Re: Web page updates following recent discussions

[David Taylor]

On 20/01/2021 17:48, Gary E. Miller wrote:
> Yo David!
>
> On Wed, 20 Jan 2021 11:35:52 +0000
> David Taylor <gm8arv@yahoo.co.uk> wrote:
>
> > On 19/01/2021 19:31, Gary E. Miller wrote:
> > > There has been lots of earlier discussion on this with ntpviz
> > > providing read data.  I suggest you setup ntpviz and collect real
> > > data.
> > >
> > > RGDS
> > > GARY
> >
> > Thanks for that pointer, Gary.  I've been collecting NTP data for
> > some years now, and I'm basically happy with this.
>
> Care to sahre your data?
>
> > - to disable the OS from setting the RTC - I gather there's an
> > 11-minute time controlling this
>
> That is certainly not the way Uputronics says to do it.  That breaks
> the RTC disciplining.  The preferred method is to let it free run
> and store the drift.  See "man hwclock".
>
> > - to have the RTC cause an interrupt (say every minute) and record
> > the PPS-disciplined system time when that happens.
>
> I don't see how the Uputrnics can do that.
>
> > There's no suggestion of using the RTC to control system time other
> > than at initial power-up.  It's more an exercise to measure the RTC
> > itself.
>
> If you just want ntpd to measure, not use, the RTC, then set it "noselect".
> Otherewise ntpd will eventually use it4.
>
> RGDS
> GARY

My simple NTP data is on my Web site, and the MRTG/Perl scripts I use. 
For example:

  https://www.satsignal.eu/mrtg/performance_ntp.php
  https://www.satsignal.eu/ntp/NTPandMRTG.html

Yes, I am trying to measure the RTC without disciplining.  As interrupts 
are not possible I will try polling, recording the time when the second 
digit (or value) changes.  This with just the i2c connection to the RTC 
- no OS or server connection.  Reading the bytes directly.  Something to 
experiment with.

The first problem with NTP reading the RTC may be that once NTP knows 
about it it may try and discipline it, preventing a long-term 
measurement, and the second that the RTC can only be read in seconds, so 
even without the disciplining you might be waiting for many weeks for 
the RTC to be a second off!

On making the RPi alone as accurate as possible, some RPI 4B show a ppm 
offset of -9 -11, and -19, so far worse than the RTC.

Cheers,
David
--
SatSignal Software - Quality software for you
Web: https://www.satsignal.eu
Email: david-taylor@blueyonder.co.uk
Twitter: @gm8arv