[ntp:questions] ESR looking for good GPS clocks

unruh unruh at invalid.ca
Tue Mar 6 00:29:30 UTC 2012

On 2012-03-05, Terje Mathisen <"terje.mathisen at tmsw.no"> wrote:
> Chris Albertson wrote:
>> 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.
> What I'm asking for is GPS unit with such an interval timer embedded on 
> it, said counter to be restarted on every PPS signal.
> The query protocol then becomes, as seen from the host:
> while (1) {
>    before = gethirestime();
>    gps_time = poll_gps();
>    after = gethirestime();
>    if (after - before < MAX_LATENCY)
>      use_timestamp(after, gps_time);
>    sleep(1);
> }

That is called ntp. (except it does it better) Why in the world should
after be a better estimate than say (before+after)/2 Ie, what you want
is for your gps chip to be an ntp server. 
I could of course be done. But not for $100.
And this is NOT a gps receiver. 

> I.e. polling the GPS (at any time) returns the current time in NTP 32:32 
> format directly. There is no need to ever interrupt anything with a 
> clock source you can poll at any time.
> Terje

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