[ntp:questions] Sole time source in Linux user space.

Michael Eder meder at whoi.edu
Wed Jun 30 13:50:44 UTC 2010

I am working on a project developing ocean observatories and we have a
highly accurate time source. Unfortunately because of the embedded nature of
our platform it is not possible have NTP interface to it in any standard
fashion.  I have some questions about what is the best mechanism to add this
(sole) time source to NTP.  The time source is composed of a date and time
string accurate to seconds, and a highly accurate Pulse-Per-Second.  I
expect the shared memory interface to NTP is the way to do this, but what is
not clear to me is if I can enter the time as a single source or I should
separate out the Pulse-Per-Second from the date and time string and
configure them as separate sources.  The Pulse-Per-Second is highly
reliable, but I can imagine a situation where the date and time string might
not be available.  This is especially a concern if the system shuts down and

My question is what does NTP do for me automatically and what do I need to
do myself to take into account some of the limitations of my time source?
Is it possible in NTP to specify that a time source (highly accurate) is
only seconds?  What is the best way with NTP to handle the situation when
the Pulse-Per-Second is available but the full time string is not?  

I have looked at the gpsd and shmpps code but still have some questions how
NTP handles sources coming in through shared memory.  From looking at these
my understanding is I would set up one source for the time string (that is
accurate to approximately a second) and the second time source for the
highly accurate Pulse-Per-Second.  

Is there any place where I can find documentation about how NTP handles
different clocks and different information that clocks may provide?  Where
do I find documentation of how to configure these shared memory time

Pointers much appreciated. 


Michael Eder

Woods Hole Oceanographic


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