What is a max_transition violation, and why is it treated as a design rule rather than a slack-based check?
From PDVerse STA Mentor Guide · pdVerse Mentor Guide
Explanation
max_transition is a design-rule ceiling on the largest transition (slew/edge rate) time a cell input or output pin is allowed to see, independent of whether the path otherwise has positive timing slack. A violation is a hard DRC failure — it's checked against a libraryspecified limit, not against a computed slack margin.
Timing-path Walkthrough
The notes stress that a path can show positive slack yet still violate max_transition, which undermines the validity of that slack number — the delay calculation feeding into slack itself assumes transitions are within characterized library ranges. If slew exceeds the library's modeled range, the delay numbers used to compute slack become extrapolated and unreliable, so the notes state max_tran violations must be fixed before fine setup work, since closing timing on extrapolated numbers is 'closing fiction.'
Expected Report Behavior
A max_transition violation shows up as a DRC failure on the affected pin regardless of the path's setup/hold slack value; fixing it (before continuing setup optimization) may change delay numbers on the path since it was previously being calculated from an outof-range slew.
Mentor Note — Common Mistake
Ignoring a max_transition violation because the path still shows positive slack, not realizing the underlying delay numbers are extrapolated and therefore unreliable.
How To Debug
Check DRC/design-rule reports separately from slack reports; treat any max_transition violation as requiring a fix (e.g., buffering, resizing) before trusting or optimizing setup slack on that path.
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