Re: [chrony-users] Get time and pps status from Java

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Tue, Dec 14, 2021 at 11:13:34AM -0800, Thangalin wrote:
> Note the lack of *PPSSIGNAL*, which is present when the GPIO port is
> carrying a signal:
> 
> 
> ntp_adjtime() returns code 0 (OK)
>   maximum error 206000 us, *estimated error 1 us*,
> 
>   status 0x2107 (PLL,PPSFREQ,PPSTIME,*PPSSIGNAL*,NANO),

This part won't work with chrony. It doesn't use the kernel PPS
discipine. But you can monitor the PPS refclock as any other source
with chronyc commands.

> When the estimated error is between 5 and 10 us, we issue a warning. When
> greater than 10 us, we take other actions. Again, this information is
> parsed from by reading from standard output, which I'd like to avoid.
> 
> From the documentation/faq, the chrony daemon looks like it can provide all
> this information.
> 
> I'd like to confirm that:
> 
>    - we can replace ntpd (ntpq/ntptime) with chronyd;
>    - chronyd can return the requisite pieces of information;
>    - it is possible to use a Java-based NTP client to obtain the data we
>    seek; and

These should be ok.

>    - 1PPS signal loss can be detected immediately (e.g., upon querying its
>    status).

There would be a delay of one polling interval if you were monitoring
the reference time in the tracking command or reachability in the
sources command. You can set the polling interval to 1 second.

> Additional pointers to information on the NTP UDP messages with respect to
> chrony that documents the fields we'd need to extract would be most
> appreciated.

Look at the candm.h file in the chrony source code. It's not NTP. It's
chrony's own protocol. You would send chronyd a request with the
REQ_TRACKING command and process response containing RPY_Tracking. You
would check if the reference time is not too old (e.g. more than 5
seconds) and check if
  
  fabs(current_correction) + root_delay / 2 + root_dispersion

is not larger than your threshold of 5 or 10 microseconds.

Note that you need to pad the request to the length of the response.
If you capture few requests from chronyc, you will see what fields
need to be set.

-- 
Miroslav Lichvar


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