[ntp:questions] Slow convergence of NTP with GPS/PPS

Unruh unruh-spam at physics.ubc.ca
Wed Oct 22 23:19:30 UTC 2008

"Richard B. Gilbert" <rgilbert88 at comcast.net> writes:

>David Woolley wrote:
>> Richard B. Gilbert wrote:
>>> The clock in a PC is basically the guts of a cheap "Quartz" watch.  It 
>>> wouldn't surprise me if the manufacturers bought the crystals rejected 
>>> by the watch makers.  I suspect that the clock exists MOSTLY so the 
>>> machine will have the correct date for things like letters and checks.
>> That describes the RTC, and may not even be valid for HPET systems.  The 
>> clock that ntpd disciplines is not based on a 32kHz watch crystal, but 
>> on a much higher frequency crystal.  Historically, the primary purpose 
>> of the latter crystal is to provide a logic clock for the processor and 
>> memory, not for time keeping.

>And it probably varies in frequency with temperature and age.  And 
>probably no one cares if the frequency is off by a percent or two, 
>especially if it's off on the high side.  Who is going to complain if 
>his 2.4 GHz processor is actually operating at 2.45 GHZ??

And from experiment it is actually off by less than .01%.(100PPM) Most
commercial computers are that good. In fact if they are off by .05% ntp is
useless and will refuse to even try to discipline it. 

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