On Fri, 16 Oct 1998, William Chops Westfield wrote:

> I once wrote:
>     Yes. Using that configuration without a series resistor is a design sin ;)
>     BTW, the books say clearly that there HAS to be a resistor there to match
>     the low Z of the amplifier to the pi input. Somewhere along the way, it
>     got rationalized away from 'standard' schematic.s
>
> Surely it is inside the chip?  Adding a resistor inside a chip is simple,
> compared to adding a cap...

  Adding a resistor at that point inside the chip adds stray capacitance
on the highest frequency clock line in the chip, extra power drain on the
gate that drives it, consumes die space, and can't be optimal as the R has
to match the actual pi. As far as I know, the only R in the clock output
is the RdsON of the respective MOS transistor, and you can't really use
that as you'd attenuate the on-chip signal. Besider, if the R would be
there, how could one destroy a crystal by over-driving it with a PIC ? ;)
Anyway bare CMOS oscillators are a hack. Fortunately, it mostly works ;)

  FYI most analog chips with crystal oscillators NEVER drive the crystal
rail-to-rail for several reasons. An example are color demodulators for TV
and VCR use where the signal on the crystal is often in the 300 mVpp range
at Vcc=9..12V. The stability and purity of these can be 3 orders of
magnitude better than a bare CMOS oscillator. F.ex. a PAL decoder's
crystal is locked to within +/- 1Hz or better to the transmitter's burst
and has a short-term drift (over 60 usec) about ten times lower (0.1Hz).
That's better than 0.03 ppm short term at 4.433619 MHz. Any drift beyond
that produces colored 'curtains' or shading in the image.

Peter