Sean Breheny wrote:

> The value specified by the manufacturer is actually HALF of what the
> crystal should really see, but since you use two caps, it IS the
> actual value of the caps themselves.  For example, if you have a
> crystal that specifies a 20pF load cap,then you use TWO 20 pF caps,
> one on each lead,which actually gives a total of 40pF.

  Eh Sean, care to have another try at that?

  The manufacturer usually specifies the parallel loading capacitance
to the crystal.  Since this is applied in practice by a capacitance to
each end, being two capacitances in series, each of these capacitances
must be *twice* the nominal load capacitance.

  *Each* actual capacitance however is composed of your external
component in *parallel* with the circuitry (inside the chip), so that
component must add to the internal (chip) capacitance to make up the
twice-load figure.  If it *happens* that the internal chip capacitance
(each of two terminals) approximates the desired capacitance, then each
of the loading capacitors will *happen* to be the same value as the
crystal load spec.

  I'd draw this, but am in a rush ;-)
--
  Cheers,
        Paul B.