Legal and Illegal PST Transition Checks: Comparison Matrix

Comparison matrix

Strict PST enforcement reduces escapes but needs stronger sequencing architecture and debug readiness.

+------------------+----------------+----------------+----------------+
| Approach         | Strength       | Weakness       | Best when      |
+------------------+----------------+----------------+----------------+
| Strict legality  | high safety    | higher debug load | early signoff stage |
| Balanced checks  | good coverage  | review discipline | ongoing development |
| Minimal checks   | quick iteration | escape risk    | temporary triage |
| Refined matrix   | risk-focused   | authoring effort | mature teams   |
+------------------+----------------+----------------+----------------+

When to choose each approach

• Choose LPV posture from escape risk, schedule stage, and owner bandwidth rather than simulator runtime alone.

Interview traps

• Selecting lower-overhead flows without proving corner-case transition behavior.

• Treating waiver volume as closure progress.

Low-power verification deep dive

Power-state correctness is a protocol contract: legal transitions, robust sequencing, and safe concurrent event handling.

Concept diagram

PST CONTROL LOOP

state request -> legality check -> handshake sequencing -> mode entry -> monitored exit

Metric graph

STATE RISK MIX

illegal transitions     ██████
sequence race bugs      █████
stable mode paths       ████████

Metrics and artifacts to collect

• PST legality matrix

• illegal transition histogram

• entry/exit handshake coverage

• mode sequencing anomaly log

Mini case study

A sporadic low-power failure closed only after proving a wake-versus-thermal race in PMU transition sequencing.

Debug branches

• Validate legal state graph first.

• Stress concurrent control events and asynchronous wakeups.

• Bind fixes to explicit transition and owner contracts.

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.

Principal LPV review addendum

Legal and Illegal PST Transition Checks should be reviewed as a transition integrity system, not just isolated checks.

Use Illegal transition escape rate, transition-checker latency to first error, and percentage of legal arcs exercised with pass/fail evidence. as alarm and Transition-arc checker specification with guard predicates, timeout rules, and illegal-arc diagnostics taxonomy. as proof.

Power-state verification is a protocol verification problem: legal transitions, ordering contracts, and corner-case concurrency. Closure quality comes from reproducible evidence and explicit owners.