[ntp:questions] Time server question

William Unruh unruh at invalid.ca
Wed Jul 24 06:07:15 UTC 2019

On 2019-07-24, Jakob Bohm <jb-usenet at wisemo.com.invalid> wrote:
> On 21/07/2019 16:02, Terje Mathisen wrote:
>> 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
> I happen to have a GPS unit (not yet connected) that is documented to do
> this too: The PPS pulse occurs at an edge of the internal crystal clock,
> but a special NMEA statement states (based on the 4D GPS solution) how
> many ns it is off for each pulse.  I have yet to find the point to pass
> this offset to ntpd after capturing the PPS arrival time.

The problem is this is largely irrelevant. The time it takes the
computer to respond to an interrupt id far far larger (and variable)
than that offset of the pulse which is on the at most 10s of nsec scale.
The computer responds on the usec scale (que the interrupt, the comp
responds to the que and loads or branches to the interrupt service
routine. The routine reads the system clock. All that takes time and a
variable amount of time. Ie, you need specialised hardware to make use
of that information, and, I thought, usually that infomation was
delivered by the gps unit a lond time after the pulse itself. Ie, it is
useful for rewriting history, not for the immediate time.

> Enjoy
> Jakob

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