[ntp:questions] Setting up a NTP Time Server

Unruh unruh-spam at physics.ubc.ca
Wed Apr 9 05:26:47 UTC 2008

hal-usenet at ip-64-139-1-69.sjc.megapath.net (Hal Murray) writes:

>>>No. All you need is refclock_nmea (127.127.20.x) for a directly
>>>connected NMEA device. Assuming that you're using a Linux kernel with
>>>PPS-kit or a BSD kernel (or another kernel which directly supports PPS).
>>OK, lets assume the kernel does NOT have the PPS support. Then what?

>If you setup a NMEA refclock, it runs off the text strings
>on the serial port.  It works, it just doesn't keep as good
>time as you normally get from PPS.

Yes. I was wondering how to use the PPS output even if the kernel does not
have pps support.

There is the shm support. 

>How good depends on your GPS device.  A lot of them have
>a lot of jitter.

The net ouput from a GArmin 18LVC into a Linux box, via my own interrupt
routine, is about plus or minus 2usec ( that includes any jitter in the GPS
pps signal, the clock reading jitter). 
I use my own parallel interrupt routine (based on the O'Reiley book by
 Alessandro Rubini and Jonathan Corbet "Linux Device Drivers"-- the short.c
driver). which is read by an adapted shm driver. 

>You also get jitter from the OS (and serial port hardware).
>There should be a simple recipe to minimize that.  I haven't
>seen one and several tries haven't produced anything that
>I'd call good-enough.  (When I run out of other things to
>work on...)

What is good enough?

>These are my opinions, not necessarily my employer's.  I hate spam.

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