[ntp:questions] Tutorial for setting up Garmin 18 LVC on FreeBSD 8.0

unruh unruh at wormhole.physics.ubc.ca
Mon Feb 22 18:13:03 UTC 2010

On 2010-02-22, ryandoyle <rd at ryandoyle.net> wrote:
> For anyone interested in setting up a Garmin 18 LVC GPS receiver on
> FreeBSD 8, I wrote up a fairly detailed tutorial of my experiences
> here: http://blog.doylenet.net/?p=145
> A couple of months ago I thought it would be a neat idea to run a
> strat 1 time server at my work. I hadn't had any experience with NTP
> before, didn't know what local clock drivers were etc... and coming
> from a Linux background I had very little experience with FreeBSD.

I disagree with your comments re the accuracy on Linux. I have
consistantly gotten 2usec accuracy on Linux with ntpd, (and with chrony, Lichvar
gets sub usec accuracy with no change in the kernel).

The important thing is to get the time into the system as rapidly as
possible from an interupt. I wrote my own interrupt driver for the
parallel port to get the data in as quickly as possible, and running
tests ( Tie an output data line to the parallel interrupt line, time
when you output on the data line, time when you get the input from the
interrupt) I get about 1-2usec delay on the interrupt on Linux spread
between output and input. 
With ntp, the other problem is that the algorithm used responds very
slowly to temp changes. This is the primary limit to the accuracy of an
ntpd driven refclock. 

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