Pulse Synchronizers: Debug Playbook

Debug playbook

Debug Playbook for Pulse Synchronizers focuses on pulse miss rate, minimum pulse width margin, duplication events. The goal is to convert issue observations into mechanism-backed closure decisions.

CDC/RDC debug is about finding the earliest violated assumption. Start with intent and context before touching low-level signal traces.

Root-cause tree

ROOT-CAUSE TREE — Pulse Synchronizers

crossing failure observed
        |
   reproducible?
     /        \
   no          yes
   |            |
stress mode   classify issue
expansion       /      |      \
            synchronizer protocol reset/reconvergence
                 |         |           |
            MTBF fit    liveness    release ordering

1. Freeze RTL/config/tool tags for reproducibility.

2. Reproduce in smallest mode/reset/traffic scenario.

3. Classify mechanism: synchronizer, protocol, reset, reconvergence, or governance.

4. Collect one decisive artifact that proves the class.

5. Pick minimal fix or bounded waiver.

6. Run targeted and full-regression matrices before closure.

Review memo template

STAFF CDC/RDC REVIEW MEMO — CDC Protocols & Handshakes / Pulse Synchronizers

1. Symptom
   - Failing metric: pulse miss rate, minimum pulse width margin, duplication events
   - Context: <mode, traffic, reset state, corner>
   - Risk class: <critical/high/medium/low>
   - Database tags: <rtl, config, assertions, tool setup>

2. Mechanism hypothesis
   - Primary mechanism: Pulse crossings require width expansion, toggle encoding, or ack-backed transport because short pulses can vanish between destination sampling edges.
   - Competing hypothesis: <false warning / protocol bug / reset order / reconvergence>
   - Missing evidence: <assertion, waveform, formal proof, stress replay>

3. Proposed action
   - Minimal reversible change: <sync/protocol/reset/waiver decision>
   - Expected metric movement: <critical count delta>
   - Regression risk: throughput, boot, latency, mode interaction

4. Signoff
   - Re-run artifact: pulse width assumptions, toggle synchronizer checks, simulation traces
   - Required owners: RTL owner, CDC owner, DV owner
   - Final decision: fix, bounded waiver, or escalate

CDC/RDC deep dive

Protocol correctness is the bridge between structural clean and functional safe.

Concept diagram

PROTOCOL FLOW

intent -> transport protocol -> synchronization -> destination acceptance

Metric graph

PROTOCOL ISSUE BURNDOWN

open issues: 20 -> 11 -> 5 -> 0

Reports and artifacts

• FIFO pointer proofs

• req/ack liveness

• pulse miss checks

• protocol assertions

Mini case study

Async FIFO empty/full logic looked correct until gray decode mismatch appeared during reset overlap.

Debug branches

• Pointer sync audit

• formal liveness checks

• reset interaction review

Senior review question

Ask: what evidence proves this risk is closed for silicon, not just tool-clean?

Key takeaways

• State crossing class, assumptions, and owner with every issue.

• Run structural and dynamic regressions after each fix.

Common pitfalls

• Treating all warnings as equivalent risk.

• Waiving issues without containment evidence.

• Skipping reset and reconvergence stress after CDC fixes.

Principal CDC/RDC review addendum

Pulse crossings require width expansion, toggle encoding, or ack-backed transport because short pulses can vanish between destination sampling edges.

Metric: pulse miss rate, minimum pulse width margin, duplication events