Low Power Verification · All levels

Detecting Redundant or Missing Isolation: Debug Playbook

Debug Playbook for Detecting Redundant or Missing Isolation.

Debug playbook

Debug Playbook for Detecting Redundant or Missing Isolation is anchored on Missing-isolation high-severity findings and redundant-cell area/timing overhead eliminated before low-power signoff freeze.. Convert observations into mechanism-backed and owner-bound actions.

Freeze seed, metadata, and boundary under investigation.
Locate first persistent low-power phase divergence.
Classify mechanism: setup, transition, boundary, retention, or X-prop class.
Apply one focused reproducer and one bounded fix.
Re-run determinism and broader regression matrix.

Review memo template

diagram

LPV REVIEW MEMO - Isolation & Level Shifting / Detecting Redundant or Missing Isolation

1. Symptom
   - Failing metric: Missing-isolation high-severity findings and redundant-cell area/timing overhead eliminated before low-power signoff freeze.
   - Trigger context: <seed/mode/sequence>
   - First failing phase: <entry/off/exit/boundary>

2. Mechanism hypothesis
   - Candidate mechanism: Missing isolation is dangerous because it permits unknowns or unsafe logic levels to leak into live domains, but redundant isolation is also costly: extra cells increase area, delay, dynamic power, and debug complexity. Effective analysis classifies every crossing by sink sensitivity, power-state overlap, and existing protection path so true missing cases are separated from safe-by-architecture paths such as retained always-on sources or protocol-gated receivers. Redundancy checks should identify stacked isolation on the same functional path, overlapping tool insertions from hierarchical UPF, and dead isolation enables that never toggle in legal modes. Teams should run iterative static analysis plus scenario-driven simulation to verify that waivers are evidence-based, ensuring no crossing is over-protected by default or left exposed by assumption.
   - Competing hypotheses: setup, transition race, boundary bug, retention drift, X-prop noise
   - Missing evidence: <trace/assertion/report>

3. Proposed action
   - Smallest reversible change: <intent/RTL/checker/flow>
   - Expected movement: <failure trend/replay stability>
   - Regression risk: compatibility, coverage, signoff delay

4. Signoff
   - Required artifact: Risk-ranked crossing ledger showing missing, justified-exempt, and redundant isolation candidates with closure action and waiver evidence.
   - Required owners: power intent signoff lead, RTL integration owner, verification triage owner, timing and power optimization owner, program quality manager
   - Final decision: ship, bounded rollout, rollback, or escalate

Low-power verification deep dive

Boundary correctness depends on timing: isolate and shift at the right crossings and right phases.

Concept diagram

diagram

BOUNDARY SAFETY VIEW

domain crossing -> level-shift requirement -> isolation control -> OFF/ON transition behavior

Metric graph

diagram

BOUNDARY BUG CLASSES

missing isolation      █████
late isolation         ████
LS misdirection        ███

Metrics and artifacts to collect

cross-domain boundary inventory
clamp-value correctness report
LS direction and threshold checks
missing or redundant boundary action list

Mini case study

A boundary bug escaped unit tests until domain-off traffic stress revealed late isolation enable timing.

Debug branches

Map each crossing to expected LS and isolation behavior.
Verify enable timing against collapse and restore edges.
Check clamp safety for protocol-facing signals.

Senior review question

Ask: what exact low-power transition boundary failed first, and which artifact proves the closure claim reproducibly?

Key takeaways

Tie each LPV claim to a concrete transition boundary and one proving artifact.
Prefer minimal reversible fixes with explicit owner and rollback criteria.

Common pitfalls

Treating power-aware failures as random before boundary classification.
Waiving X-prop failures before proving impact and root cause.
Declaring closure without deterministic replay across key modes.

Debug ladder

Sequence: reproduce -> classify -> isolate boundary -> prove mechanism -> bounded fix.

Avoid mixed fixes before first-principles classification.

Practice this lesson