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

[Martin J. Bligh]

>> > 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.

Does positive vs negative error make a difference to the timer subsystem?
Nish was telling me they had to add 1 extra tick to timer inaccuracies
because of the errors ... does changing the polarity of the error 
affect that (seems like it would ... but I got lost by now ;-))?

M.

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