[ntp:questions] xGPS/oPPS

unruh unruh at invalid.ca
Sat Mar 10 22:38:25 UTC 2012


On 2012-03-10, alex n <alexvm22 at gmail.com> wrote:
> 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?

The two drivers appear to behave differently. The atom driver uses
/dev/pps0 while the nmea driver it appears does not-- not sure what it
does do to read the pps signal. 
But I have no idea what is wrong. Nor will most people care since using
the same source many times is not what most people want to do.

>
> Thanks,
> Alex.



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