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Signoff vs Implementation Views: Interview Drills

Interview Drills for Signoff vs Implementation Views.

Interview drills

Interview Drills for Signoff vs Implementation Views focuses on view delta WNS, parasitic corner mismatch, SPEF vs signoff RC gap. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

diagram

PROMPT
You see view delta WNS, parasitic corner mismatch, SPEF vs signoff RC gap on Signoff vs Implementation Views. Walk through root cause and fix.

STRONG ANSWER
1. Names corner/mode/view and path group.
2. Explains Implementation and signoff use different parasitic corners and analysis views; comparing them without mapping causes false panic or false confidence.
3. Requests view comparison report, SPEF tag manifest, delta slack log.
4. Proposes one reduced sequence and one system regression.

WEAK ANSWER
Jumps to upsizing cells, widening routes, or blaming tools without reading constraints and path delay.

Diagram to draw on the whiteboard

View comparison

diagram

SAME NETLIST — TWO VIEWS

impl_view SPEF (typical)     WNS = +50ps
signoff_view SPEF (cworst)   WNS = -30ps

Delta is expected; panic only if delta exceeds historical bound.

Root-cause tree to narrate

diagram

ROOT-CAUSE TREE — Signoff vs Implementation Views

view delta WNS, parasitic corner mismatch, SPEF vs signoff RC gap looks wrong
        |
   same netlist tag?
     /        \
   no          yes
   |            |
 view/SDC     real path or
 mismatch     constraint bug
 /    \           |
lib   SPEF    delay breakdown
diff  diff    cell vs net

STA deep dive

Signoff WNS is the minimum across the full scenario matrix.

Concept diagram

diagram

MMMC

corners (ss/tt/ff) x modes (func/test) x views (signoff RC)
= scenario set -> min(WNS) is signoff

Metric graph

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WNS HEATMAP BY CORNER

        ss    tt    ff
setup  -80   +10   +5
hold   -15   -2    -8

Reports and artifacts

per-corner WNS
mode summary
lib map audit
view delta

Mini case study

Passed TT typical but failed SS hold — signoff matrix was incomplete; added low-voltage mode.

Debug branches

Isolate failing scenario first
Never compare unlike SPEF tags

Senior review question

Ask: what corner/mode/view proves this topic is closed or failing?

Key takeaways

State corner, mode, view, and database tag with every slack claim.
Run setup and hold plus MMMC regression after every ECO.

Common pitfalls

Comparing STA runs with different SPEF or SDC tags.
Broad false_path to green-wash violations.
Setup-only ECO without hold check.

Principal STA review addendum

Implementation and signoff use different parasitic corners and analysis views; comparing them without mapping causes false panic or false confidence.

Metric: view delta WNS, parasitic corner mismatch, SPEF vs signoff RC gap

Practice this lesson