[ntp:questions] ESR looking for good GPS clocks

Terje Mathisen "terje.mathisen at tmsw.no" at ntp.org
Sat Mar 3 18:11:53 UTC 2012

unruh wrote:
> On 2012-03-03, Terje Mathisen<"terje.mathisen at tmsw.no">  wrote:
>> 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.
> ?? How does anything know what the offset is between the time the GPS
> emits the PPS pulse and the time the computer receives it? The polling
> of the pseudo DCD signal on the usb device is going to take time. The
> encoding of the PPS pulse as a usb message is going to take time. USB
> has two lines-- send and receive, over which everything goes. Thatt
> pseudo DCD is simply going to be yet another usb message sent over the
> usb lines-- waiting its turn (you cannot interrrupt some other message
> already on the bus AFAIK) to be sent, then received, then decoded, and
> then the computer polls the usb and finally timestamps it.

The idea is of course to NOT use the USB-RS232 emulation driver, instead 
(or on top of) you want a private command to poll for the latest PPS 

The reply you get from this command can then contain the actual time at 
the point of polling, instead of just what it was at the latest PPS.



PS. Afair the Trimble Palisade (or some other old GPS) had a serial 
protocol which was very similar to this; the GPS and the computer would 
cooperate in figuring out the actual offset from the most recent PPS event.
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"

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