Why might the globally worst-WNS scenario not be enough, by itself, to decide which views can be removed from a signoff plan?
From PDVerse MMMC Interview Masterclass, part of the pdVerse Mentor Series
Direct answer
Different analysis views have different critical paths. The worst-WNS scenario may constrain the high-speed functional datapath, while another scenario with lower WNS may be the only view that exercises scan test chains, asynchronous interfaces, or low-power retention paths.
Mentor explanation
Signoff coverage requires checking all unique path endpoints, not just the single worst WNS.
Key terms
- Path dominance vs WNS — a view with small WNS can be the dominant and only constraint for a specific sub-block or test mode.
- Endpoint coverage matrix — auditing that every timing endpoint is constrained across its worst operating condition.
- Asymmetric datapath activation — test modes and low-power modes activate logic paths that are completely inactive during functional mode.
Removing a view based purely on a WNS ranking table leaves unique mode-specific paths completely unverified, risking silicon failure.
Practical example
Dominance Migration Example:
Baseline Run:
• View 1 (func_ss_125c_rcmax): WNS = -150 ps (Rank 1 - Worst)
• View 2 (func_ss_m40c_cmax): WNS = -40 ps (Rank 2)
• View 3 (scan_ff_m40c_rcmin): WNS = -15 ps (Rank 3 - Hold Only)
After fixing ALU paths in View 1:
• View 1 WNS -> +10 ps (Clean)
• View 2 WNS -> -40 ps (Now the new failing critical limiter!)
• If View 2 had been pruned based on baseline ranking, this failure would be missed!Interview trap
Pruning signoff views based exclusively on a WNS ranking table. A view with minor WNS may be the only view testing critical test-mode or interface paths.
Key takeaways
- WNS only reflects the single worst path at a specific design snapshot.
- Different views cover unique clock domains, test modes, and endpoint populations.
- Optimizing the worst path causes secondary views to become the new critical limiters (dominance migration).
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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