# [ntp:questions] Something for distributing a PPS signal? (was: Re: Notions of Time in Computer Architecture)

Rick Jones rick.jones2 at hp.com
Fri Jul 1 20:37:45 UTC 2011

```Over in comp.arch, this was mentioned as how one might get time to the
far reaches of a chip with equal delay.  I recall that someone not too
long ago wanted to distribute a PPS signal to a bunch of systems, and

rick jones

[ This is a repost of the following article:                               ]
[ From: jsavard at excxn.aNOSPAMb.cdn.invalid (John Savard)                   ]
[ Subject: Re: Notions of Time in Computer Architecture                    ]
[ Newsgroups: comp.arch                                                    ]
[ Message-ID: <4e0e2a2f.820481 at news.aioe.org>                              ]

On Thu, 30 Jun 2011 14:13:23 +0200, Terje Mathisen <"terje.mathisen at
tmsw.no"> wrote, in part:

>With the sub-ns time scales in modern cpus, any large cluster (even if
>presented as a single image CC-NUMA system to the programmer) will get
>into trouble here:
>
>You cannot have a globally consistent, totally distributed clock: Unless
>you designate a single point of control, there's no way to always be
>able to figure out the absolute order of two or more distributed events,
>right?

Actually, no. This is not really a problem. If you want to have a clock
distributed on a chip that's accurate to a small fraction of the time
that light takes to travel across the chip, you *can* do it.

Think of the letter H. Assume you have a pin on the edge of the chip
where an external high-frequency clock signal comes in. Take that to the
exact center of traces having the form of the letter H. Clearly, when
the signal reaches the four corners of the H, it will have travelled an
equal distance in each of the four cases.

Now, make each of those four corners the center of a smaller letter H.
Repeat (but not infinitely, as we don't need a _true_ fractal.)

In fact, this actually *is* how clock signals are distributed on some
chips.

John Savard