A setup ECO improves WNS by 100 ps in one view but creates hold violations in two other views. What actually happened?
From PDVerse MMMC Interview Masterclass · pdVerse Mentor Series
Direct answer
Nothing is wrong with the timing engine. The physical change you made — a resize, a buffer insertion, whatever it was — is global. It affected every view that touches that cell or net, not just the setup view you were optimizing.
Mentor explanation
A view is a lens: it's a particular way of measuring the design under one mode and one corner. But the ECO itself doesn't live inside that lens — it's a physical change to the actual silicon, and every other view that includes that same cell will see the consequence too. Upsizing a cell to speed up a setup-critical launch path adds delay margin in one direction, but that same larger cell might now push a different view's hold check over the edge, because hold is stressed by paths being too fast, and you just made the wrong stage slower somewhere it mattered. The lesson isn't "don't fix setup." It's "never evaluate a setup fix in isolation from the other active views that share the same physical structure."
Example or conceptual chain
Setup view: −120 ps → (resize) → −20 ps. Same physical change, evaluated in the hold view: +15 ps → −45 ps. Same physical change, evaluated in a second mode's views, can shift again in either direction.
Interview trap
Reporting an ECO as a pure win because the targeted view improved, without re-checking every other active setup and hold view it touches.
Key takeaways
- Nothing is wrong with the timing engine.
- The physical change you made — a resize, a buffer insertion, whatever it was — is global.
Self-check: can you answer this aloud?
Try a 45-second answer using this structure:
- State the direct answer.
- Explain the timing or physical reason.
- Name one caveat.
- Say how you would verify it in a real flow.
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