Andrew,
        Been there did that....... The formula you are using is correct for a different application. Microchip has not spec'ed
a formula for this purpose. The RC formula does not apply here. The OSC is mostly frequency dependant on the resister.
The capacitor plays less of a role in deterring the frequency. It is used more for stability and the charge cycle.
However you must have it. To solve for frequency you must try it imperially. Others may be able to tell you what values
they used to get the frequency they wanted.

Gordon Varney
www.iamnee.com

>
> Which brings up another point:
>
> I've been using 1/(2*pi*R*C) to determine the PIC frequency using an RC
> oscillator circuit.  Not being an EE, I'm not sure if this is entirely
> correct, but it seems to be in agreement with what I see happening with the
> PIC when I use it in my application.
>
> Anything wrong with doing it this way?
>
> e.g., an 18pF cap with (2) 4.7Kohm resistors in series gives me
> approximately 4.25us per cycle, or 941 KHz input frequency, at least by my
> calculations.
>
> Please correct me if I'm mistaken.
>
> --Andrew
>
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