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I think Fmax should have never been reported, as it's trying to over-simplify something that is often more complicated.
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slack is only part of the overall story, consider other cases like io speed violation and this case below which will certainly fool anyone focussed on slack only:
A module was failing in some builds. The design was timing clean regarding setup or hold slack and so suspicion fell on some wrong multicycle path. After painful efforts it turned out that though every multicycle path was correct but in some builds the maximum delay caused by multicycle was causing minimum pulse width violation due to a counter forced to zero at some point. This was not reported as slack failure and the integration team insisted the design was timing clean but then I looked at the report and found out restricted fmax.
Fmax can cover slack and all others and thus it is far more useful concept.
Honored Contributor
