On Sat, Mar 04, 2017 at 07:41:57PM -0700, Brian Gieryk wrote:
> Running latest chrony(d)(c) with a raspberry Pi 3(B) and a DS3231 (DS1307 
> with temp comp) and GPS with PPS on GPIO pin 18.
> PPS and GPS work fine (thanx to the list!).
> Now I added an RTC module.
> Still have all good PPS etc.
> After restarting chronyd first then gpsd everything starts up fine.  Good 
> data on gpsmon, ppstest.
> When doing a chronyc sources, starts out with both RTC and PPS having a ?.  
> After a short time, it will pick up the RTC (*) then the PPS (*) and RTC(-).  
> Then very shortly thereafter both (RTC and PPS) switch to falseticker (X) 
> status, never to recover.

If I understand it correctly, the RTC is not synchronized to anything
but is used as a time source (a PPS refclock?) mixed with the GPS PPS?
That won't work as the sources don't agree on what time it is. A
better way would be to use it as a tempcomp input to adjust the
frequency of the system clock.

Dan Drown has some very interesting posts about that:


Miroslav Lichvar

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