[SI-LIST] Re: Fibre channel interconnect margins

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: Hal Murray <hmurray@xxxxxxxxxxxxxxx>
  • Date: Tue, 04 Jul 2006 18:00:42 -0700

Hal, Gaussian noise is thermal.  The thermal noise amplitude isn't 
what tends to bite in links unless they are very long like undersea 
optical links.  In back planes and boards the noise will usually get 
us as jitter.

I don't have numbers that would be useful to you for CPUs.  You might 
be able to find the energy numbers for the required amplitudes by 
searching on SEUs which are becoming a perceivable problem.  Then we 
could work that back to the bulk resistance of the material and 
probably come up with a reasonable number for thermal noise, and an error rate.



At 05:41 PM 7/4/2006, Hal Murray wrote:

> > Hal, the Gaussian model really does apply and eventually your CPU
> > does make mistakes.  It tends to happen at such a low rate we all
> > blame it on the software or input devices that make mistakes millions
> > of times more frequently.  When it comes to CPUs I am not even sure
> > where to begin sifting transistor switching errors from other sources
> > because the event frequency is so low.
>I believe it will happen if I wait long enough, but I don't have a good feel
>for the numbers.
>I expect that I would have to wait something like the age-of-universe rather
>than just a short time like several/many years which is the time scale for
>most error mechanisms.  (at least in a well designed system)
>What's the SNR for a normal CMOS signal?  (either on chip or between chips)
>I'm only interested in the Gaussian part of the noise.
>Where does real-life Gaussian noise come from?  Thermal/Johnson?  If so, is
>the R the output resistance of the driving transistor?  What's a typical
>What's the (Gaussian) SNR on a Fibre Channel link?  (copper and fiber)
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