Variations on XTAL clock frequency

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

Reply to
<Sunwaesh>
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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.

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Hans-Bernhard Broeker (broeker@physik.rwth-aachen.de)
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Reply to
Hans-Bernhard Broeker

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.

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Jerry G.
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Jerry G.

The specifications of the crystal will tell you.

That would be a gigantic drift for even a very cheap crystal. I think

1/10,000 is more typical of what would be a large drift from a low-quality crystal.
Reply to
mc

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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Reply to
Joerg

drifting about

will be

think

low-quality

The battery-powered RTC clock is frequently VERY inaccurate, though. For reasons I've never had explained to me, they almost always have a trimcap on that xtal. Their Q/C processes to set that trimcap appear to be FUBAR.

Reply to
larwe

What trimcap? I've never seen one in a PC RTC circuit and, golly, does it need it!

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Graham W   http://www.gcw.org.uk/ PGM-FI page updated, Graphics Tutorial
WIMBORNE   http://www.wessex-astro-society.freeserve.co.uk/ Wessex
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Reply to
Graham W

though.

a

appear

does

Maybe your motherboard collection differs from mine. But every board I've owned (except some of the laptops) has had a trimcap.

Mystery why they bother to put such fine-tuning on there, since it is very common to get as much as 10 seconds drift per day!

Reply to
larwe

I would not have expected an un-trimmed Crystal to drift more than 100 Parts per Million or 86.4 mS per day, the Laptop I am using at the moment is still within a Second of the Talking clock and I set it 3-4 Months ago. I am sure I would have noticed if Clocks on any PC's I used were losing or gaining 10 Seconds a day

Reply to
Richard Freeman

You're fortunate... I've only had one laptop out of 4 that didn't drift in time too much. My current one loses about 4 seconds per day., my previous one gained about 2 seconds per day. The PC battery-backed-up clock system was 'adequate' when it was created back in the mid 1980's, but unfortunately that part of the PC architecture hasn't changed much since then.

Reply to
MC

It's just there because the appnote says to put it there. And it allows a demanding customer to calibrate the clock himself if he wants to. ;-)

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Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)

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Reply to
Stef

There are clock correction programs that either apply a daily correction to the time or query the NIST clock in Colorado.

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Nicholas O. Lindan, Cleveland, Ohio
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Reply to
Nicholas O. Lindan

Do they use crystals that are much lower-grade than even the cheapest RF crystals?

Ceramic resonators, maybe?

Do they have firmware that is incorrect? Maybe the problem isn't just frequency.

Reply to
mc

Who knows ?. Every manufacturer will use what they want at the time.

It's not a firmware issue, more a situation that the hardware timekeeper is not intended to be super accurate and therefore providing things to make it more accurate (such as temperature control and accurate oscillators) can become expensive.

Reply to
MC

crystals?

frequency.

The reason that PC clocks drift is that the crystal is cut for a different mode of vibration than most higher frequency crystals. To save power, the crystal oscillator that runs off of battery in your PC operates at 32768 Hz (or something nearby). The crystal vibrates in a flexing mode rather than the more stable shear modes because that is the only way to get the frequency down without having to make a truly huge device that would require more power to keep going.

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--Larry Brasfield
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Larry Brasfield

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