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ECO Regression Matrix: Interview Drills

Interview Drills for ECO Regression Matrix.

Interview drills

Interview Drills for ECO Regression Matrix focuses on corners re-run, new violations introduced, signoff delta. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

diagram

PROMPT
You see corners re-run, new violations introduced, signoff delta on ECO Regression Matrix. Walk through root cause and fix.

STRONG ANSWER
1. Names corner/mode/view and path group.
2. Explains Every ECO must re-run the agreed MMMC matrix plus SI; regression hygiene prevents shipping a fix that breaks another corner.
3. Requests regression manifest, corner comparison table, signoff approval.
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

Post-ECO regression

diagram

ECO REGRESSION CHECKLIST
[ ] all MMMC scenarios
[ ] SI re-run
[ ] power/leakage delta
[ ] equivalence / LVS if logical ECO
[ ] signoff memo updated

Root-cause tree to narrate

diagram

ROOT-CAUSE TREE — ECO Regression Matrix

corners re-run, new violations introduced, signoff delta 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

ECO without regression is a coin flip.

Concept diagram

diagram

ECO LOOP

failing path -> fix -> setup check -> hold check -> SI -> MMMC matrix

Metric graph

diagram

ECO PROGRESS

WNS
 0 |              o target
-50|        o
-100|   o baseline
    +-----------------> ECO #

Reports and artifacts

pre/post WNS
hold delta
leakage delta
ECO ticket log

Mini case study

48-hour tapeout crunch: VT swap fixed setup; hold fixed with route on short path — order mattered.

Debug branches

Never ship setup-only proof
Logical ECO needs equivalence

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

Every ECO must re-run the agreed MMMC matrix plus SI; regression hygiene prevents shipping a fix that breaks another corner.

Metric: corners re-run, new violations introduced, signoff delta

Practice this lesson