I don't remember if there were capacitors on the PWM lines for
the LED's, but if so, they would have a much lower impedance at the
higher clock frequency and might have been acting as enough of a load
to cause the read to always return the same state, messing up the
following write.  That is the only reason I can think of if the PIC is
running at a clock frequency it is supposed to run at, to behave
differently at 20 MHZ than it does at 4 MHZ.

	If there was asymmetry in the wave form, then that would tend
to always end up in one state or the other, either LED's full on or
full off.  The flash when disconnecting a LED and reconnecting it is
another indicator (puns, who me?), that the output probably tried to
function when the LED was disconnected, but then reverted to its
failure mode as soon as the load was re-applied.

	It might even have been the capacitance in the lines from the
PIC to the LED that became significant at the higher frequency and
loaded down the output enough to confuse the RMW.

	It's good to keep stories like this in the back of one's mind
when building a project.

Martin McCormick WB5AGZ  Stillwater, OK 
OSU Information Technology Division Network Operations Group
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