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

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