Re: [PATCH] i386: Selectable Frequency of the Timer Interrupt

Con Kolivas
2005-07-12T12:43:14Z

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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).

Cheers,
COn

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