DenseChip

CDC / RDC · All levels

Scenario: Async FIFO Pointer Mismatch

An async FIFO between ISP and memory controller reports occasional empty when writes are active. Pointer sync and gray decode assumptions differ across blocks.

Scenario

An async FIFO between ISP and memory controller reports occasional empty when writes are active. Pointer sync and gray decode assumptions differ across blocks.

diagram
OBSERVED METRIC
sporadic underflow with no functional traffic violation

45-MINUTE INTERVIEW FLOW
0-5: classify crossing/reset scope
5-15: explain mechanism and assumptions
15-25: request and interpret evidence
25-35: propose fix/waiver/escalation
35-45: define regression + signoff

Common pitfalls

  • Debug data-path only; ignore pointer crossing logic

  • Waive gray-code warning without transition proof

  • Fix full/empty logic without reset alignment review

Scenario debrief

Score each response on mechanism clarity, evidence quality, and signoff decision discipline.

diagram
INTERVIEW FLOW

classify -> explain mechanism -> propose fix -> state regression
diagram
PRACTICE SCORE

session1 62%
session2 78%
session3 89%

Debrief prompts

  1. Which crossing class or reset dependency failed first?

  2. What evidence changed your decision?

  3. Fix, waive, or escalate and why?

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.

Practice this lesson