[ntp:questions] Panic stop captured

Rob nomail at example.com
Sat Dec 15 10:43:08 UTC 2012


A C <agcarver+ntp at acarver.net> wrote:
> Yes, the SHM is configured currently as the preferred peer and PPS is 
> the system peer usually.  SHM is coming from gpsd and PPS is coming 
> directly from a serial port (separate from the one used by gpsd).

I take it that you then use the first SHM provided by gpsd, the one
that is calculated from the serial messages from the GPS receiver?
It is quite usual that it wobbles by several hundred milliseconds
on some receivers.

The second SHM provided by gpsd is the PPS one, but in reality it is
not independent of the first one, because of a limitation of SHM.
It is not possible to convey PPS information via SHM, only absolute
time information.  So gpsd works around this by adding the current
system time to the relative time provided by the PPS signal.  As
this is a circular definition, gpsd attempts to keep sanity by only
putting valid time into the second segment when the system time is
within some defined offset from the time received in the GPS serial
messages.



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