[ntp:questions] Basic MSF clock setup

Chris Hastie chris at nospam.oak-wood.co.uk
Tue Nov 11 09:09:41 UTC 2003


I recently picked up a soldering iron for the first time in about twenty
years and to my astonishment managed to build the simple MSF clock at
<http://www.buzzard.org.uk/jonathan/radioclock.html> without burning out
any components or setting fire to the house :)

This provides a raw MSF signal at the DCD pin of the serial port.

The radioclkd software at that page I couldn't get to compile on
FreeBSD, but managed with the code from <http://www.jonatkins.com/radioc
lk2/>, so now have an MSF receiver set up using the SHM driver. Now I'm
wondering what the best way of running this clock is.

It strikes my that it shouldn't be too difficult to adapt the GENERIC
driver for raw MSF, based on the existing raw DCF mode. Would there be
any advantage to doing this over using radioclkd2 and the SHM driver?
Has anyone already done it? If not, is there a blindingly obvious reason
why not?

The other thing I'm wondering about is PPS. In effect I have a PPS pulse
at DCD, with the proviso that pulse width varies and occasional pulses
are missing. I know radioclkd2 makes use of the PPS capabilities of
FreeBSD, although I don't really understand how or what the implications
of this are. Reducing the poll interval of the SHM driver to try to
improve synchronisation doesn't work as it ends up being reported
unreachable for most of the additional polls and then being discarded.
So I tried adding a PPS driver:

|# ln -s /dev/pps1 /dev/ttyd0

|ntpd.conf:
|server 127.127.22.1 minpoll 4
|fudge 127.127.22.1 flag2 1 time1 0.0213

This sort of works. Except every so often a poll of the PPS driver gives
an unexpectedly high offset (40 to 45mS on the whole). Which leads ntpd
to 'clock hop' between PPS and SHM somewhat, with PPS reporting quite
large jitter at times. I presume this is caused by the occasional
missing pulse.

Adding

|fudge 127.127.22.1 flag2 1 time1 0.0213 flag3 1

seems to mean that I never manage to catch these unusual offsets, but I
do see PPS jitter shooting up every so often and the PPS signal being
marked as an outlyer.

Is there something else I should be doing? am I trying to do to much?
Have I completely missed the point? Is using the PPS driver on a raw MSF
signal a complete waste of time?
-- 
Chris Hastie



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