[ntp:questions] LOCL clock reachability not 377?

Martin Burnicki martin.burnicki at meinberg.de
Fri Aug 1 08:22:34 UTC 2014


Rob wrote:
> William Unruh <unruh at invalid.ca> wrote:
>> On 2014-07-31, Rob <nomail at example.com> wrote:
>>> William Unruh <unruh at invalid.ca> wrote:
>>>> On 2014-07-31, Martin Burnicki <martin.burnicki at meinberg.de> wrote:
>>>>>
>>>>> Unlike otherwise stated in this thread I don't think it's a good idea to
>>>>> leave the 1 PPS signal alone disciplining the time of the NTP server.
>>>>> This can easily yield unforeseen problems, similarly as if you use an
>>>>> IRIG time reference which only provides day-of-year and time-of-day, but
>>>>> no year number. If you don't take care then such signal can be accepted
>>>>> and provide a "valid" time which is off by an integral number of years.
>>>>
>>>> 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.
>>
>> A reboot is a restart and on a restart you need an external source for
>> the seconds.
>
> Why?  The time is copied to the CMOS clock regularly, and one could
> expect that during the short reboot the CMOS would not drift away so
> much that the time could not be synced to the PPS unambiguously.

This is only the case for Linux, and AFAIK only if the system has been 
configured to do these updates. I don't know if this is also true for 
*BSD and other Unix-like systems, but at least for Windows it is 
certainly not.

So I think the concept doesn't have to rely on this by default, but on 
the other hand there should be ways to configure this for particular 
cases where the user is sure the system time is set to +/- 0.5 s when 
ntpd starts.


Martin
-- 
Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont
Germany



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