[ntp:questions] LOCL clock reachability not 377?

Rob nomail at example.com
Fri Aug 1 12:09:31 UTC 2014


Martin Burnicki <martin.burnicki at meinberg.de> wrote:
>>> My point is that most of the internal clocks on computers are well able
>>> to maintain the time to better than a second for a long time, even if
>>> they were freewheeling, and if disciplined by a PPS, they are able to
>>> maintain the time forever (well, until the next power failure anyway).
>>
>> There are complications.
>> While the clock would probably be capable of maintaining the time
>> within a second, it cannot be set to a reasonable accuracy.
>>
>> On the system-under-test, i.e. with the locked PPS source, the LOCL
>> clock, and the unreachable external references, I did a reboot.
>
> With my proposal you wouldn't need the local clock driver for this.

Of course :-)
But now we have a running system and encountered a problem, so I was
investigating a bit...

> In context of the PPS input this means that ntpd must be sure the 
> absolute time has been set to within some bounds from some absolute 
> source, like (ntpdate used to do before the -g flag was invented ;-)) 
> before the PPS signal is initially accepted. But if that time source 
> fails afterwards the PPS signal would be accepted for a particular trust 
> time which could be 0, some dedicated time interval, or unlimited, 
> depending on the configuration of the PPS source.

That is right!
At the moment, one of my problems is that the LOCL clock cannot bring
the time back to sufficient accuracy for the PPS signal to be accepted
as trusted in ntpd (after a reboot).  Of course it is no problem when
the system remains running or when there is an external reference.
I will probably work around it in the way I described elsewhere.
(combine the time from the GPSDO with the date from the system to form
a timestamp sent to ntpd via SHM)

That is, until a nice solution for all this is incorporated in ntpd.
I think your suggestions are very good.



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