Re: [chrony-users] Automotive usage of chronyd

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On Sat, 4 Apr 2015, Olivier Delbeke wrote:


For an AGL (Automotive-Grade-Linux) project, I'm evaluating if we could use chronyd on IVI
(In-Vehicle-Infotainment) systems. At first sight, it looks much closer to our needs than

The usual automotive clock references are
* RTC chip (but maybe not on all systems)
* GPS sensor (not always ; can be with or without PPS when present (usually without))
* NTP over Wifi (on some systems ; even then, the connections are very occasional)
* RDS (on FM signal, with ~100ms precision)

The good news is that a precision of ~0.5s will be enough for most OEMs.

Two questions :
1. Can chronyd use non-precise references like a GPS sensor without PPS (+-200ms) or an RDS
FM timestamp (+-100ms)?

Sure. What you need is some driver which actually takes the signal from the
source and delivers it to the system. Then chrony can do its fitting.

2. Can chronyd set the system time when it has not been initialized yet (Jan 1, 1970) ?

It of course needs a source from which to get the time. But yes, you can tell
it to jump the clock if it is far off initially (or at any time) rather than

I made a first test in a near worst-case scenario : system without RTC and without network,
but with only a USB GPS sensor (without PPS). Time/date after boot is Jan 1, 1970, 00:00:00.
I send the timestamps of the GPS sensor to chronyd (over a socket), but chronyd rejects
them. Even if I use the first GPS timestamp to force the system time, the subsequent
timestamps are still rejected by chronyd ("refclock sample not valid age=-0.41" from
refclock.c - valid_sample_time())). What is the condition for a valid sample ? I can
understand that samples are rejected, but the "< 0.0" conditions looks like the drift is
only allowed in one direction. Is that right ?

Well, time does only flow in one direction. Why would the next sample be
earlier than the first from a GPS? GPS time is unidirectional.

Thank you very much,


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