[ntp:questions] Re: Linux PPS triggering with NMEA

Bjorn Gabrielsson bg at lysator.liu.se
Mon May 9 22:37:31 UTC 2005


"David Schwartz" <davids at webmaster.com> writes:

> "Bjorn Gabrielsson" <bg at lysator.liu.se> wrote in message 
> news:m3acn4ozpe.fsf at lysator.liu.se...
> 
> > I know what TIOCMIWAIT gives. Is there a user-space function that can
> > return time with better than than the 1us resolution that
> > gettimeofday() gives?
> 
>     No. But unless you have some specialized hardware, you're just kidding 
> yourself if you think you can get more than that. Even with a PPS input 
> accurate to 200nS and the PPS kernel patches, accuracy is still only about 3 
> microseconds.
> 
>     DS

David, I am not kidding myself. Lets skip the absolute accuracy. Say
you want to measure the time between two positive edges of a
signal. Let this signal be a 1PPS, 10PPS or 100PPS generated by a
high quality GPS, specified to <50ns, and better if sawtooth error is
corrected for...

If I then measure the time between these edges to 1sec +-1us. The
median beeing 1us. The 1sigma error being 1us. Am I entitled to look
for sub us resolution???

--

        Björn



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