[ntp:questions] 1s offset

unruh unruh at invalid.ca
Thu Mar 7 17:36:14 UTC 2013

On 2013-03-07, David Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
> On 06/03/2013 20:22, unruh wrote:
> []
>> That should all be in the same second. The nmea sentence cannot come
>> before the PPS, so the "one second" should all be between the PPS pulse
>> and the next one. The problem occurs if the end of the sentence comes
>> after the next pulse occurs.
>> The code should probably use the beginning of the sentence, not the end,
>> to mark the time. It can always throw away the timestamp if it is not
>> needed. Or at least once it knows that the appropriate sentence is
>> coming. (the first four or 5 characters received).
> []
> Yes, it /should/ be the same second, but it depends on how NTP may, or 
> may not, round the value 0.6, for example.  I haven't looked at the code 
> to check the actual behaviour.

It does not round the time value. It uses it all ( to themicrosecond).

> The problem of going into the "next" second, requiring a fudge of over 1 
> second, did occur with one lot of Garmin firmware, but was fixed.


> Yes, it was whether the beginning or end of the sentence was used that 
> made me wonder about the different behaviour of different NTP versions. 
> I recall that in the Windows version it is the end of the first sentence 
> received which has the PPS timestamp substituted as the time of 
> reception.  I am less familiar with the UNIX versions.

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