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 8.3.1.1.2 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.
RGDS
GARY
---------------------------------------------------------------------------
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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