DenseChip

STA Mastery · All levels

MMMC Debug Playbook: Interview Drills

Interview Drills for MMMC Debug Playbook.

Interview drills

Interview Drills for MMMC Debug Playbook focuses on scenario-specific violation clusters, corner isolation time. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

diagram
PROMPT
You see scenario-specific violation clusters, corner isolation time on MMMC Debug Playbook. Walk through root cause and fix.

STRONG ANSWER
1. Names corner/mode/view and path group.
2. Explains MMMC failures cluster by corner, mode, or library; triage isolates the scenario before touching RTL or layout.
3. Requests per-corner WNS heatmap, mode failure matrix, scenario replay script.
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

Corner isolation heatmap

diagram
WNS BY CORNER (mV)

        ss   tt   ff
reg2reg -80  +20  +5
hold    -12  -3   -15
io      +10  +30  +25

Cluster guides which scenario to debug first.

Root-cause tree to narrate

diagram
ROOT-CAUSE TREE — MMMC Debug Playbook

scenario-specific violation clusters, corner isolation time 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 failures cluster by corner, mode, or library; triage isolates the scenario before touching RTL or layout.

Metric: scenario-specific violation clusters, corner isolation time

Practice this lesson