What do positive slack and negative slack mean?
From PDVerse STA Mentor Guide · pdVerse Mentor Guide
Short Answer
Positive slack means a path has margin to spare and passes its timing check; negative slack means the path violates its check and fails.
Example
In the worked setup example, a required time of 900 ps against an arrival of 915 ps gives a slack of 900 - 915 = -15 ps, meaning the path fails by 15 ps.
Why It Matters
Whether slack is positive or negative is the pass/fail verdict that signoff is built on — designers hunt for the most negative slack paths first because those are the ones most likely to cause real silicon failures.
Equation
setup slack = required - arrival = 900 - 915 = -15 ps -> FAILS by 15
psMentor Note — Common Mistake
Assuming a small negative slack, like -15 ps, is negligible — even a tiny negative number means the check technically fails and needs to be fixed before signoff.
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