[LinuxPPS] ntpd

[Rodolfo Giometti]

On Wed, Aug 02, 2006 at 08:08:56PM +0200, Udo van den Heuvel wrote:
> 
> BTW: I was looking at the time_pps_findpath function but do not yet
> really understand it. There is a PPS device defined like "/dev/ttyS2" or
> similar but where does that info enter the code? What does the function
> look for to know what port/device I want ntpd to use as a timesource?

The time_pps_findpath() can be use to say to the NTPD daemon which is
the PPS source you wish to use. Example (maybe it can help Folkert :),
I have a GPS antenna connected to ttyS1, if the NTPD uses:

   fd = time_pps_findsource(-1, path, 40, id, 40);

the system will return the _first_ available PPS source (due "-1") so,
using 8250 client, it is ttyS0! This because the client defines a PPS
source for _each_ serial line in the system (it cannot know which
serial line the user will use for his/her GPS antenna).

The only way to force time_pps_findsource() to return the PPS source
on ttyS1 is by using:

   fd = time_pps_findsource(1, path, 40, id, 40);

or whatever "id" ttyS1 has into /proc/pps/sources. This imply to
modify the code...

On the other hand if you use time_pps_findpath() as described into the
wiki you can say to the NTPD which is your PPS source by doing a
simbolic link. I report below the example in the wiki:

   giometti at jeeg:~/linuxpps$ cat /proc/pps/sources 
   id	mode	echo	name			path
   ----	------	----	----------------	----------------
   00	1151	yes	ktimer		
   01	1133	no	pps_8250_0		/dev/ttyS0
   02	1133	no	pps_8250_1		/dev/ttyS1
   03	1133	no	pps_8250_2		/dev/ttyS2
   04	1133	no	pps_8250_3		/dev/ttyS3
   giometti at jeeg:~/linuxpps/test$ sudo ln -sf /dev/ttyS1 /dev/gps0
   giometti at jeeg:~/linuxpps/test$ sudo ./ppstest /dev/gps0
   found PPS source #2 "pps_8250_1" on "/dev/ttyS1"
   giometti at jeeg:~/linuxpps/test$ sudo ln -sf ktimer /dev/gps0
   giometti at jeeg:~/linuxpps/test$ sudo ./ppstest /dev/gps0
   found PPS source #0 "ktimer" on ""

As you can see I can switch PPS sources by simply redefining the
symbolic link _without_ changing the code! :)

Ciao,

Rodolfo

-- 

GNU/Linux Solutions                  e-mail:    giometti at enneenne.com
Linux Device Driver                             giometti at gnudd.com
Embedded Systems                     		giometti at linux.it
UNIX programming                     phone:     +39 349 2432127
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