[ntp:questions] Assistance with PPS on Windows

Chris Albertson albertson.chris at gmail.com
Thu May 5 22:59:24 UTC 2011

On Thu, May 5, 2011 at 1:45 PM, Richard B. Gilbert
<rgilbert88 at comcast.net> wrote:

> Getting the time into the computer takes X microseconds where X depends on
> the hardware and software.  Windows' clock ticks every 17 milliseconds or
> something like that.  Other operating systems also take a toll!

The way it is typically done, that is to get sub-nanosecond
measurements into a computer, is to do the actual measurement outside
of the computer using some flavor of time interval counter and then
send the data back to the computer over a serial line.  In the simple
case here of NTP.  the PPS signal from the GPS gates a hardware clock
to start.  then later the interrupt enable line on the CPU chip stops
the clock and some very long time later, perhaps milliseconds later
the CPU reads the time from the hardware counter.  People were
reliable measuring nanoseconds this way 10 years ago.   It works
because we don't need to care how bad or unpredictable the computer's
interrupt processing is because we measure it.

Now days one can buy a time interval counter that can get down to
about 250 picoseconds on ebay for $250.    This is a 4U tall chuck of
rack mounted gear made by HP in the 1980's.  the new stuff is better.
I read about one lab here at work using the term "femtoseconds" for
the first time lately and he had graphs in those units on a computer.
  That's not me.  My work uses uSeconds at most.

Chris Albertson
Redondo Beach, California

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