[ntp:questions] xGPS/oPPS

unruh unruh at invalid.ca
Sat Mar 10 21:17:28 UTC 2012

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. It is like two little children, each asking their mother, one
right after the other "Are we there yet?" hoping for a different answer
than their sibling, but never getting it. 

> Thanks,
> Alex.

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