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Disabling single signals in expression coverage?

cmautner

I am running expression coverage on a design. There are many ATPG multiplexers implemented in RTL, such as

wire signal = aptg_mode ? something : something_else;

Expression coverage complains that, since atpg_mode is always 0 in RTL simulations, the ternary expression is only 50% tested. Is there something I can do about it? Like, a pragma that tells the coverage scoring that atpg_mode is considered to be constant '0'?

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  • You can ignore named (or pattern-matched) signals from toggle coverage, but I don't believe this will have any effect on expression coverage, because the ignore is simply saying you don't care about the toggle, not that you expect it to be stuck at a certain value.

    You might wish to consider using the formal coverage reachability app; this takes a simulation (merged) coverage database, and produces a cover property for each uncovered block / expression / toggle. The formal engine then looks to see if it's even possible to reach that hole given any kind of stimulus. If formal says the line can't be reach by any means, it automatically creates ignore marks for you. It works really well, even for quite large designs that you might not normally consider for formal.

    Depending on how the D input to your scan control flop is driven you might need a simple constraint to prevent the scan mode being enabled.

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  • You can ignore named (or pattern-matched) signals from toggle coverage, but I don't believe this will have any effect on expression coverage, because the ignore is simply saying you don't care about the toggle, not that you expect it to be stuck at a certain value.

    You might wish to consider using the formal coverage reachability app; this takes a simulation (merged) coverage database, and produces a cover property for each uncovered block / expression / toggle. The formal engine then looks to see if it's even possible to reach that hole given any kind of stimulus. If formal says the line can't be reach by any means, it automatically creates ignore marks for you. It works really well, even for quite large designs that you might not normally consider for formal.

    Depending on how the D input to your scan control flop is driven you might need a simple constraint to prevent the scan mode being enabled.

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