Digital Signal Processing Reference
In-Depth Information
CHAPTER 2
The Signal Integrity Process
2.1 Introduction
Although modern signal integrity (SI) CAD is sufficiently sophisticated to predict
with great accuracy the shape and noise content of signal waveforms, this is not the
usual intent of SI analysis (advertisements from SI CAD companies notwithstand-
ing). Instead, an SI analysis defines the boundaries of operation. By creating a group
of worst-case simulations, the SI engineer can confidently predict the absolutely
best and worst signal behavior and noise. No matter when or where it is manufac-
tured, any given sample of the actual hardware is expected to fall within the bounds
defined by the worst-case analysis.
An experienced SI engineer, especially one who in the past has correlated SI
simulation results with laboratory measurements on hardware, will instinctively
know when a simulation contains too much margin or when the simulation results
are physically unreasonable. A less experienced SI engineer, particularly one with
no correlation experience, takes the simulation results more literally. These simula-
tions are likely to show excessive margin or unrealistic signal characteristics.
There are special instances when the signaling environment is known in great
detail or where the electrical characteristics can be specified to within very small
tolerances. These cases often involve very high-performance signaling, or high-re-
liability systems signaling at more modest rates. In other instances electrical mea-
surements of actual hardware can be used to fine-tune an existing SI model. In
these cases modern SI CAD will predict the actual signal wave shape and other
attributes with great precision.
2.2
Why Perform a Signal Integrity Analysis?
Perhaps the most common expected outcome of an SI analysis is to insure sig-
nal quality and validate system timing. Simulations are performed to quantify the
amount of timing margin and to determine that signals have adequate noise margin,
do not overshoot by too much, and have manageable crosstalk.
Signal integrity analysis can also determine if I/O drivers are subjected to stress-
es causing them to operate outside of their safe zone. Some stresses can permanently
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