Puzzling clock offset spikes

Gary E. Miller gem at rellim.com
Sat Jun 30 06:19:47 UTC 2018

Yo Paul!

On Fri, 29 Jun 2018 18:27:30 -0700
Paul Theodoropoulos <paul at anastrophe.com> wrote:

> Raspberry Pi 3B+, Adafruit GPS hat., Raspbian Stretch, ntpsec 1.1.1, 
> gpsd 3.18~dev.

I have a very similar setup.  Only difference is Raspbian -> Gentoo64.

> Everything runs quite lovely, notwithstanding temperature variations, 
> which I'm working on smoothing out.

I dealt with that in two ways.  

First added a TEMPer thermometer to the RasPi.  Then had the RasPi turn
on and off a 12V light bulb in the box with the Raspi.

But I still had frequency variations on CPU load changes.

Second, I added a CPU job that would vary computations to keep the CPU
temerature stable.  I don't care so much about the CPU temp, but the
XTAL is on the backside of the PCB from the CPU.  Thos two flattend out
the frequency variations.

> *Except* that at regular intervals, there is a spike in clock offset, 
> followed a smaller interval by another spike in offset.

Interesting graphs.  You seem to have eliminated all my guesses already.

> I briefly latched onto this - 
> http://www.ntp.org/ntpfaq/NTP-s-trbl-spec.htm - based on my earlier 
> timings of eleven minutes, as the example within suggested
> an eleven minute interval for RTC updates - but these later timings
> dash that.

Do you have an RTC on your RasPi?  That is not standard.

Otherwise I go nothing...  Good luck with it.

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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