[ntp:questions] is USB inherently evil, or only if something else is on the bus?

Dave Hart davehart at gmail.com
Thu Aug 9 22:16:25 UTC 2012


On Thu, Aug 9, 2012 at 20:29 UTC, Charles Elliott wrote:
> The process shown in the graph repeats indefinitely using my BU-353W.  In
> other words, the offsets always begin at about 300 ms, rise slowly to about
> 360 ms, vary erratically between 260 and 360 ms for about two hours, settle
> at 260 ms, and then rise slowly to 360 ms to repeat forever.
>
> It is not clear to me if the problem is the USB interface or the GPS device.

Drift of that magnitude must be due to the GPS not USB-triggered
latency.  Moreover, the real problem is you're assuming the GPS is
designed to provide the NMEA sentence(s) at a consistent delay
relative to the top of the UTC second.  Very often, they are not --
the timing of the NMEA sentences wanders by 100 msec or even more.  To
get better-than-WAN-NTP performance out of such a GPS, you need to be
using its PPS signal.  Essentially all USB-interfaced GPSes do not
wire the PPS signal through to DCD or another suitable input
handshaking line on their serial-to-USB chip.  Thanks to Eric
Raymond's bufferbloat-related efforts, there is hope we will see PPS
exposed on more USB GPSes in the future, however.

Cheers,
Dave Hart


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