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

[Vojtech Pavlik <vojtech@suse.cz>]

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