Why can enabling signal integrity (SI) analysis change which corner or view is actually the worst one?
From PDVerse MMMC Interview Masterclass · pdVerse Mentor Series
Direct answer
Because SI introduces a new dependency that plain nominal timing doesn't capture: coupling between neighboring nets, which depends on aggressor switching alignment, direction, and effective loading — and that dependency doesn't scale uniformly with the same PVT or RC assumptions that make a corner look "worst" without SI.
Mentor explanation
Without SI, the worst corner is usually the one with the most punishing standalone cell and wire delay assumptions. Once you turn on coupling analysis, a completely different mechanism enters the picture: how much delta delay a net picks up depends on whether its neighbors are switching at the same time, in the same or opposite direction, and how exposed the net is to those neighbors given the actual routing. A corner that looked mild on nominal delay can become the true worst case once you add in a coupling scenario that happens to align badly for it, and a corner that looked worst on nominal delay might not be the worst once SI- driven delta delay is layered on top. This is exactly why SI-aware and SI-unaware results for the same design can disagree about which condition dominates.
Key takeaways
- Because SI introduces a new dependency that plain nominal timing doesn't capture: coupling between neighboring nets, which depends on aggressor switching alignment, direction, and effective loading — and that dependency doesn't scale uniformly with the same PVT or RC assumptions that make a corner look "worst" without SI.
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