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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="";>Eric S. Raymond</a>

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