[ntp:questions] SNTP with 1ms of precision?

unruh unruh at wormhole.physics.ubc.ca
Sun Jun 20 08:29:09 UTC 2010

On 2010-06-18, Richard B. Gilbert <rgilbert88 at comcast.net> wrote:
> David J Taylor wrote:
>> "Richard B. Gilbert" <rgilbert88 at comcast.net> wrote in message 
>> news:fcGdnXzbxfVY7IfRnZ2dnUVZ_sadnZ2d at giganews.com...
>> []
>>> If you have a GPS timing receiver (different from navigation 
>>> receiver), with a PPS output, one edge of that pulse should be 
>>> accurate to within 50 nanoseconds.  A serial output will tell you 
>>> which second is marked by the pulse.
>> .. although the very popular GPS 18x LVC specifies:
>>   "Accuracy: Measurement Pulse Output Time: +/- 1 microsecond at [the] 
>> rising edge of the pulse."
>> so not quite as good as 50 nanoseconds.
>> Cheers,
>> David
> The difference is not significant unless you can somehow take advantage 
> of the greater accuracy!  The difference in price between the Motorola 
> device whose specifications I was citing and the GPS 18x LVC is quite 
> significant; ISTR that I paid about $300 for mine.  If I had to replace 
> it, I'd go with the GPS 18X LVC.

Agreed. The interrupt serviced routines are simply not accurate enough
to make use of the accuracy better than about 1us. In tests I ran I got
roughly 1us random variations in the itnerrupt servicing. 


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