> I was having a problem the other day where my 20MHz 16C63/JW seemed
> to be running at more like 6-7MHz.  I wrote up a description of
> the problem and almost submitted it to the list, but then I decided
> to spend $0.75 of my own money buying another 20MHz crystal first.
> Sure enough, the new crystal works fine.
>
> I suspect and hope that the first crystal was just mislabelled.
> Or is this something that could happen again?  Can damage to a
> crystal change its frequency, rather than just break it?

A crystal, like many other resonant systems, can either oscillate at
a "base" frequency, or at (approximate) multiples thereof.  For example,
an undamped "A" string on a violin may resonate at 440Hz, 880Hz, 1320Hz,
etc.  The third harmonic may be produced by playing the string while
lightly touching the string 1/3 of the way up the fingerboard (pushing
down firmly would generate 660Hz, not 1320Hz); even after releasing the
lightly-touching finger, the string will often continue to vibrate at
the higher pitch.

In the case of a 20Mhz crystal, most manufacturers design the crystal
so it's fundamental mode of operation is at 6.667MHz (20/3) but then
modify the shape so that it prefers to oscillate at the third harmonic
rather than the fundamental (analagous to the light finger touch on the
violin string).  Apparently, however, that particular crystal favors the
fundamental instead for some reason.  This may result from a manufacturing
defect, or from damage the crystal has received somehow (either mechanical
shock or excess drive level).

FYI, the only time I've had trouble with a crystal oscillating at the wrong
mode was when a 32Khz crystal wanted to run at about 160Khz.  Same phenom-
enon but the other direction.