Scenario: Retiming Debate: Silicon PPA Impact

Silicon PPA and closure impact

Interview scenarios often mirror real signoff war-room pressure.

Area drivers

• Buffer growth

• Cell mix shifts

• Retiming duplication

Power drivers

• Leakage from LVT

• Clock power from gating/skew choices

Timing impact

• Critical path depth changes

• Hold/setup coupling

PD consequences

• Congestion ripple

• Handoff predictability

Verification burden

• Equivalence burden

• Regression matrix depth

PPA SNAPSHOT — Scenario: Retiming Debate
area / dynamic / leakage / WNS / runtime

PPA takeaways

• QoR wins must survive signoff checks

• Correlation beats one-off gains

QoR snapshot

BEFORE / AFTER QOR — Scenario: Retiming Debate

QoR score
  0 |                     --- target zone
 -1 |        ● regression
 -2 |             ● baseline
 -0.5|                  ● after fix
    +----------------------------------> iteration

Validate timing + area + power, not one number.

Synthesis deep dive

Interview readiness comes from repeatable QoR reasoning under pressure.

Concept diagram

INTERVIEW ANSWER SHAPE

context -> metric -> mechanism -> fix -> regression -> ownership

Metric graph

PANEL SCORING

clarity      ███████
mechanism    ██████
regression   █████

Reports and artifacts

• mock scorecard

• scenario debrief

• Q&A coverage

• framework adherence

Mini case study

Candidate improved by replacing tool-command answers with evidence-driven closure narratives.

Debug branches

• State assumptions first

• Defend option tradeoffs

• Close with rollback/regression

Senior review question

Ask: what evidence proves this QoR move is real and stable?

Key takeaways

• State exact run context (RTL, SDC, libs, switches) with every QoR claim.

• Re-run timing, area, and power regressions after each synthesis ECO.

Common pitfalls

• Comparing runs with mismatched constraints or library views.

• Timing-only fixes that violate power or area budgets.

• Skipping equivalence checks after structural changes.

Principal synthesis review addendum

Retiming debate evaluates whether register movement is safer than RTL pipeline edits under schedule and formal constraints.

Metric: frequency target miss versus latency and verification budget