How do you validate clock period and waveform?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Compare reported period and edge waveform with the interface specification for each mode, including non-50-percent duty cycles when relevant.
Technical Explanation
The period tells you when a clock repeats; the waveform tells you where its active edges occur. Correct period with wrong edges is still a wrong clock.
What To Check
• Warning sign: The period is correct but active edges or duty cycle are wrong. • Inspect: Compare one named object across the related reports; the same object should tell a consistent story. • Correct: Correct the owning clock definition and inspect paths using both intended edges.
Command Checks & Actions
• report_clocks: shows clock definitions and relationships
Run the commands in order. Each line answers a separate part of the check.Healthy, Suspicious & Hard-stop Results
• Expected: Compare reported period and edge waveform with the interface specification for each mode, including non-50-percent duty cycles when relevant. • Investigate: The period is correct but active edges or duty cycle are wrong. • Stop before floorplanning when required logic or timing coverage is missing or unexplained. Correct the owning clock definition and inspect paths using both intended edges.
Common Mistake
Do not assume the check passed just because ICC2 continued: The period is correct but active edges or duty cycle are wrong. Fix or narrowly justify the named objects, then rerun the same command.
What The Interviewer Is Testing
Be ready to explain why this matters before floorplanning: The period is correct but active edges or duty cycle are wrong.
Follow-up Question & Model Response
Model response: “I would save the report, inspect one affected object in the correct block and scenario, and make or request this correction: Correct the owning clock definition and inspect paths using both intended edges. Then I would reopen the clean checkpoint and rerun the same evidence.”
Practical Example
Symptom: The period is correct but active edges or duty cycle are wrong. Corrective path: Correct the owning clock definition and inspect paths using both intended edges.
Continue practising