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

folkert folkert at vanheusden.com
Mon Mar 9 09:52:45 UTC 2015


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

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