Re: [gpsd-dev] Final revisions to the Time Service HOWTO

[Hal Murray]

address@hidden said:
> That's an interesting datum but I'm not sure enough about the context to be
> sure I can use it. The TSC is *not* a crystal source, correct?  What does
> its frequency variation look like?

It starts with a crystal.  There there is a PLL to get up to the CPU 
frequency.

Some CPUs broke TSC for timekeeping when they introduced changing the CPU 
frequency to save power.  I think that's been fixed on modern CPUs.

There is another worm in this can: spread spectrum clocking to keep the FCC 
happy.  The PLL is fuzzy.  The output varies from the nominal target to a 
1/2% or 1% slower.  It's FM modulated with ballpark of 30 KHz.


address@hidden said:
> > Note that a low drift contributes to stability, not necessarily accuracy.
> 
> I'm not sure I understand this part. When a clock has 100ppm drift, it
> doesn't mean it's not stable, or not?
> That sentence is Gary's and your response has confused me further.  Maybe
> he'll have something enlightening to say. 

The key idea is that there are two properties: stability and accuracy.  If 
the clock is off by 50 ppm, that's not a problem since we can correct that in 
software.  If the clock is not stable, it's hard to keep good time.


-- 
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