You could consider turning the power off completely and let the PIC restart
each time it was needed. Whether the restart time takes too much of your
processing power depends on duty cycle and application. A possible advantage
of this method is that when the PIC IS awake it can be run at full speed if
you so desire (or at least at >> 32 KHz). This of course takes extra
circuitry and would probably only be sensible in extreme applications. The
startup time also draws current unproductively and for higher duty cycles
this may push up the mean current unnaceptably. In your application you
would also need an external "waker upper" which may negate any gains.

I am making a training device for "special needs" children (the prototype is
under test by a user at present) which is apparently always on but actually
goes to sleep after a small period of inactivity. User input wakes it up by
turning on a pnp transistor in the positive rail of the power supply and
simultaneously supplying a logical high to the PIC's switch input. Off
current should theoretically be 0 - actually I see the barest trace on a
junko analog meter on the 250 uA range (<<1uA?). For my purposes this is
fine. A bit more care should allow (almost) 0 0 current draw when off.


-----Original Message-----
From: Mike Harrison <wwl@NETCOMUK.CO.UK>
Date: Tuesday, 16 December 1997 08:42
Subject: Re: Zero-power


>Here's my two cents worth:
>
>    I'm doing a 3V app with a 16C54A that sleeps 99.9% of the time.  WDT
off.
>The databook says average drain should be about 1 microamp.  Using 2
alkaline
>button cells @ 190 mah.
>    190 milliamp-hours is 190,000 microamp-hours, so at 1 microamp they
should
>give me 190,000 hours, or about 20 years, eh?

Remember shelf-life - you need lithium for this!
Battery mfr's often give curves for life at given loads. There are
some issues at very low powers - maximum battery efficiency can depend
on drain, and I'm told it can decrease at very low drains!.
Adding a capacitor to sink impulse loads when the thing wakes can also
help.
>    I'm in the process of measuring this, but don't fully trust my
>instrumentation.
A 1K or 10K resistor inline with the supply and the mV range on a DMM
helps here!
>How do I a test for ten-year life in less than ten years?
Measure the AVARAGE current, including wake-up current - a while ago I
designed an integrating microammeter for this purpose (basically a
current shunt, precision opamp, integrator, and sample/hold) - it's
otherwise hard to measure avarage power for devices which wake up
occasionally. I'd expect an asleep PIC to take well under 1 uA.

Looks like I'll be using a NEC 4-bit uC for my product!
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