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RE: [Xen-devel] HPET/PIT timer accuracy

To: "Petersson, Mats" <mats.petersson@xxxxxxx>, "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx>
Subject: RE: [Xen-devel] HPET/PIT timer accuracy
From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Date: Mon, 1 Aug 2005 15:42:24 +0100
Cc: xen-devel List <xen-devel@xxxxxxxxxxxxxxxxxxx>, Michael Hohnbaum <hohnbaum@xxxxxxxxxx>
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Thread-topic: [Xen-devel] HPET/PIT timer accuracy
> You have to consider that the HPET and TSC are both driven by 
> a separate PLL that is based of some sort of 
> crystal/oscillator. These are often not as precise as we'd 
> want them to be. If you do a google with "crystal oscillator 
> ppm" (or some such), you will find that most of these are 
> within the 50-100 ppm ranges [these are the guaranteed 
> values, which probably means that the typical is around 25-50 
> ppm, much along the lines of your findings]. 

The figures you are quoting are for frequency *accuracy*, not
*stability*. Crystals typically have good stability, varying mostly as a
result of temperature changes.

The only time I've ever seen such high instability on a crystal was when
the load capacitors were a couple of orders of magnitude out and it was
a miracle the circuit was oscillating at all...


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