What is useful skew? How can it be used to fix setup violations? Command for useful skew?

o   Useful Skew: The deliberate introduction of a controlled amount of clock skew between specific launch and capture flops to help meet timing requirements (either setup or hold). Instead of minimizing skew everywhere (the default CTS goal), useful skew leverages available timing margin from one path to fix a violation on another.

o   Fixing Setup with Useful Skew:

o    Equation: Setup Slack ≈ (Clock Period + Skew) - Data Path Delay - Uncertainty - Lib Setup. Increasing positive skew directly increases setup slack.

o    Caution: Adding useful skew borrows margin. You must ensure that adding this skew doesn't cause:

§  Hold violations on the same path (as positive skew hurts hold).

§  Setup violations on other paths connected to the launch or capture flops

o   Command for Useful Skew:

o    setOptMode -usefulSkew true

o    setAnalysisMode -usefulSkew true