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

Richard B. Gilbert rgilbert88 at comcast.net
Wed Oct 22 20:32:40 UTC 2008


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??




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