[ntp:questions] ESR looking for good GPS clocks

Terje Mathisen "terje.mathisen at tmsw.no" at ntp.org
Sat Mar 3 15:09:49 UTC 2012

Michael Deutschmann wrote:
> It's silly of him to ask only on his blog and not here, since many here
> would want the same thing.  The only difference between what he wants
> and the NTP hobbyist's dream is that he requires a USB interface.  An
> NTP hobbyist would prefer raw RS-232 to cut conversion overhead, but ESR
> wants to connect his reference clocks to router devices that only have
> USB ports.  He claims to know the USB conversion impact and that his
> error budget can take it.

USB is perfectly OK if the GPS device can use its internal 10 MHz clock 
to time the offset between the PPS position and the polling time.

I.e. even if regular USB is limited to 1 ms resolution, it should be 
possible to improve this to a sub-us timestamp with such a protocol.


- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"

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