Bryan,
        Crystals are usually modeled as LC Tank circuits with very stable
components.  But changing the external capacitance (or inductance) will
change their operating frequency.  I've seen some Ham Radio circuits
which are designed to change the crystal frequency a few kilohertz.
Makes a nice and stable variable oscillator.  If you are concerned about
crystal accuracy consider adding about 30pF variable capacitor in
parallel or series with the crystal (one slows it down one speeds it up),
and trim it to the needed frequency.

Fred Thompson (AA0CA)
fthompso@mail.win.org


On Sun, 12 Jan 1997, Bryan Mumford wrote:

> I have a circuit that runs a PIC16C57 with the XT oscillator. When I
> program an erasable part, the 4 MHz clock crystal runs at a certain rate.
> When I burn the same code into an RC OTP part, the crystal runs at a rate
> that's different by several parts per million.
>
> Does anyone know why that would be?
>
>
> Bryan Mumford
> Santa Barbara, California
>