User`s guide

14
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28 Di
g
ital Simulation
Trackin
g
Timin
g
Violations and
Hazards
When there are problems with your design, such as setup/hold
violations, pulse-width violations, or worst-case timing hazards,
PSpice A/D logs messages to the simulation output file and/or
Probe data file. When using Probe, messages can be selected
causing associated waveforms and detailed message text to be
automatically displayed.
PSpice A/D can also detect persistent hazards which may have
a potential effect on a primary circuit output or on the internal
state of the design.
Persistent hazards
In the digital domain, the types of problems identified by
PSpice A/D are generally classified as either timing violations
or timing hazards. Timing violations include SETUP, HOLD,
and minimum pulse WIDTH violations of component
specifications. The occurrence of such a violation may result in
a change in the state behavior of the design, and potentially the
answer. However, the effects of many of these errors are short-
lived and don’t influence the final circuit results.
For example, consider an asynchronous data change on the input
to flip-flop FF1 in Figure 14-2. The data change is too close to
the clock edge e1, resulting in a SETUP violation. In a hardware
implementation, the output of FF1 may or may not change.
However, some designs won’t be sensitive to this individual
missed data because the next clock edge (e2 in this example)
latches the data. The significance of timing errors such as this
one must ultimately be judged by the designer, accounting for
the overall behavior of the design.
Timing hazards are most easily identified by simulating a design
in worst-case timing mode, usually close to its critical timing
limits. Under such conditions, PSpice A/D reports conditions
such as AMBIGUITY CONVERGENCE hazards. Again, these
may or may not pose a problem to the operation of the design.
However, there are identifiable cases that cause major problems.
An example of one such case is shown below. Due to the
simultaneous arrival of two timing ambiguities (having
not
included
in:
The messa
g
in
g
feature is
discussed further in
Tracking
Digital Simulation Messages on
pa
g
e 17-41 of Chapter
17,Analyzing Waveforms in
Probe.