> I tried a resistor of 15K and a Cap of 0.1 mfd.
> But it does not work.

100nF is waaaaaaaay too much

> Could someone advise me on how to get this working
> and how to calculate the frequency for a given value of
> R and C ?

http://home.clear.net.nz/pages/joecolquitt/0f84rc.html

I made this graph using 4 x 16F84A's from different batches
and taking the average

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