[ntp:questions] rasperry pi, pps, false ticker

dan at drown.org dan at drown.org
Mon Mar 9 15:05:49 UTC 2015


Quoting folkert <folkert at vanheusden.com>:
...
> Linux has this PPS interface for a while so now that it is easy
> configurable in config.txt on the rpi, I thought I give that a try. The
> strange thing now is that using that I get very bad jitter. So bad that
> ntp discards it as a false ticker!
>
>      remote           refid      st t when poll reach   delay   offset jitter
> ==============================================================================
> x127.127.22.1    .PPS.            0 l    2   16  377    0.000    0.161 0.001
>
> x127.127.22.1    .PPS.            0 l    7   16  377    0.000    0.148 0.006
>
> x127.127.22.1    .PPS.            0 l   13   16  377    0.000    0.147 0.007
>
> x127.127.22.1    .PPS.            0 l    2   16  377    0.000    0.146 0.006

Your kernel pps RPI has a ~147us offset compared to your other time  
sources. I assume you have the two configured as peers over a local  
lan.  I think ntpd is rejecting the local source because your other  
source is low enough latency (probably in the 100-200us range).

You could try adjusting the fixed offset of the userland pps to  
account for the latency between the pps happening and the user process  
being run.



More information about the questions mailing list