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

ramadog at gmail.com ramadog at gmail.com
Tue Feb 26 00:38:07 UTC 2013

On Tuesday, February 26, 2013 6:32:53 AM UTC+10, unruh wrote:
> When I ran a test many years ago I used a program to put out a rising
> pulse onto one of the printer port output pins, and then connected that
> to the parallel port nack pin. I timestamped the time just before I put
> out the pulse, and the time at which the interrupt occured and got
> roughtly 1 to 2 microseconds delay. Ie, the interrupt service was of the
> order of 1us.  12us seems really long. Now this might be a difference
> between the serial and parallel interrupts or a difference in the
> interrupt sevice routine, but 12us still seems a long time. 

Awhile back when I did similar using a parallel port pin to trigger dcd with 
the p3-600 I am currently using for my ntp server there was an average of 2.6us 
difference between the 2 time stamps.

For pps I use a time stamp taken at the beginning of handle_irq() in 
arch/x86/kernel/irq_32.c. Using a 3.5.7 kernel there is a 11us difference 
between that time stamp vs the serial pps time stamp taken in the ldisc code. 
Add some cpu load and the difference climbs to 14us. The 12us delay mentioned 
above is close to what I am seeing.

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