[ntp:questions] help with setting up NTP on windows with a USB GPS

David J Taylor david-taylor at blueyonder.not-this-bit.nor-this-part.co.uk.invalid
Tue Dec 1 18:15:56 UTC 2009

"jack" <> wrote in message 
news:72491452-f89d-454b-8bcc-f945c825915f at j19g2000yqk.googlegroups.com...
> David,
> I would like to add that, from my own experiences, the GPS has to be
> configured to output $GPRMC sentences only. Otherwise the PPS part of
> NTP doesn't work.

Good point - added.

> I have a related question: given that my system clock is synced to an
> external GPS within ms, how do I read time that gives me the same
> accuracy? When I tried to read time at 60Hz, I found that the times
> returned are not periodic, sometimes even the same.
> Jack

That was my point about using Windows!  By default, the clock only returns 
new values at either 16ms or 10ms intervals - depending on the OS version. 
You can enable the Multimedia timers which increase the clock frequency, 
but I can't remember off-hand whether that increases the read-out 
precision in /all/ versions of Windows, or just Vista and Windows-7.  Use 
the timeBeginPeriod and timeEndPeriod functions, but be aware that 
switching these functions on and off can result in timekeeping transients, 
so with NTP and pre-Vista systems it's best to use NTP to keep the MM 
timer enabled.

This source looks interesting for XP, but old:

On my XP system, with the MM timer enabled, I see a resolution for a 
timeGetTime call of just under a millisecond.  I wrote a small program 



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