STA Mastery · All levels

Corner / Mode Matrix Design: Interview Drills

Interview Drills for Corner / Mode Matrix Design.

Interview drills

Interview Drills for Corner / Mode Matrix Design focuses on corner coverage completeness, mode cross-product size, signoff WNS per scenario. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

diagram

PROMPT
You see corner coverage completeness, mode cross-product size, signoff WNS per scenario on Corner / Mode Matrix Design. Walk through root cause and fix.

STRONG ANSWER
1. Names corner/mode/view and path group.
2. Explains MMMC runs STA across PVT corners and functional modes; the matrix must cover real silicon without exploding runtime.
3. Requests MMMC scenario table, signoff matrix spreadsheet, scenario WNS dashboard.
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

MMMC scenario cube

diagram

MMMC MATRIX

           mode: func   test   low_power
corner ss     [ ]       [ ]      [ ]
       tt     [ ]       [ ]      [ ]
       ff     [ ]       [ ]      [ ]

Each checked cell = full STA run.
Signoff = worst WNS across all checked cells.

Root-cause tree to narrate

diagram

ROOT-CAUSE TREE — Corner / Mode Matrix Design

corner coverage completeness, mode cross-product size, signoff WNS per scenario 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

diagram

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

MMMC runs STA across PVT corners and functional modes; the matrix must cover real silicon without exploding runtime.

Metric: corner coverage completeness, mode cross-product size, signoff WNS per scenario

Practice this lesson