> I get the impression that a 0.1uF capacitor from PIC to gnd
> can't hurt?  Is the capacitor ok on its own, or should it have a
> resistor in series too to make some kind of RC constant (wouldn't
> know what I should choose here)?
> 
> Regards,
> Mark

Are you referring to a series resistor like the 220R in the Mclr
circuit ? That has a different function to RC, one I've wondered
about and asked the list about before, with no answer. The d/s
says"R1 (the 220R) will limit any current flowing into Mclr from
external capacitor C in the event of Mclr/Vpp pin breakdown
due to Electrostatic Discharge or Electrical Overstress"

My question is - if Mclr has already broken down, what's the
point of protecting it ? However, a manufacturing friend has
told me of Mclr breaking down in several PICs that did not
have a series R

An RC on the input is a passive low-pass filter that passes the
lower frequencies (ie the 0V -> 5V DC of the switch transition)
and attenuates the higher frequencies that are contact noise

http://www.phys.unsw.edu.au/~jw/RCfilters.html

http://www.seas.upenn.edu/~ee206/labs/Filterlab.html

I arrived at my own values by observing noise reduction on the
'scope

If you're interested, Microchip have free filter design s/w at their
site. Search for "filter" to find the FilterLab download page. It's
about 10MB for the whole thing

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