Cost-effectivenesss of highly-accurate clocks for NTP

Eric S. Raymond esr at
Sun May 15 10:25:15 UTC 2016

Bruce Simpson <bms at>:
> On 13/05/16 20:39, Eric S. Raymond wrote:
> >In 2012, nearly three years before being recruited for NTPsec, I
> >solved this problem as part of my work on GPSD.  The key to this
> >solution is an obscure feature of USB, and a one-wire
> >patch to the bog-standard design for generic USB that exploits
> >it.  Technical details on request, but what it comes down to is
> >that with this one weird trick(!) you can mass-produce primary time
> >sources with a jitter bounded by the USB polling interval for
> >about $20 a pop.
> >
> >The USB 1 polling interval is 1ms.
> What about USB 3.1 (assuming the device is not intended to be backwards
> compatible with the polling model) ? I should point out Intel intend to
> retire EHCI/UHCI and implement only xHCI.

Nobody makes GPSes with even USB 2 or 3 yet, and it is unlikely to happen
for a long time.  Cost reasons - USB GPSes are cheap consumer-grade hardware
and the manufacturers care about fractions of a cent on the BOM. 
		<a href="">Eric S. Raymond</a>

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