Corner / Mode Matrix Design: Worked Example

Worked example

Worked Example 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.

A signoff run shows corner coverage completeness, mode cross-product size, signoff WNS per scenario. The first review mistake is to blame implementation without checking constraints. A better review starts by pinning one critical path, proving where slack was lost, and checking whether the SDC rule is correct for Corner / Mode Matrix Design.

Sequence under inspection

TIMING PATH — Corner / Mode Matrix Design

launch FF --logic stages--> capture FF
     |                           |
  launch clock                capture clock
     |                           |
  required: arrival < capture_edge - setup
  metric: corner coverage completeness, mode cross-product size, signoff WNS per scenario

MMMC scenario cube

MMMC MATRIX

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

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

1. Capture report_timing and the relevant SDC excerpt.

2. Tag corner, mode, view, and path group.

3. Find the first cell or net that dominates critical delay.

4. Compare against MMMC scenario table, signoff matrix spreadsheet, scenario WNS dashboard.

5. Choose one reversible ECO and write the regression matrix before editing netlist or constraints.

Did the fix work?

BEFORE / AFTER ECO — Corner / Mode Matrix Design

WNS (ps)
  0 |                    --- target
-50 |    ● regressed
-80 |         ● before ECO
-20 |              ● after ECO
    +-------------------------------> ECO iteration
Prove setup AND hold moved safely.

STA deep dive

Signoff WNS is the minimum across the full scenario matrix.

Concept diagram

MMMC

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

Metric graph

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