[LinuxPPS] Motherboards with decent Xtals.

[Cirilo Bernardo]

On Wed, Nov 12, 2008 at 4:20 AM,  <clemens at dwf.com> wrote:
> Since there has been some related discussion on the DFC77-PPS thread,
> Im going to ask a question of everyone who reads this list.
>
> Namely.
>
> Does anyone know of motherboards (preferably for Intel Chips) that actually
> have decent Xtals on them?  From my point of view, the optimal solution would
> be a board with a decent Xtal, that also allowed (by changing some jumper) the
> use of an external frequency standard (say 5 or 10MHz).
>

A good oscillator is not the only problem; the temperature of the
oscillator will vary widely on a modern CPU if the CPU does 'frequency
stepping'.  If you really want very good pps precision, you have to
get a rubidium reference (~$2k).

> It would seem that the machine I was using back when I was testing the Ulrich
> PPS implementation was much more stable than the machine I am using now.
> If one plots Loopstats/clock-offset, the previous machine was smooooth, while
> the current machine jumps all over the place.

Part of the problem may be with the kernel's implementation of its
timers.  There was some discussion over a year ago about using the
"high performance timers" - sounds good, especially with the phrase
"high performance" in there - but unfortunately stepping the CPU
frequency affected those clocks and this was a nightmare.  There was
some talk of sorting out the timer code (especially the high-res timer
code) but I decided to forget about all that and look into it again at
least a year later - just hoping someone else would do all the work.
:)

>
> I know of no such boards, but I have heard rumors that they exist, but are
> very expensive.  Anyone have any first (or second) hand info on such a beast?

I don't know of any specific boards, but I'd be surprised if other
people haven't done it even if only for a pet project.  Personally, if
I wanted to get as close as I can to nanosec precision, I'd build a
custom board with registers, a Rubidium clock to provide a a precise
PPS signal (and possibly also a high-frequency high-stability timer),
and a GPS unit to provide the GPS and/or UTC time.  Such a setup would
allow me to at least achieve the GPS claimed jitter of <1ns on my
hardware board.  The computer clock of course will never get there
thanks to a variety of other reasons so such a board has somewhat
limited use for managing computer time; it would be excellent for
providing precise time codes for observing other phenomena though.

- Cirilo