Re: [PATCH] USER_HZ & NTP problems

[Vojtech Pavlik <vojtech@suse.cz>]

On Tue, Sep 10, 2002 at 08:37:17AM +0200, Rolf Fokkens wrote:
> On Tuesday 10 September 2002 08:09, Vojtech Pavlik wrote:
> > Actually, the clock true frequency is 1193181.8 Hz, although most
> > manuals say 1.19318 MHz, which, because truncating more digits, also
> > correct. But 1193180 Hz isn't. If you're trying to count the time
> > correctly, you should better use 1193182 Hz if staying in integers.
> 
> I copied the clock frequency from the kernel source, timex.h defines:
> 
> #define CLOCK_TICK_RATE 1193180
> 
> If what you're saying is correct, timex.h uses the wrong value as wel I guess.

Yes, it uses the wrong value.

It's not too big a difference to notice, the error in the frequency from
the ideal value in most cheap mainboard crystals is greater than 2 Hz.
So it's lost in the noise.

The clock is generated from a base crystal of 14.3181818 by division by 12.
The base crystal is the frequency found on ISA for the MDA/CGA adapters,
and is a NTSC dot-clock. All other frequencies on a motherboard (except
the RTC, which has a separate 32.000 kHz clock) are derived from it.

It's just if you try to get the computations correct, you may also want
to have this value correct. :)

-- 
Vojtech Pavlik
SuSE Labs