[ntp:questions] Time server question

Terje Mathisen terje.mathisen at tmsw.no
Sun Jul 21 14:02:27 UTC 2019

William Unruh wrote:
> On 2019-07-19, Chris <xxx.syseng.yyy at gfsys.co.uk> wrote:
>> On 07/18/19 11:13, William Unruh wrote:
>>> Sure, but I do not have faith in the "averaging" If one is always 30us
>>> after the other, then the average will always be out by 15us.
>> One would expect a difference, but how can you tell which one is right
>> using just 2 pps ?. With three, you could choose the closest to average
>> and discard the outlier, or if it was outside a defined window. Ok,
>> it's a bit nitpicking, but would still be interesting to try it.
> No. The mechanism is clear. While one is answering its interrupt the
> other gets to wait. So, it is the earliest one that is closest to
> "right" Ie, do not try to use more than one interrupt on the same
> computer. It does not work

A good timing-optimized gps unit, like the original Oncore, have a sw 
mechanism to offset the PPS event away from the actual top of the 
second, as well as a way for the sw protocol that numbers the PPS 
signals to also specify how far away this particular pulse is from the 
actual event.

I.e. with an internal 10 MHz clock, PPS signals will be synced to one of 
those 100 ns-wide periods, so it can/will be at least up to +/-50 ns 
away from the proper moment, but when the driver knows about this, it 
can adjust perfectly for that effect.


- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"

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