Timing-Driven Synthesis: Interview Drills

Interview drills

Interview Drills for Timing-Driven Synthesis focuses on critical path WNS, violating endpoint count, compile effort distribution. The goal is to connect observed QoR movement to mechanism, ownership, and regression risk.

PROMPT
You observe critical path WNS, violating endpoint count, compile effort distribution in Timing-Driven Synthesis. Walk through root cause and closure plan.

STRONG ANSWER
1. Names run context and affected QoR lane.
2. Explains Timing-driven compile focuses optimization where slack is most constrained, balancing global QoR instead of over-optimizing non-critical logic.
3. Requests critical range report, path-group WNS report, effort histogram.
4. Proposes one reversible fix and full regression scope.

WEAK ANSWER
Jumps to random tool switches without mechanism evidence.

Whiteboard diagram

Timing-driven effort allocation

path groups ranked by slack
      |
      v
high effort on critical groups
medium effort on near-critical
low effort elsewhere

Root-cause narration

ROOT-CAUSE TREE — Timing-Driven Synthesis

critical path WNS, violating endpoint count, compile effort distribution regressed
        |
   same RTL/constraints tag?
     /              \
   no                yes
   |                  |
input drift      transform side-effect
 /    \             /         \
SDC    libs      mapping      physical estimate
diff   diff      choice       mismatch

Synthesis deep dive

Mapping converts logic intent into real PPA outcomes.

Concept diagram

MAPPING LOOP

boolean net -> library mapping -> optimization -> report and iterate

Metric graph

MAPPING IMPACT

depth reduction   ███████
fanout cleanup    █████
runtime overhead  ███

Reports and artifacts

• mapping summary

• critical path cell list

• fanout/slew report

• library coverage

Mini case study

Cell-heavy critical path improved after restructuring, while blanket buffering had worsened power.

Debug branches

• Depth issue -> structure

• Fanout issue -> buffers

• Library mismatch -> view audit

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

Timing-driven compile focuses optimization where slack is most constrained, balancing global QoR instead of over-optimizing non-critical logic.

Metric: critical path WNS, violating endpoint count, compile effort distribution