[SI-LIST] Re: Bit pattern for high speed serial link simulation

Pat:

You raised some very good points. I'm concerned that I may miss some
resonance frequency due to reflection or whatever and did not capture the
resulted ISI. On the other hand, I don't want to be overly conservative in
designing the link.

Thanks

Perry  


======================================= 

Perry Qu 

Design & Qualification, Alcatel Canada 

600 March Road, Ottawa ON, K2K 2E6 

DID: 613-7846720  Fax: 613-5993642 

Email: perry.qu@xxxxxxxxxxx 

======================================= 


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Zabinski, Patrick J.
Sent: Monday, November 28, 2005 3:40 PM
To: Perry.Qu@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Bit pattern for high speed serial link simulation

Perry,

Although your question was rather specific, I cannot help myself but respond
to a few of your points in more generic terms.

The question of "worst case" pattern comes down to the
specific circuit/link being considered.   If the 
link is AC-coupled, then the link will be sensitive to lower frequency
content.  In these cases, the worst case will involve long strings of
consecutive 1's and 0's.

Beyond a simple repeating pattern with a run-length of five bits (e.g.,
0000011111), you can accentuate even lower frequencies with repeating-word
patterns.  For example, FF-FF-00-00-FF-FF-00-00... (translated from 8B to
10B form) will raise the 3 Gbps * 1/10 (for 10 bits) * 1/4 (for four-words)
= 75 MHz peak.  Even longer word patterns will further lower the frequency
of interest.

There's also an upper frequency bound.  In these cases, a lonely-bit is
often the worst case.  For example, a 0000010000 pattern can give the link a
real headache.  Or, you can inject a 1010101010 pattern.

In between these two extremes (low-freq and high-freq), you might consider
mid-frequencies.  In particular, some passive interconnect have resonant
points of high attenuation due to discontinuities like vias, connectors,
etc.  If you simulate or measure S-parameters of the overall passive
interconnect, you can often find a particular frequency in the middle bands
that have poor transmission.  When these resonances exist, then the worst
case pattern is often one that produces high spectral density near these
resonances.

In some instances, it's not easy to pre-determine the worst case pattern.
On these occasions, a generic approach of injecting a long pseudo-random
pattern (like PRBS31) works albeit at a much longer run-time.  Or, you can
create a custom pattern that is a collection of the above "corner" cases
(low-, mid-, and hi-freq).

Regards,
Pat

> 
> Hi,
>  
> What will be a realistic bit pattern to use for simulation of a serial 
> link
> (3Gbps) with 8B/10B encoding ? Right now I used PRBS7 pattern but I'm 
> not sure whether it captured the worst case pattern compared to 
> reality, where the live data traffic is a lot more random. 8B/10B has 
> a run length of 5 but if it's fed with say a PRBS31 bit stream, will I 
> get more spectrum peaks than just a pure PRBS7 pattern without 
> encoding which repeat itself every
> 127 bits ?
>  
> Thanks
>  
> Perry
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