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Re: [gpsd-dev] Final revisions to the Time Service HOWTO
From: |
Eric S. Raymond |
Subject: |
Re: [gpsd-dev] Final revisions to the Time Service HOWTO |
Date: |
Fri, 22 Nov 2013 06:30:37 -0500 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Hal Murray <address@hidden>:
>
> 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.
If you think this merits a patch to the HOWTO, send me one; your writing
is pretty good.
--
<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>