[ntp:questions] ESR looking for good GPS clocks

unruh unruh at invalid.ca
Mon Mar 5 20:18:31 UTC 2012

On 2012-03-05, Chris Albertson <albertson.chris at gmail.com> wrote:
> 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.

He is suggesting that the on board gps chip have ntp on board, so that
your computer timestamps the request, the gps chip then timestamps when
that trequest was received and returns it. No gps chip that I know of
has an onboard ntp server. So if you want to get a chip fab to make you
a special purpose chip which contains a gps receiver and ntp onboard,
you could probably do it-- but it might cost a bit more than $100 ieach
lots of 100.
> 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.

The problem is that parallel ports are far rarer than serial ports these
days, and even rarer than usb ports. And you would then have to get the
output of that counter into the computer. A bit more than $100 for the
whole thing I suspect. 

> Chris Albertson
> Redondo Beach, California

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