[ntp:questions] Red Hat vote for chrony

Paul tik-tok at bodosom.net
Sat Dec 6 19:46:53 UTC 2014

On Sat, Dec 6, 2014 at 7:27 AM, Charles Swiger <cswiger at mac.com> wrote:

> Um, certainly?  Are you concerned about the quality of the XO which
> shipped with a Soekris board?
> That'd be important if you were running a stratum-2+ timeserver.

If you don't do the reading you won't understand the problems.  Here's a
salient quote from Ackerman

"*To implement high-performance timekeeping, you need to modify the net4501
board to accept an external clock signal*, as well as use the high
resolution timer. These steps require fairly good soldering skills as the
surface mount components on the board are tiny; if you're unsure of
yourself, find someone with SMT soldering experience to help you.
Locate and remove the Soekris clock crystal (X1). A small heat gun is the
best way to do this, but with care a soldering iron and solder wick can be
used to lift the part. The only pin on X1 that we'll use is the one in the
upper left corner, so make sure you don't damage that solder pad. "

And regarding the Clock-Block

"The Clock-Block is a flexible frequency synthesizer that can be used for
many timing and clocking purposes.It accepts an *external reference signal*
from 2 MHz to 50 MHz, and can be programmed via DIP switches to generate
output frequencies in the range of about 500 kHz to 250 MHz. The output is
a square wave at either 3.3 or 5 volts peak-to-peak. An on-board divider
circuit allows the output frequency to be reduced by factors of 16 to 16384
for specialized applications.* One application for the Clock-Block is to
replace the crystal oscillator of a PC motherboard to allow synchronizing
the PC's clock to an external high-precision reference*."

This isn't about modifying a 4501 for testing or measurement.  It's about
modifying it for high resolution production service.  By the way in the
current era the 4501 is poor choice.  Common SoC based systems like the
BeagleBone Black have everything you need (and more), are cheaper and no
un-soldering is required.

You setup a high quality clock somewhere and pull timestamps from the
> general purpose computer you want to measure.

Perhaps you're unfamiliar with the problem PTP solves.  That is one of
things you have to deal with if you're going to use external measurements
to assess the quality of your clock discipline.

Maybe you're also unfamiliar with what NTP is doing:  It disciplines a
virtual (system) clock which is derived from the on-board (usually
uncompensated) crystal
.  That's why NTP can act as a thermometer.  The big problem is temperature
induced wander and that's something (back to RedHat and Chrony) Chrony
claims to do better.

I suspect fiddling something into an external TIC is a way to go but
something both simpler and more clever is what I'm curious about.

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