> >(I was considering one
> > of those 2.5V super caps as a backup source 1F should last a while).
> > Current draw is 90ma max at the 1.8V core voltage.
> 
> Did you mean 90 mA ???
> That's only about 1000/90 x (Vmax-Vmin)
> If micro will run on 1.8v max and say 1.5v min that's about 3.3
> seconds at 90 mA.
> If that's 9mA then that's about 33 seconds.
> 
> Even ay 50 uA standby, 1F will give you 1E6/50 = about 20,000 seconds
> per volt of drop in the cap or under 2 hours with 0.3v of droop.
> 
> If using a supercap you would probably be better off using it with a
> low dropout, low quiescent current regulator to allow it to droop
> more.
> 
> Two AAA alkaline plus a regulator would allow 1000 mAH or 20,000 hours
> or about 2.5 years at 50 uA. Or half that if the regulator added 50 uA
> to the load.
> 
> Even the very smallest of 3v Lithium coin cells would far exceed the
> capacity of a supercap. (A 1F supercap with 1 volt drop gives about
> 0.3 mAH).

What Russell said, plus this:

I considered using a "supercap" for awhile, but in my case, when the
power loss occured (say, the battery was pulled) the desired behavior
was to save some data in flash, then switch off until further notice.  I
realized quite quickly that in order to meet the current/timing demands
of my flash chip, the size and cost of adding the capacitance to the
project would outweigh the benefit.  The result?  A standard "removing
the battery may cause data loss" warning, plus measures to ensure
that only a minimal amount of data could be lost.

I think supercaps are mainly intended to keep SRAM from losing its
mind during off cycles; for example, a sleeping nanowatt PIC draws
<1 uA.  As long as it doesn't wake up and start trying to do something,
a supercap will help it retain its SRAM contents for a long time.

YMMV, and your application may require different considerations.

Mike H.
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