[ntp:questions] Windows time question.
unruh at wormhole.physics.ubc.ca
Wed May 4 14:49:46 UTC 2011
On 2011-05-04, David J Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
> "unruh" <unruh at wormhole.physics.ubc.ca> wrote in message
> news:slrnis0nra.4on.unruh at wormhole.physics.ubc.ca...
>> Unfortunately , AFAIK, usb is terrible for delivering a PPS-- ie no
>> interrupt lines-- the problem with having only two data lines (one beign
>> signal ground)
> USB is not as good as a direct connection, but it is not "terrible" - see
> my tests here:
> PPS delivered over a serial-USB convertor which carries the DCD line shows
> significantly better performance than syncing over a LAN connection to a
> stratum-1 server. Jitter was around 45 microseconds with the PPS over
> USB, and about 160 microseconds when LAN synced. It could be that the
> faster polling with the PPS source also helped reduce the jitter. PPS
> over USB is likely to be significantly better than a WAN sync as well.
Jitter and accuracy are not the same thing. It would be interesting to
attach the PPS to a real interrupt (ie one which can be serviced on the
1usec level) and see what the offset of the usb pps actually is.
>From what I read, the usb interrupts have a minimum latency of 125us but
more likely much larger ( but less than 100ms).
> Although I have a GPS on a USB stick, it's a logging device and I don't
> think that it supplies a NMEA feed. Perhaps I could test this later this
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