> but I seem to remember a URL (no idea what it was) where
> someone used two pins on a pic to measure an rc time constant

I use that method occassionally to measure pots, but it may not
be suitable for your small capacitance, bearing in mind the top
speed of a PIC

http://home.clear.net.nz/pages/joecolquitt/0pots.html

Taking your example of a 35pF cap, to get around a full-byte
count at 10MHz

R = (150nF / 35pF ) x 10k

(150,000 / 35 ) x 10,000 = 42.86 megohm

At 20MHz this would come down to 21.43

Using the loop modification (25% faster), 16M

I think this is getting into a do-able region if the 35pF range can
be measured repeatably in practice, considering stray resistances,
capacitances and humidity

I don't have the reference to hand but I believe an op-amp can
be used as a capacitance multiplier - if so, that would help

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