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[time-freq] Supported operating system questions for

John Ackermann N8UR jra at febo.com
Fri Jun 29 00:47:55 UTC 2007

Gregg Levine said the following on 06/28/2007 08:37 PM:
> Hello!
> Our Owner has his TAPR FatPPS connected to a Soekris system running
> FreeBSD, and is making use of a Clock Block, for a better
> understanding of what I am getting at and why I am bringing up the
> merely obvious as opposed to the incredibly obvious, please revisit
> http://www.febo.com/pages/soekris/ it explains in far greater detail
> as to why I am doing that.
> Now the question, will the pair work with the same hardware but with
> it running Linux?

Hi Gregg --

I think the answer is "no", or at least not as well, on one general and
one specific count.

The general reason is that Linux doesn't have the same level of support
for the PPS signal that FreeBSD does; I don't understand all the
history, but the short version of the story is that the kernel patches
that are necessary to implement PPS functionality have never been merged
into the Linux kernel tree.  Search on the web for "PPSkit light" or
something similar, and you will find a patch that you can apply to a
fairly recent Linux kernel to add the PPS API; if you do that, you will
be able to support a hardware PPS signal, but as I understand it, Linux
still poses some challenges to really high performance NTP work.

The specific reason is that only FreeBSD, to my knowledge, supports the
special timer hardware on the Soekris, so only that OS will take
advantage of the really high resolution timer system.

However, I should mention that if you're using a Clock-Block with a good
reference oscillator, you may not need to use a PPS signal at all if
you're willing to rely on an external server for time sync.  The
Clock-Block will stop your machine from drifting once it's set, which
makes NTP's job very easy, so you may be able to get very good
performance just by syncing to a good external server.  However, that's
not nearly as cool as using a PPS signal to get the ultimate time machine!

Hope this helps.


> I freely admit that I had known at the TAPR site because they were the
> first to offer in the US a board that translates a CF card into an IDE
> form facter gizmo, it was mentioned in a copy of either LinuxJournal,
> or their late ELJ newsletter that I have here. But that is an
> inredible job of making three objects work together and do something
> special. I should also mention that the Soekris people were also
> brought to my attention in an article in the same magazine on the
> subject of network managing a sump pump.

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