[SI-LIST] Re: SSN and Crosstalk in Package
- From: Chris Cheng <Chris.Cheng@xxxxxxxxxxxx>
- To: "'si-list '" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 15 Apr 2005 22:34:22 -0700
It depends on what kind of topology you have.
If you just have some high impedance tightly coupled short traces in a
package with nothing outside and no termination, the switching current could
be very small and your edges can be super sharp to generate saturated
crosstalk that bounce around and increase in amplitude with little di/dt and
little SSO noise.
If you tie it with some meaningful terminator and well spaced signal trace
outside the package and your signal edge starts to be reasonably slow, you
xtalk will not get saturated and your di/dt will drastically increase and
hence your SSO.
-----Original Message-----
From: Zhangkun
To: si-list
Sent: 4/15/2005 1:48 AM
Subject: [SI-LIST] SSN and Crosstalk in Package
Dear all
Recently, I was simulating SSN in package. I found out that in the "SSN"
noise, there are two parts:
1. The noise because of the inductance of power or ground net.
2. The crosstalk between signals.
Moreover, I noticed that the second part is much larger than the first
one. The verification method is to simulate twice:
1.Regular simulation.
2.Simulation with perfect power supply. Therefore the inductance of
power or ground net is eliminated.
Comparing these two SSN simulaiton results, I notice that the difference
is not great. Therefore, I concluded that the main source of "SSN" is
the crosstalk in package.
Let the above conclusion be correct. I think that the SSN would be not
very critical. My explanation is following. Based on the "SSN" waveform,
it is clear that the crosstalk is mainly inductive and the capacitive
coupling is small. When the inductive and capacitive coupling is not
equal, the farend crosstalk is very large. The nearend crosstalk would
be very small for the trace in package is very short.
Now let us consider several SSN situation. There are N IO ports in the
IC.
1.(N-1) net is output and aggressive. Victim net is output and
quiet.
Because the crosstalk is inductive, there would be great "SSN"
in the victim net. However, the "SSN" is of high frequency. When the
"SSN" is transfered along the transmission line, it would be damped and
would not damage the signal integrity of victim net.
2.(N-1) net is output and agressive. Victim net is input and quiet.
Because the nearend crosstalk is small, there would be no great
damage for the input net.
3.(N-1) net is input and aggressive. One net is input/outpu, quiet,
and victim.
Normally, the risetime and falltime at input buffer is large for
the damp of transmission line or loading effect. Therefore, there would
be no critical crosstalk.
At last, there would be no critical "SSN" problem if the crosstalk in
package is dominating.
The above is my viewpoint about SSN in package. Any comment is welcome.
The basic is that crosstalk in package is much larger that the noise
because of inductance of power supply system.
Best Regards
Zhangkun
2005.4.15
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