Why check clock transition before CTS?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
An explicit pre-CTS transition can be a modelling assumption for cell delay; it is not a measured routed-clock slew.
Technical Explanation
Clock transition is the rise or fall time assumed at a clock pin. Before CTS it is an input assumption used for delay calculation.
What To Check
• Warning sign: The design uses an unrealistic default or a post-CTS value copied from another block. • Inspect: Compare one named object across the related reports; the same object should tell a consistent story. • Correct: Apply the characterised methodology assumption and keep it distinct from later propagated results.
Command Checks & Actions
• report_clocks: shows clock definitions and relationships
• report_clock_settings: shows latency, transition, uncertainty, and clock-analysis settings
Run the commands in order. Each line answers a separate part of the check.Healthy, Suspicious & Hard-stop Results
• Expected: An explicit pre-CTS transition can be a modelling assumption for cell delay; it is not a measured routed-clock slew. • Investigate: The design uses an unrealistic default or a post-CTS value copied from another block. • Stop before floorplanning when required logic or timing coverage is missing or unexplained. Apply the characterised methodology assumption and keep it distinct from later propagated results.
Common Mistake
Do not assume the check passed just because ICC2 continued: The design uses an unrealistic default or a post-CTS value copied from another block. 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 design uses an unrealistic default or a post-CTS value copied from another block.
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: Apply the characterised methodology assumption and keep it distinct from later propagated results. Then I would reopen the clean checkpoint and rerun the same evidence.”
Practical Example
Symptom: The design uses an unrealistic default or a post-CTS value copied from another block. Corrective path: Apply the characterised methodology assumption and keep it distinct from later propagated results.
Continue practising