Hi Russell,

I probably should wait until I have read the links in more detail but I
can't resist engaging the mouth ;-)

I don't see why this should have to be "black magic". Isn't it (to good
approximation) a simple pi network with the xtal as the inductor? Yes, I
suppose you would have to include the resistive part of the xtal impedance,
too, but it shouldn't be that hard.

I also don't see why you agree with the "series" idea. It has nothing to do
with ground being independent, but rather that the xtal/caps combo is a
three terminal device (gnd, input, output) so it does not look symmetrical
if C1 is not equal to C2, so you can't just (AFAICT) say it's a two
terminal device with a single effective cap across the xtal.

Sean

At 05:22 PM 2/25/2003 +1300, you wrote:
> > Also, the first of the two links you sent, Russell, claims that the cap on
> > the output of the inverter, along with the drive limit resistor, form a
> > simple RC voltage divider, and you can compute the voltage level at this
> > point based on this. I don't think this is an accurate way of describing
>it
> > since it is also connected to the (inductive) xtal which may alter the
> > impedance at that junction considerably.
>
>Yep. It's all black magic. Alas, the spells vary depending on what book you
>look in. It is possible for the oscillator signal to get outside the rails
>slightly. Add a parallel inductor (as I have had to do on one design to
>compensate for a manufacturers inability to build microprocessors which meet
>specification)(not PIC)  and the output can get substantially outside rail.
>
>I agree with the "add the two capacitors in series" idea, with ground being
>independent for small signal models. But, as you note, in the real world
>other factors enter the model. If you have a near rail to rail swing and the
>capacitors are substantially unequal and you ground their midpoint that will
>skew the signal wrt the supply rails and carry one extreme outside the
>supply rail. Get a diode drop too high or too low and interesting results
>will occur.
>
>
>         Russell
>
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