184.108.40.206.3. Simulation Glitch Filtering
The Power Analyzer defines a glitch as two signal transitions so closely spaced in time that the pulse, or glitch, occurs faster than the logic and routing circuitry can respond. The output of a transport delay model simulator contains glitches for some signals. The logic and routing structures of the device form a low-pass filter that filters out glitches that are tens to hundreds of picoseconds long, depending on the device family.
Some third-party simulators use different models than the transport delay model as the default model. Different models cause differences in signal activity and power estimation. The inertial delay model, which is the ModelSim default model, filters out more glitches than the transport delay model and usually yields a lower power estimate.
Glitch filtering in a simulator can also filter a glitch on one logic element (LE) (or other circuit element) output from propagating to downstream circuit elements to ensure that the glitch does not affect simulated results. Glitch filtering prevents a glitch on one signal from producing non-physical glitches on all downstream logic, which can result in a signal toggle rate and a power estimate that are too high. Circuit elements in which every input transition produces an output transition, including multipliers and logic cells configured to implement XOR functions, are especially prone to glitches. Therefore, circuits with such functions can have power estimates that are too high when glitch filtering is not used.
Enable glitch filtering in the .vcd that you generate for use in power analysis by turning on Perform glitch filtering on VCD files.
The .vcd file reader performs glitch filtering that is complementary to simulation glitch filtering, but is often less precise. While the .vcd file reader has the ability to remove glitches on logic blocks, the file reader cannot determine how a given glitch potentially affects downstream logic and routing. Filtering the glitches during simulation avoids switching downstream routing and logic automatically.
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