[ntp:questions] Setting up a NTP Time Server
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
What is good enough?
>These are my opinions, not necessarily my employer's. I hate spam.
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