"You can find a low-frequency, stable clock source and use it to
calibrate a baud-rate generator or event timer. One source of a
low-frequency, stable clock is the line voltage. This voltage is
a good source 50- or 60-Hz frequency that you can easily interface
to the microcontroller's 16-bit timer. By counting CPU cycles for
a half cycle of the external clock, you can determine the frequency of
the microcontroller's internal RC oscillator and calibrate the baud rate."

Link to EDN story: http://www.devrs.com/pic/

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