[time-nuts] Re: PPS latency? User vs kernel mode

[Adam Space]

I am using a Raspberry Pi GPS/RTC Expansion board sold by Uptronics.

> What do you mean by "kernel mode"?

I am referring to this guide
<http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html> that another user here
recommended to me a bit ago. I am not too sure myself since I am relatively
new to this.

Thanks for clarifying the stuff about NTP. Regarding the program to figure
out the amount of time needed to access the time signal from GIPO, how
would I go about doing that? I am using pps-tools to get the PPS signal to
go to /dev/pps0 by the way.

On Sat, Dec 11, 2021 at 8:43 PM Hal Murray <halmurray at sonic.net> wrote:

>
> Adam Space said
> > I have a PPS with Raspberry Pi setup going. Of course, in terms of
> precision,
> > it's quite good (for me it's as good as I'd ever care for), and for
> accuracy
> > it's quite good too. Although I guess the problem is, I don't really
> know how
> > good the accuracy is, nor am I sure how I would go about finding out.
>
> You can write a hack program that does:
>   wait until X
>   grab time
>   turn on GPIO or modem control signal
>   grab time
>
> The 2 time stamps are your error bars.  With a scope, you can compare the
> output signal with a PPS signal.
>
> > I am not using kernel mode right now,
>
> What do you mean by "kernel mode"?
>
> > When I run ntp..
>
> Which OS/distro?  Which vesion/flavor of ntp?
>
> > and averaging over long periods of time
>
> NTP doesn't really work that way.  The crystal frequency is temperature
> dependent.  The box temperature follows daily cycles and
> air-conditioning/heater cycles and CPU load.  ntpd polling interval has to
> work fast enough to track those changes.  Slower can average some noise.
> It
> tries to adjust the polling interval to balance the gain from averaging
> against the environmental changes -- looking for the bottom of the ADEV
> curve.
>
> The CPU/header is close to the crystal so CPU work can result is sudden
> changes in crystal temperature.  A cron job that does significant CPU work
> is
> likely to show up on daily graphs.
>
> Longer polling intervals reduce the load on the network and servers.
>
>
> > With regards to kernel mode, I was looking to give it a try. However, the
> > guides I've found on this, some posted several years ago on this list,
> are
> > pretty outdated
>
> Do you have a GPS HAT?  Which one?
>
>
> --
> These are my opinions.  I hate spam.
>
>
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