VT Mix Strategy: Interview Drills

Interview drills

Interview Drills for VT Mix Strategy focuses on HVT/SVT/LVT cell mix, setup/hold slack distribution, leakage budget. The goal is to connect observed QoR movement to mechanism, ownership, and regression risk.

PROMPT
You observe HVT/SVT/LVT cell mix, setup/hold slack distribution, leakage budget in VT Mix Strategy. Walk through root cause and closure plan.

STRONG ANSWER
1. Names run context and affected QoR lane.
2. Explains VT assignment trades delay against leakage; poor mix causes either timing misses or unnecessary static power burn.
3. Requests VT distribution report, leakage breakdown, timing split report.
4. Proposes one reversible fix and full regression scope.

WEAK ANSWER
Jumps to random tool switches without mechanism evidence.

Whiteboard diagram

VT mix triangle

LVT: speed ++ leakage --
SVT: balance
HVT: speed -- leakage ++

mix per criticality bucket

Root-cause narration

ROOT-CAUSE TREE — VT Mix Strategy

HVT/SVT/LVT cell mix, setup/hold slack distribution, leakage budget regressed
        |
   same RTL/constraints tag?
     /              \
   no                yes
   |                  |
input drift      transform side-effect
 /    \             /         \
SDC    libs      mapping      physical estimate
diff   diff      choice       mismatch

Synthesis deep dive

PPA closure is a constrained tradeoff problem, not a timing-only exercise.

Concept diagram

PPA TRIAD

timing target <-> power budget <-> area cap

Metric graph

PARETO PROGRESSION

early runs      o o
balanced point    o
over-tuned          o (risk)

Reports and artifacts

• VT mix

• leakage trend

• dynamic power trend

• PPA Pareto table

Mini case study

LVT-heavy fix recovered setup but violated leakage target; balanced VT strategy closed both.

Debug branches

• Timing-only claims need power check

• Validate hold on VT swaps

• Use Pareto memo

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

VT assignment trades delay against leakage; poor mix causes either timing misses or unnecessary static power burn.

Metric: HVT/SVT/LVT cell mix, setup/hold slack distribution, leakage budget