[ntp:questions] TrueTime refclock Users?

Jason Rabel jason at extremeoverclocking.com
Tue Apr 17 15:44:05 UTC 2007


Harlan,

Yes, I checked out those links, nobody else listed with a NTS and there
isn't any additional information, but that's okay.

I was swearing up and down the other day wondering why the driver wasn't
working. I knew my C skills were rusty, but the driver code isn't *that*
complex. I must of re-written the driver routines about 3 or 4 different
ways. Not knowing what all the debug references were (I think a few led me
down a false path), I was having to dig through even more code trying to
figure out their meaning.

I finally tossed the true driver completely and re-wrote it based on the
nmea driver. This eliminated a lot of the old testing routines for ancient
TrueTime devices and was much cleaner.

Finally a light dawned upon me when I was staring at the time output from
the NTS... It was in local time, not UTC time! DOH! lol....

I switched the TZ on the NTS and BAM, it worked! Unfortunately I was less
than impressed with the timing accuracy. I tried both the continuous
once-a-second automatic output and the time on request polling method (which
returns the time with milliseconds). I think I could get a little bit better
resolution if I knew exactly how the time routines worked and could set the
point in time that the received data represents. The automatic output seemed
to give slightly better performance, probably because there isn't the delay
from the whole request, process, and receive.

Part of the poor accuracy could be due to the serial port on the Linux box,
I'm about to recompile the code on my geode box that is running FreeBSD (it
just takes a long time to compile NTP, which is why I do the testing on a
faster machine). The time came out to be about 1-2ms off (it states accuracy
within 1ms in the manual), but I want to compare it with my GPS on the same
machine before finally calling it quits.

I know this was a pretty round-about way of doing things, and there are much
easier methods for getting more accurate time, but I just wanted to see if
it could be done. At the very least I learned a lot about how the refclock
drivers worked. It's entirely possible with a little more hacking (both on
the NTS and the driver), PPS support could be added. But at that point it
would just be easier (and cheaper) to use a regular GPS receiver.

So that's my story about getting time output via the serial port on a
NTS-100. :)

Jason




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