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ECO Physical vs Logical: Interview Drills

Interview Drills for ECO Physical vs Logical.

Interview drills

Interview Drills for ECO Physical vs Logical focuses on ECO turnaround time, leakage delta, area delta, timing recovery. The goal is to connect the observable symptom to timing mechanism, ownership, and regression risk.

diagram

PROMPT
You see ECO turnaround time, leakage delta, area delta, timing recovery on ECO Physical vs Logical. Walk through root cause and fix.

STRONG ANSWER
1. Names corner/mode/view and path group.
2. Explains Logical ECO changes netlist; physical ECO changes placement/routing; each has different cost, risk, and regression surface.
3. Requests ECO ticket, LEF/DEF delta, netlist diff, timing before/after.
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

ECO types

diagram

LOGICAL ECO          PHYSICAL ECO
netlist change         place/route change
synthesis-driven       PD-driven
faster for logic fix   faster for wire fix
re-verify equivalence  re-extract SPEF

Root-cause tree to narrate

diagram

ROOT-CAUSE TREE — ECO Physical vs Logical

ECO turnaround time, leakage delta, area delta, timing recovery 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

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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

Logical ECO changes netlist; physical ECO changes placement/routing; each has different cost, risk, and regression surface.

Metric: ECO turnaround time, leakage delta, area delta, timing recovery

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