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- To: chrony-users@xxxxxxxxxxxxxxxxxxxx
- Subject: Re: [chrony-users] Multi-PPS+RTC Setup Questions
- From: Matt Corallo <ntp-lists@xxxxxxxxxxxxxxx>
- Date: Sun, 27 Sep 2020 12:48:02 -0400
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Ah! I'd hit send on this before reading the chrony 4.0 update list - initial testing shows latest git HEAD to not have
the issue of selecting the RTC. Warning on maxlockage may still be of use, but that's trivial.
Matt
On 9/27/20 11:34 AM, Matt Corallo wrote:
I have a setup with two GPS receivers with PPS inputs as well as a local RTC that is (roughly) disciplined to GPS time
as a sanity check.
From the docs, I'd expect to set this up as, roughly
refclock ...RTC precision 0.002 trust require
refclock PPS refid GPS1 ... prefer
refclock PPS refid GPS2 ... prefer
but on chrony 3.4 this seems to result in lots of reselection, with chrony often selecting the RTC as the current clock
source (with or without the prefer set on the PPS refclocks), even when the GPS PPS refclocks are well within the 2ms of
the RTC and the stddev of the GPS clocks is 1.5 orders of magnitude lower on the GPS refclocks.
If I drop "trust" on the RTC, it seems chrony 3.4 will happily decide NTP time is more than 2ms away from the RTC even
though it may not update the system clock for it (separately, I should probably figure out why my two independent,
different-model GPS receivers occasionally decide to jump 2-5ms for 10-20 seconds at a time).
If this is just an old-chrony-version issue I'm happy to try to build 4.0, but I figured I'd ask what the right settings
are first.
Finally, it would be nice to get a warning if "maxlockage" is not set when rate is >1 - if I'm understanding it
correctly it doesn't make sense to have a rate > 1 if the lock source doesn't have a negative dpoll (which should
roughly be the common setting for eg a GPS set to >1HZ PPS, as in my setup).
Thanks,
Matt
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