How do you debug a generated clock's source and master?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
The reported master, source object, target pin, divide/multiply factor, phase, and edge mapping must match real logic connectivity.
Technical Explanation
A generated clock is derived from another clock through real logic such as a divider or MUX. Its source and master relationship must follow that logic.
What To Check
• Warning sign: The clock name exists but derives from the wrong pin or an unrelated master. • Inspect: Compare one named object across the related reports; the same object should tell a consistent story. • Correct: Correct the generated-clock definition and prove the source path and sink coverage.
Command Checks & Actions
• report_clocks: shows clock definitions and relationships
• check_timing -include {generated_clock no_clock}: finds missing or inconsistent timing setup
Run the commands in order. Each line answers a separate part of the check.Healthy, Suspicious & Hard-stop Results
• Expected: The reported master, source object, target pin, divide/multiply factor, phase, and edge mapping must match real logic connectivity. • Investigate: The clock name exists but derives from the wrong pin or an unrelated master. • Stop before floorplanning when required logic or timing coverage is missing or unexplained. Correct the generated-clock definition and prove the source path and sink coverage.
Common Mistake
Do not assume the check passed just because ICC2 continued: The clock name exists but derives from the wrong pin or an unrelated master. 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 clock name exists but derives from the wrong pin or an unrelated master.
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 generated-clock definition and prove the source path and sink coverage. Then I would reopen the clean checkpoint and rerun the same evidence.”
Practical Example
Symptom: The clock name exists but derives from the wrong pin or an unrelated master. Corrective path: Correct the generated-clock definition and prove the source path and sink coverage.
Continue practising