> 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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