[time-nuts] Time stamping with a PICPET

Hal Murray hmurray at megapathdsl.net
Sat Oct 26 17:47:01 EDT 2013


tvb at LeapSecond.com said:
> As far as NTP is concerned, I wonder if you've ever considered turning the
> tables on how it works. AFAIK, NTP gets an external time reference (e.g.,
> 1PPS) as an interrupt. With modern computers, this is horrible. There are
> far too may layers of h/w jitter (pipelines, multiple levels of cache, TLB
> misses, etc.) and s/w jitter (interrupt handling, spinlocks, interrupt
> masking, priority levels, etc.).

You have to have your time-nut hat on before that makes a difference.

Has anybody measured it?  It should be easy to hack the kernel PPS interrupt 
routine to flap a printer port signal and measure the delay between the PPS 
signal and printer port.


> For best timing, perhaps a better solution is not to have NTP receive a 1PPS
> as *input* and try to pretend to measure it internally in s/w but for NTP to
> *output* a 1PPS and measure that externally with sub-microsecond h/w (like a
> picPET). 

The Trimble Palisade and Acutime work that way.



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