[time-nuts] raspberry pi, adafruit gps & ntp

[David J Taylor]

From: folkert
[]
I'm surprised that the jitter goes down to 0.005 as I'm now measuring
the PPS from userspace. My program runs with "real time" scheduling and
maximum priority but still the kernel needs to do a context switch etc.
when it receives the pps pulse.

Folkert van Heusden
================================================

That sounds good, Folkert - perhaps you might publish the details somewhere? 
I'd like to try it myself, but my Linux and C knowledge is limited.

For comparison, on three RPi cards here with modified kernels to get PPS 
from a GPIO pin, using ntpq -pn I see jitter values of 0.002, 0.002 and 
0.002/0.004.  The 0.002 seems to be near the sys_jitter limit as reported 
from ntpq -c rv.

Cards 1 and 2 are just doing NTP, the third card is running a data collector 
processing signals from a DVB receiver stick, and sending the derived data 
over a Wi-Fi link to a PC running Plane Plotter using the dump1090 program. 
This gives a CPU load around 35%.  It would be interesting to know how your 
version handles a busy RPi.

  http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html
  http://www.satsignal.eu/Radio/dump1090.html
  http://www.satsignal.eu/mrtg/performance_raspi-3.php

Cheers,
David
-- 
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Email: david-taylor at blueyonder.co.uk