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SubjectRe: linux-kernel-digest V1 #4223
> On Thu, 29 Jul 1999 23:44:33 +0200 (NST) <jelle@flying.demon.nl> wrote:
>
> A 14.318 crystal, which could be in the range 14.3175..14.3185Mhz would
> result in extremes of -2 minutes to +0.96 minutes difference per month. A
> 14.31818 crystal, would result in less than a second difference per month.
>
> So, the conclusion is that if you find the accuracy of the timekeeping
> important, then look for the oscillator on the motherboard (tiny metal
> box), and make sure it says "14.31818" on it...
>
> Jelle

I apologize for taking some of your text somewhat out of context but I
wanted to clear up a technical misunderstanding regarding crystals so that it
does not result in code incorrectly written based upon that understanding.

The number of digits to which the frequency is specified is NOT an
indication of a crystals tolerance, the tolerance might be .001% for a good
crystal and .1% for a really tacky one irregardless of how many digits are on
the case, and rarely on the case do they stamp the tolerances.

Further, external factors strongly affect the crystals tolerance,
primarily temperature and capacitance in the circuit they are used. If your
computer bakes, chances are the oscillator will drift significantly off
frequency, adequately cooled it will be less inclined to do so.

It's common practice for manufacturers not to stamp the full number of
digits on the case. A crystal that is 14.31818 Mhz might well be stamped as
14.31 mhz. If you look in color TV's, they'll usually have a crystal for the
chroma burst frequency, 3.579545 mhz. Usually the frequency stamped on the
crystal case is 3.58mhz. The crystal really is 3.579545, it's just easier to
abbreviate and vendors that buy the crystals understand that it is really
3.579545 or 14.31818.

Since there is some variation, it would be good for the software to
include a tweekable fudge-factor so that with experience of whether the clock
looses or gains a small amount each day, the fudge factor can be adjusted to
compensate, so that given a crystal that is off by a constant amount, once
adjusted the clock will keep time.

Heat affects crystals frequency so if the computer doesn't stay at a
constant temperature, the clock will wander. This is why they put crystals
that control broadcast transmitters in a temperature controlled crystal oven,
and the use of vacuum crystals (crystals mounted in a vacuum) are often use to
reduce the crystals sensitivity to short-term external temperature variations.



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