| Re: [chrony-dev] temperature compensation |
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On Tue, Apr 13, 2010 at 12:31:40PM -0700, Bill Unruh wrote:
> On Tue, 13 Apr 2010, Miroslav Lichvar wrote:
> >I don't think they are in conflict. The temperature corrections are
> >not visible to upper layers as the local module adjust the frequency
> >in both directions. When sourcestats sets the frequency to f and local
> >module corrects it by c, it will get only f when reading frequency
> >back. Also, the temperature corrections are done independently from
> >source updates and they may (or should?) happen more frequently. One
> >of the main points is that reading temperature is cheaper than
> >querying NTP server.
>
> I am not sure how the temp corrections are actually done. Let us say you
> measure the temp at some time you find it is 26000 (26 degrees), so there is
> "no correction". Does this mean that you set the clock rate to 0
> (adjtimex setting tickvalue and newfreq to 0)? That would certainly fight
> chrony which has say set newfreq to 7000.
No, the freq will stay at 7000.
> Now the temp changes to 27000 (it is
> quantized in units of 1000). Your formula now gives 1.8PPM. Do you now change
> newfreq to this 1.8PPM value, or do you add that 1.8PPM to the current rate of
> the clock?
It will be added to the current rate. (it's only 0.183 ppm actually)
> Now at the next time, the temp is again 27000. Do you leave the
> rate alone, or do you adjust it by 1.8PPM again, or do you set the clock rate
> to 1.8PPM?
There will be no change in rate.
> If you use only the changes in temp to adjust the clock, then T0 (and k0) are
> irrelevant. If you use the absolute temp to adjust the clock rate then I
> cannot see how it cannot fight with chrony.
I believe T0 is needed for the quadratic coefficient. And k0 is there
only to get the result below 10 ppm if T0 is very high or low. If the
correction is outside +-10 ppm, it will be ignored. This is to avoid
accidents if the sensor fails, noise, etc.
--
Miroslav Lichvar
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