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SubjectRe: [PATCH] i386: Selectable Frequency of the Timer Interrupt
On Tue, Jul 12, 2005 at 10:39:00PM +1000, Con Kolivas wrote:
> On Tue, 12 Jul 2005 22:10, Vojtech Pavlik wrote:
> > The PIT crystal runs at 14.3181818 MHz (CGA dotclock, found on ISA, ...)
> > and is divided by 12 to get PIT tick rate
> >
> > 14.3181818 MHz / 12 = 1193182 Hz
> >
> > The reality is that the crystal is usually off by 50-100 ppm from the
> > standard value, depending on temperature.
> >
> > HZ ticks/jiffie 1 second error (ppm)
> > ---------------------------------------------------
> > 100 11932 1.000015238 15.2
> > 200 5966 1.000015238 15.2
> > 250 4773 1.000057143 57.1
> > 300 3977 0.999931429 -68.6
> > 333 3583 0.999964114 -35.9
> > 500 2386 0.999847619 -152.4
> > 1000 1193 0.999847619 -152.4
> >
> > Some HZ values indeed fit the tick frequency better than others, up to
> > 333 the error is lost in the physical error of the crystal, for 500 and
> > 1000, it definitely is larger, and thus noticeable.
> >
> > Some (less round and nice) values of HZ would fit even better:
> >
> > HZ ticks/jiffie 1 second error (ppm)
> > ---------------------------------------------------
> > 82 14551 1.000000152 0.2
>
>
> Most interesting... Would 838 Hz be a much better choice than 1000 then?
> (apart from the ugliness).

No, 838 isn't significantly better. 864 and 627 would be better
candidates:

HZ ticks/jiffie 1 second error (ppm)
---------------------------------------------------
627 1903 0.999999314 -0.7
838 1424 1.000109105 109.1
864 1381 1.000001829 1.8
A good HZ value would make ntpd significantly happier, if the crystal is
of reasonable quality.

152ppm (1000Hz) is 13 seconds a day,
0.7 ppm (627Hz) is 22 seconds a year.

--
Vojtech Pavlik
SuSE Labs, SuSE CR
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