Timekeeping oddities on MacMini G4s

Gary E. Miller gem at rellim.com
Sun Feb 5 01:01:24 UTC 2017

Yo Fred!

On Sat, 4 Feb 2017 16:19:00 -0800 (PST)
Fred Wright <fw at fwright.net> wrote:

> When you think about it, the manufacturer knows perfectly well the
> nominal frequency of the crystal being stuffed, and is also
> programming onboard nonvolatile memory (typically EEPROM) with
> various parameters,

I used to do ATE on motherboards.  Most manufacturers never tested their
crystals.  Sometimes they'd get a bad lot of crystals, thye usually did
not know, and if they did they figure'd the customer would never figure
it out.

Apple was better than most, but crystals also age.

> You are aware, aren't you, that frequency errors reported by NTPd
> have the wrong sign?  I.e., a negative value in the driftfile means
> that the frequency of your local clock oscillator is too high.  I
> imagine it's too late to fix that now, by decades.

Or thnk of it the other way.  ntpd is not reporting the frequency error,
it is reporting the correction to the frequency error.  So if the clock
is 100ppm fast the correction is -100ppm.

> > ntpq -c "rv 0 frequency" <host-name, defaults to localhost>
> > will get you the fudge-factor that ntpd passes to the kernel to get
> > the clock ticking accurately.  Units are parts-per-million.  
> And three decimal places is at least two too few if you're using a
> rubidium-based frequency reference. :-)

I just fixed that.  Got an Rb clock I can borrow for testing?

    # ntpq -c 'rv 0 frequency'

> > Over 100 is getting suspicious but could
> > easily be due to some round off someplace.  
> Generally, yes.  Tolerances on run-of-the-mill crystals are usually
> 100ppm or better, with 50 being quite common.  I imagine that the
> 500ppm limit is intended as a fairly loose sanity check, on the
> theory that if it's that far off, it's unclear whether it's due to
> frequency confusion or general brokenness.

Then you have to take aging into account.  A 10 year old Mac may have
difted a lot.  Or maybe the crystal got bumped.

Also a lot of crystals are now surface mount.  The soldering process
may have bend to crystal a lot.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588

	    Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin
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