[ntp:questions] NTP with GPS and RTC

Biebaut Sven sven.biebaut at be.thalesgroup.com
Fri Apr 26 12:25:45 UTC 2013


thank you for these enlightening explanations. 

The idea behind using the RTC as reference clock was that it would drift less than the local clock for the periods without external synchronisation.

If I drop the idea of the RTC as a reference clock, am I correct in stating that, when there is no external synchronisation:
- my local clock and my RTC will drift away from each other, but at least my RTC will be closer to the mark (the DS3231 is chosen for its precision)
- the kernel stops updating the RTC
- there is no other existing mechanism that disciplines the local system clock

Thanks in advance,


Sven Biebaut
Senior Software Engineer

Rue Freres Taeymans 28
B-1480 Tubize

-----Original Message-----
From: questions-bounces+sven.biebaut=be.thalesgroup.com at lists.ntp.org [mailto:questions-bounces+sven.biebaut=be.thalesgroup.com at lists.ntp.org] On Behalf Of unruh
Sent: 26 April, 2013 03:22
To: questions at lists.ntp.org
Subject: Re: [ntp:questions] NTP with GPS and RTC

On 2013-04-25, Harlan Stenn <stenn at ntp.org> wrote:
> I gather it returns time to the nearest second - that will not be very
> useful for OS or NTP timestamps.  If you have to set the time to the
> nearest second you will also need to take special care to make sure that
> you set the clock at the "correct" moment of the second in order to get
> the "seconds" transition to happen ar the right time.

Well, actually, If you could set it so that it really kept the second
properly, and it had an interrupt when the second turned, that would be
just like a PPs clock with the seconds also delivered. Of course those
careates are pretty big ones. I know with the normal rtc, you have to
send it the set signal exactly .5 sec before the second transition
actually occurs.

> As I said above, I do not know if the OS is that careful about setting
> the time of the RTC every 11 minutes.

That of course makes is a very jumpy clock. every second its time
suddenly jumps. You would be far better off disabling the 11 min process
(chrony does that for you for example). 

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