Variations on XTAL clock frequency

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 (broeker@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

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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 wrote in message
news:4217077d$0$1023$afc38c87@news.optusnet.com.au...
Is there any published research/report about how much "uncertainity" and/or
variations must be expected on a PC clock frequency ?

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.

Peter

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