[ntp:questions] timing issue with a HP 58534A

unruh unruh at invalid.ca
Fri Feb 3 21:38:21 UTC 2012


Just ran a test in which I turned off chrony and ntpd (not running) and
monitored the offset of a Sure GPS connected to the ACK pin of my
parallel port, and a parallel interrupt service module I wrote. 
I got brief blips in which the offset was clarly 10-40us late. Ie,
something is eating up interrupt time-- delaying the interrupt by
10-40us.  and these occurred at clustered times. 
The usual fuzz in the offset was in the few us range, and those
popcorn blips were pretty rare. (maybe 20 in half a day-- interrupts
were once per second. ) They were also isolated (ie, you would not have
two blips side by side. ) The median filters in at least some of the
refclock drivers would have removed those blips.  And with a poll of 4,
(ie, about 10 samples after median filtering), the level of the offset noise per 
poll would be at the 1us level. 




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