Variations on XTAL clock frequency

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Is there any published research/report about how much "uncertainity" and/or
variations must be expected on a PC clock frequency ?

Re: Variations on XTAL clock frequency
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I doubt there'll be much about PC clocks in particular --- but I'm
sure there's plenty about crystal oscillator stability in general,
which will apply to PCs quite seamlessly.

Hans-Bernhard Broeker (
Even if all the snow were burnt, ashes would remain.

Re: Variations on XTAL clock frequency
What you are asking would be dependent on the grade crystal materials that
they select for manufacturing them. The crystals have a thermo, and internal
pressure coefficient factor to deal with. They are also a bit voltage
sensitive, as in part of the feedback circuit employed in the design of the
oscillator that they are part of.

For home computers, they do not need to use an expensive high stability type
crystal, as such that is used for precision instrumentation. I would not be
surprised if the clocking frequency in a home PC machine is drifting about
1% to 2%. As long as everything keeps properly synchronized there will be no
problem, no matter what the master clocking frequency is.

The time keeping crystal in a PC is different from the one used for the main
system. The actual real time clock is a separate operation. It is read by
the main system, only at the times where it needs to get the time of day,
and the date data. The time of day accuracy of most computers is about the
same as any low cost quartz watch. I found the time of day on most computers
to drift as much as several minutes a month, if not corrected. A typical
Timex or Casio watch can do better than 15 seconds per month.

There are softwares available to re-set the clock automatically from some of
the various time standard services around the world. I believe that XP comes
with such a software. This can be done over the internet.

If you have the budget you can install a GPS time standard system, and
install the hardware and software in your computer to work with it. The GPS
antenna would have to be installed at a location where it can clearly see
the sky to receive the GPS satellite data. This type of installation would
result in the most possible accurate time of day standard for a PC computer.


Jerry G.

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Re: Variations on XTAL clock AND time synchronization

<Sunwaesh> wrote in message
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You should have mentioned your project in your first post.

For the networked PCs you can use Network Time Protocol. Run an NTP server
on your time standard PC and run clients on all the others. This learns the
error rate of the PC clock and gently slews the time of the client to keep
it in sync. This ensures that the time is monotonic (always goes forwards)
and doesn't have discontinuities. It also has the great advantage that you
just have to install it as everything has been written for it already.

For the standalone PCs the simplest model that I can think of is that the PC
clock runs at fairly constant but slightly incorrect speed. When you have
measured the rate of time drift you can estimate what the offset you need
for the correct time. You will have to average it out over a long time to
get an accurate value. You should always be measuring the drift so that you
can adapt to change in the rate of drift. This should be stable enough for
anything that a PC needs to do.


Re: Variations on XTAL clock frequency
Hello Sunwaesh,

If it is a modern PC or laptop keep in mind that clock frequencies are
often purposely dithered. That way the spectral energy spreads and the
manufacturer often gets around an EMC nightmare.

Regards, Joerg

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