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

folkert folkert at vanheusden.com
Tue Mar 17 15:36:00 UTC 2015


Config is as follows:
	driftfile /var/lib/ntp/ntp.drift

	## kernel pps driver
	server 127.127.22.1 minpoll 4 maxpoll 4 prefer 
	fudge 127.127.22.1 refid PPS flag3 1

	# NMEA source
	server 127.127.28.0 minpoll 4
	fudge  127.127.28.0 refid GPPS

	# other time sources on LAN with GPS/GLONASS/DCF77/etc
	peer neo
	peer hetlicht
	peer clara
	peer nanosg20
	peer china

	server agps iburst minpoll 1 maxpoll 4
	server artc iburst minpoll 1 maxpoll 4

	# upstream servers
	server ntp.xs4all.nl iburst
	server ntp.nmi.nl iburst
	server chime1.surfnet.nl iburst
	server ntp0.nl.uu.net iburst

	restrict -4 default kod notrap nomodify
	restrict -6 default kod notrap nomodify

	restrict 127.0.0.1
	restrict ::1

	disable auth

	broadcast 172.29.0.255


> You don???t give your config, but as using the shared memory driver works, do you have a prefer peer active when you are using the atom driver? If there isn???t one, that will cause the ref clock to be falseticker???d?. Your atom jitter isn???t the issue.

So what you're saying is that the atom should not be the preferred one?
I'll give that a try.

Thanks

> "Ceux qui sont prêts à abandonner une liberté essentielle pour obtenir une petite et provisoire sécurité, ne méritent ni liberté ni sécurité."
> Benjimin Franklin
> 
> > Le 9 mars 2015 à 10:52, folkert <folkert at vanheusden.com> a écrit :
> > 
> > Hi,
> > 
> > I have a raspberry pi with a gps connected to it.
> > It runs linux kernel 3.18.5+ and ntp version 1:4.2.6.p5+dfsg-2.
> > When I use my own rpi_gpio_ntp which is a userspace program, I get
> > something like:
> > 
> >     remote           refid      st t when poll reach   delay   offset jitter
> > ==============================================================================
> > *127.127.28.1    .PPS.            0 l    8   16  377    0.000   -0.010 0.008
> > 
> > *127.127.28.1    .PPS.            0 l   11   16  377    0.000    0.005 0.012
> > 
> > *127.127.28.1    .PPS.            0 l   12   16  377    0.000    0.005 0.012
> > 
> > *127.127.28.1    .PPS.            0 l    1   16  377    0.000    0.001 0.011
> > 
> > (I omitted the peers)
> > So the jitter is around 8us. Not bad for userspace.
> > 
> > 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
> > 
> > This is odd: the pps interface should work way better due to lower
> > latency etc.
> > 
> > What could be going wrong here?
> > 
> > The one at the top (28.1) uses GPS and the one at the bottom uses a
> > combined Glonass/GPS receiver. Note that I also swapped tried using pps
> > with 28.1 and rpi_gpio_ntp on 22.1 and then 28.1 fails and 22.1 is fine.
> > 
> > 
> > Folkert van Heusden
> > 
> > -- 
> > Curious about the inner workings of your car? Then check O2OO: it'll
> > tell you all that there is to know about your car's engine!
> > http://www.vanheusden.com/O2OO/
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Folkert van Heusden

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