Bizarre question:

I'm driving my PIC16F877 with a 19.6608 MHz oscillator (Digikey didn't have
the 20.000 MHz ones in stock :-).  Everything worked happily, until I
started experimenting with the hardware USART, at 9600 baud.  I set the BRG
to high speed, with its divisor at 127:  19.6608 MHz / (16 * (127 + 1)) =
9600, exactly.  Fine.  But when I ran my program, I got a slew of framing
errors, in both directions.  After much experimentation and pulling out of
hair, I discovered that the setup worked perfectly if I set the divisor to
121.  But, if we reverse the equation:  9600 baud * 16 * (121 + 1) = 18.7
MHz.

So, the question:  Why on earth does my crystal seem to be oscillating at a
frequency 5% less than it should?  Now that I've got everything working the
question is solely academic, but nevertheless I'm quite curious what could
be going on.  It seems unlikely (albeit not impossible) that Digikey
shipped me the wrong oscillator.

Thanks and best regards,
Jake

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