[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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