DenseChip

Interface Protocols · All levels

Compliance & Coverage Plan: Interview Drills

Interview Drills for Compliance & Coverage Plan.

Interview drills

Interview Drills for Compliance & Coverage Plan focuses on spec feature coverage, compliance pass rate, waiver count. The goal is to connect the observable symptom to protocol mechanism, ownership, and regression risk.

diagram
PROMPT
You see spec feature coverage, compliance pass rate, waiver count on Compliance & Coverage Plan. Walk through root cause and fix.

STRONG ANSWER
1. Names the layer and transaction identity.
2. Explains compliance maps spec requirements to tests, assertions, coverage bins, exclusions, and signed waivers.
3. Requests requirement trace matrix, coverage dashboard, waiver log.
4. Proposes one reduced sequence and one system regression.

WEAK ANSWER
Jumps to widening the interface, increasing FIFO depth, or blaming firmware without evidence.

Diagram to draw on the whiteboard

Requirement-to-coverage trace

diagram
COMPLIANCE TRACE MATRIX

spec clause ──> test(s) ──> assertion(s) ──> coverage bin ──> status
  4.2.1          t_axi_07     a_resp_ok        cov_resp        PASS
  4.3.5          t_axi_12     a_order          cov_order       WAIVED (id=W3)
  4.4.0          (none)        (none)           (none)          GAP  <- find these

A gap row is the most valuable output of the plan.

Root-cause tree to narrate

diagram
ROOT-CAUSE TREE — Compliance & Coverage Plan

spec feature coverage, compliance pass rate, waiver count looks wrong
        |
   reproducible?
     /        \
   no          yes
   |            |
 flaky env   same first transaction every time?
 / seed         /            \
              yes             no
               |               |
        protocol rule     timing/reset/PVT
        or config bug     or load-dependent

Protocol deep dive

Verification closes the gap between 'works in directed test' and 'legal under all stressed traffic'.

Concept diagram

diagram
VERIFICATION CLOSURE LOOP

spec clause -> test -> assertion -> coverage -> waiver -> signoff
                  ^                           |
                  +--------- gap found --------+

Metric graph

diagram
COVERAGE vs ESCAPE RATE

escapes
  |*
  | *
  |  *
  |   **  <- knee: more random helps
  |     ****
  +----------------> constrained-random depth

Metrics and artifacts to collect

  • compliance pass rate

  • coverage closure

  • scoreboard mismatch rate

  • assertion fire count

  • waiver log

Mini case study

Block passed VIP compliance but chip failed: system test omitted cross-master ID reuse through a bridge. Scoreboard key did not include upstream port ID.

Debug branches

  • If VIP pass but silicon fail, check integration assumptions.

  • If scoreboard noisy, fix key (ID+port+addr).

  • If coverage plateau, add compliance gap analysis not more repeats.

Senior review question

Ask: what is the first transaction that deviates, and which spec rule does it test?

Key takeaways

  • Connect every protocol claim to a transaction identity and measurable metric.

  • Store the artifact (waveform, log, counter) next to every signoff decision.

Common pitfalls

  • Debugging timeouts without finding the first bad transaction.

  • Quoting peak bus width without payload efficiency and retry overhead.

  • Treating VIP compliance as a substitute for system integration replay.

Interview whiteboard

Draw layers first, then place the failing transaction on the diagram.

Practice this lesson