[ntp:questions] pps coming in, not received by ntpd

folkert folkert at vanheusden.com
Tue Jul 2 19:58:39 UTC 2013


> >> []
> >>> To timestamp the pps, you have to have some module (ie kernel level)
> >>> timestamp process. shmpps, gpsd, the kernel pps, write your own,....
> >>
> >> Thanks for the information, Bill.  My only additional comment is that 
> >> the code to timestamp the PPS /can/ be in user-mode, although 
> >> kernel-mode is preferable.  Additional Linux modules are /not/ required.
> >
> > ?? To timestamp the interrupt, you need kernel land. Ie, you need a
> > module. Now you can use one of the modules others have written or you
> > can write your own. But userland cannot get any sort of accuracy trying
> > to figure out when and if a hardware line has been pulled up or not.
> 
> You may think that, but gpsd does it that way and it works surprisingly
> well.

I'm not entirely sure of that.
On Linux it uses the kernel pps-api. So the kernel makes a note when an
interrupt came in and gpsd requests these values from the kernel. That's
why it opens /dev/ppsX


Folkert van Heusden

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