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

unruh unruh at invalid.ca
Mon Feb 25 20:32:53 UTC 2013

On 2013-02-25, Miroslav Lichvar <mlichvar at redhat.com> wrote:
> On Mon, Feb 25, 2013 at 01:44:02PM +0100, Kasper Pedersen wrote:
>> From the PPS arrives, and to the kernel timestamps it, is a very long time.
>> I wrote this to measure it:
>>  http://n1.taur.dk/edgetest.c
>> (you will need a linux machine, gcc, and kernel-headers to compile)
> Very interesting, thanks! For my machine it shows that the interrupt
> latency is around 12 us.
> I'm wondering if the kernel module could have an option which would
> enable a polling method to time stamp the PPS events.

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. 


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