake up and immediately start into
the needed preparations for power loss.

I'd really like it if the PIC were able to log the time the power
failed and set a flag in its internal EEPROM, and issue a write
buffer command to the external DataFlash where it stores the
data it collects.  My problem is simply one of providing power
to the system long enough for it to do these things.

The DataFlash requires 14 mS max and 35 mA max to write
from buffer to flash page.  If I throw in 1 mA to cover the
PIC, which seems reasonable per the datasheet, my
i=C dV/dT calculation is indicating over 1300 uF to keep the
3.3V system voltage above the 2.7V the DataFlash needs to
operate long enough for the write to proceed.  That's a
fairly unreasonably large capacitor for this circuit.

So, my question is this:  what other spiffy little tricks are there
to give just enough juice to shutdown gracefully?  I'm thinking
about using a 3V lithium button cell that I can switch into the
circuit using an NMOSFET between its negative pole and the
system ground.  Good idea, bad idea, indifferent?

Mike H.

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