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Re: [gpsd-users] Garmin 18X-5Hz

From: Miroslav Lichvar
Subject: Re: [gpsd-users] Garmin 18X-5Hz
Date: Wed, 17 Aug 2016 08:36:06 +0200
User-agent: Mutt/1.6.2 (2016-07-01)

On Tue, Aug 16, 2016 at 11:39:30AM -0700, Gary E. Miller wrote:
> On Tue, 16 Aug 2016 09:32:37 +0200
> Miroslav Lichvar <address@hidden> wrote:
> > No, it's an Intel machine with active cooling. On the graph you can
> > see the frequency changes rapidly in both directions. On a Pi with no
> > cooler the temperature goes up quickly, but it cools down slowly, so
> > the offset swings only to one direction.
> My take is totally different, the ints on the Pi are slow, so when the LF
> goes up, the PPS pulses get delayed.  I can easily force this condition
> and see it.  The time spike is way to short to have any relation to
> cooling.

If the error was due to a delay in capturing of PPS timestamp, I think
the offset would change in steps in both direction. In your graphs the
offset is changing relatively slowly. To me it looks like a frequency
error, most likely due to the temperature change.

Also, in your graphs I can see the spikes are smaller when the
temperature is highest.

> > Well, yes, that's the point I was making. You need to keep the
> > temperature stable (e.g. not running any CPU-intensive tasks) if you
> > want ntpd to work well. 
> Once again, not related to temps.  The time constant of the temps is
> much longer than 10 seconds.  If you notice the thermometer at the bottom
> you can see that.  others have used the onboard Pi temp and got the same
> results.

It seems the temperature in your graph is collected by the cron job
itself (every 30 minutes), so it won't show the short-term variations
that ntpd is chasing. The temperature needs to be logged independently
from the cron job and at a higher rate. You know what Nyquist said...

Miroslav Lichvar

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