[ntp:questions] ntpd connect gpsd shared memory driver

David Taylor david-taylor at blueyonder.co.uk.invalid
Wed Jun 19 05:14:18 UTC 2013

On 18/06/2013 20:28, Richard Cagley wrote:
> I'm having a hard time understanding this. Let's consider two cases:
> 1.) I have pps support built into my kernel:
> In addition to the to grab nmea time data You suggest using
> the pps line driver instead of You're saying that gpsd will
> not do this for me? I'm not sure what "some software" refers to. Should I
> be using ever?
> 2.) I don't have pps support:
> Well, I'm not sure what to do in this case but others have indicated that
> gpsd can grab both nmea and pps. Hence, I would think I'd use
> and

To distinguish between 1 and 2, run the PPS test, which shows the 
transition times.  I saw this:

$ sudo ppstest /dev/pps0 # press Ctrl-C to cancel..
trying PPS source "/dev/pps0"
found PPS source "/dev/pps0"
ok, found 1 source(s), now start fetching data...
source 0 - assert 1351501153.999956346, sequence: 47481 - clear 
0.000000000, sequence: 0
source 0 - assert 1351501154.999954601, sequence: 47482 - clear 
0.000000000, sequence: 0
source 0 - assert 1351501155.999951856, sequence: 47483 - clear 
0.000000000, sequence: 0

Sorry about the wrap!  The "some software" refers to the Raspberry Pi 
I'm using, where a 3rd-party program supplies the PPS data to the gpsd 
memory segment number 1.  I don't know whether gpsd can time the DCD 
line or not - it's outside the scope of my experience.  If it can, use 
28.1, if not, use 22.0.
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