[ntp:questions] xGPS/oPPS

alex n alexvm22 at gmail.com
Sat Mar 10 21:45:24 UTC 2012


2012/3/11 unruh <unruh at invalid.ca>

> On 2012-03-10, alex n <alexvm22 at gmail.com> wrote:
> >>
> >> >> Are both drivers using the same PPS signal?
> >> >
> >> >
> >> > Yes, it is so.
> >> >
> >> >
> >> >> If so, what do you hope to gain?
> >> >
> >> >
> >> > Why the same source (PPS)  may not be shared?
> >>
> >> You show it does. But why? It is the same source.
> >> Note that if this is not the same source-- ie is attached to  different
> >> interrupts ( parallel and serial ports for example) you will have
> >> interrupt contention. One of the interrupts will be seen by the computer
> >> as occuring first, and will be serviced first. Interrupts are turned off
> >> while the interrupt is serviced. Thus there is a 10-20us delay before
> >> the other interrupt is serviced. Ie, the two interrupts will not be
> timed
> >>  to  occur at
> >> the same time. They will have timestamps that differ by 10s of us.
> >> Now if it is the same interrupt that you are sending to two separate
> >> handlers, the question really is "why". There is no extra information
> >> that the two can give you.
> >>
> >>
> > As I understand ntp uses /dev/gpspps0 as pps source so all interrupts
> > things are placed into lower levels. For example I can run ppstest
> > /dev/pps0 at the same time with ntp and it works fine.
>
> If you are using /dev/pps0 for both sources, then as stated this is
> pretty pointless. It is the same source. It cannot give different
> answers.
>
You get no more information by reading it twice than by reading
> it once.
>

But why we need different answers? nmea+pps (127.127.20.0 flag1 1 flag3 1)
and pps (127.127.22.0 flag3 1) drivers receive the same timestamp. ok. why
it is wrong?

Thanks,
Alex.


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