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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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