The CHEAPEST implementation would be to connect a 32.768 kHz oscillator
(w/a couple of caps) to the external clock pin of one of your timers.  I
am not sure which PIC you are using, but most do have at least one timer
that can have its own external oscillator.

The reason for using the 32 kHz oscillator is that its pulses can be
evenly divided by powers of two so as to arrive at 1us units. (2^15 =
32,768)  By doing a little bit of bit shifting, all you have to do is
count the number of microseconds from a base point in time, e.g., when
the clock is set.

A good example is available in a Microchip app note (AN582):

http://www.microchip.com/download/appnote/devspec/16cxx/00582b.pdf

Jen'



Would you then not have power issues (RTC memory retention) Would the
PIC have to go to sleep with a lithium?
Can you connect the square wave out from a cheap RTC to the timer input
so you can still use the timers with 1 second resolution?





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