[ntp:questions] ESR looking for good GPS clocks

Chris Albertson albertson.chris at gmail.com
Mon Mar 5 19:44:43 UTC 2012

On Sat, Mar 3, 2012 at 10:11 AM, Terje Mathisen <"terje.mathisen at
tmsw.no"@ntp.org> wrote:

> 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 timestamp.
> 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.
> OK?

No.  That can't work.  the whole point of the time stamp is to sample
the PC's clock so that it ca be corrected.   When the PPS happens it's
the computer's INTERNAL time that we need to snapshoot.

The problem with USB is the large uncertainty in the time data goes in
to when the interrupt happens.   The serial port works well because it
is very simple with no buffering.

How to measure?   You need to modify the interrupt handler to raise a
pin on a (say) parallel port. then measure the time from PPS to that
pin going high with a time interval counter.  Such counters are cheap
now on eBay.

Chris Albertson
Redondo Beach, California

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