[ntp:questions] How do I validate my PPS clocks?

ramadog at gmail.com ramadog at gmail.com
Thu Feb 28 07:47:35 UTC 2013


On Thursday, February 28, 2013 3:53:22 PM UTC+10, unruh wrote:
> On 2013-02-28, ramadog wrote:
> > Up to that 8us point it is the same behaviour I have been seeing since 2008.
> > I did this graph April 11 2008 http://users.on.net/~boddingt/irq-pps.png
> > Blue line is a time stamp taken at the beginning of the irq code and the red
> > line taken in the serial code.
> 
> You have an offset from 0 for the irq.c timestamp. How do you know that
> it took 1/2 a us or so Until that point?

The graph represents the actual time stamps for the pps using getnstimeofday()
I was getting at the time. It is possible I had stopped ntp so the system was
free running while I collected that data. I look at it as the difference
between grab the event time as soon as possible and decide later if it can
be used vs wait until we are sure then grab the time. It was a 2.6.something
kernel and I still see similar with the 3.5.7 kernel I am currently using.

What got my interest was the code posted earlier in this thread to estimate the 
delay. 1 person had a delay similar to what I get and that code was close for
me and my p3-600 which is a little faster than a 8088.



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