[ntp:questions] xGPS/oPPS

unruh unruh at invalid.ca
Sat Mar 10 19:16:42 UTC 2012

On 2012-03-10, alex n <alexvm22 at gmail.com> wrote:
> 2012/3/10 Dave Hart <hart at ntp.org>
>> On Sat, Mar 10, 2012 at 13:08, alex n <alexvm22 at gmail.com> wrote:
>> > xGPS_NMEA(0)  .NV24.           0 l    9    16  377    0.000    0.021
>> 0.004
>> > oPPS(0)             .PPS.            0 l    7     16  377    0.000
>> 0.020   0.004
>> >
>> > GPS_NMEA is nmea driver with pps support.  Why GPS is falseticker
>> > though it has a same offset as PPS?
>> 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.

> Thanks,
> Alex.

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