ModelSim SE User's Manual - Electrical and Computer Engineering

Weighting coverage directives
Configuring functional coverage directives UM-387
As shown in the dialog above, you can assign weights to coverage directives. Weighting
affects the aggregated coverage statistics in the currently selected design region. A
directive with a weight of 2 has twice the effect of a directive with a weight of 1.
Conversely, assigning a directive a weight of 0 would omit the directive from the statistics
calculation. See "Understanding aggregated statistics" (UM-393) for more details.
Weighting is a decision you make as to which cover points are more important than others
within the context of the design and the objectives of the test. Weightings might change
based on the simulation run as specific runs could be setup with different test objectives.
The weightings would then be a good way of filtering how close the test came in achieving
its objectives.
Example scenario
Choosing "AtLeast" counts
The likelihood that each type of bus transaction could be interrupted in a general test is very
low as interrupted transactions are normally rare. You would probably want to ensure that
the design handles the interrupt of all types of transactions and recovers properly from
them. Therefore, you might construct a test such that the stimulus is constrained to ensure
that all types of transactions are generated and that the probability of transactions being
interrupted is relatively high. For that test, the weighting of the interrupted transaction
cover points would probably be higher than the weightings of uninterrupted transactions (or
other coverage criteria).
The AtLeast count is a minimum threshold of coverage that gives you some confidence that
the run was meaningful. You don’t need to set this threshold on every directive, but you
should understand which minimal thresholds make for a useful simulation run based on
your design and the objectives of the verification session.
For example, say your test program requires a certain level of PCI traffic during the
simulation. 30 PCI STOP transactions might be a proxy measure of sufficient PCI traffic,
so you would set an AtLeast count of 30 on the "PCI STOP" coverage directive. Another
example might be that a FIFO full should have been achieved at least once as that would
indicate that enough activity occurred during the simulation to reach a key threshold. So,
your "FIFO full" directive would get an AtLeast count of 1.
ModelSim SE User’s Manual