> I'm not saying you can't implement an accurate clock with a crystal, I'm =
just saying that it frequently doesn't seem to happen.  I'm pretty sure I'v=
e seen a fair number of crystal-controller micros that have quite noticeabl=
e drift when used as clocks (which is, after all, a pretty common hobbyists=
 application=85)

If you "oven" a 32 kHz watch type crystal with an adjacent tightly
coupled 38.x degrees C wrist, and then trim it manually for notional
zero drift, you can get a good wristwatch accurate to about 1 second
per week*, in my experience and AFAIR. That's about 1.5 ppm over 1
week. ie long ago I monitored my wrist watch's drift and at a battery
change had the tech adjust the trimmer by the amount I suggested and
thereafter it was essentially 'spot on'. I wore it on the inside of my
wrist (note to self: ensure non radioactive dial) and never took it
off ("100 metre" waterproof essential). I think that was a metal cased
Pulsar - which is a Seiko made watch  in a case cheap enough that
Seiko don't want to let you know that they make it.

* I originally wrote "one second per month" but I'm not 100% sure that
was correct. (It may have been 30+ years ago). I know it was very
accurate indeed once trimmed. The tech was surprised when I told him
how much adjustment I wanted. He had a frequency meter calibrated in
seconds per xxx error and my claimed drift matched what he measured
almost exactly. As you'd expect - but he can't have met many engineers
:-).



                       Russell

--=20
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