> Could anyone recommend a good capacitive value used with a 2.2K resister
> to filter out noise.

That of course depends completely on the frequency you want to start
filtering at.  The rolloff frequency for an RC filter is:

  F = 1 / (2 * Pi * R * C)

where F is the frequency in hertz, R the resistance in ohms, and C the
capacitance in farads.  I keep the constant 10**6 / (2 * Pi) = 159155
permanently in a variable in my calculator.  It can be divided by any two
of the resistance in ohms, the frequency in hertz, or the capacitance in
microfarads to yield the third.  Very handy.

> I have some limits switches which are NO and
> connected to a 2.2K pullup resistor.  However the moment the limit
> switches are closed to ground, they cause my PIC processor to reset.  I
> am thinking that the limit switches are somehow picking up some high
> frequency noice, which is not appearing on my 20MHz scope.  I know that
> the pins are set for input mode, so it is not shorting the system.

There is something else going on, like inductance in the line causing a
kickback.  If the switch is external to the board, then you should protect
the PIC inputs.  After the 2.2Kohm pullup, put a 1Kohm resistor in series
with diodes to ground and supply.  After that put another 10Kohm resistor
in series to the PIC pin.  That should protect from over and under voltage
spikes to a few tens of volts.


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