Async FIFO CDC: Interview Drills

Interview drills

Interview Drills for Async FIFO CDC focuses on overflow/underflow risk, pointer synchronization integrity, latency throughput. The goal is to convert issue observations into mechanism-backed closure decisions.

PROMPT
You see overflow/underflow risk, pointer synchronization integrity, latency throughput in Async FIFO CDC. Explain root cause and signoff decision.

STRONG ANSWER
1. Names crossing/reset class and context.
2. Explains Async FIFOs decouple domains with gray-coded pointers and synchronized status logic; correctness depends on pointer math, full/empty rules, and reset behavior.
3. Requests FIFO architecture diagram, pointer proof checks, formal safety properties.
4. Chooses fix/waiver/escalation with regression plan.

WEAK ANSWER
Lists tool commands or generic advice without mechanism and risk framing.

Diagram to draw on whiteboard

Async FIFO core

write clk domain                       read clk domain
wr_ptr(bin->gray) ---sync---> rd domain compare
rd_ptr(bin->gray) <---sync--- wr domain compare

full/empty must be computed with synchronized gray pointers.

Debug tree to narrate

ROOT-CAUSE TREE — Async FIFO CDC

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

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

Async FIFOs decouple domains with gray-coded pointers and synchronized status logic; correctness depends on pointer math, full/empty rules, and reset behavior.

Metric: overflow/underflow risk, pointer synchronization integrity, latency throughput