[SI-LIST] Re: Bit pattern for high speed serial link simulation
- From: "Zabinski, Patrick J." <zabinski.patrick@xxxxxxxx>
- To: perry.qu@xxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Mon, 28 Nov 2005 14:40:17 -0600
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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