When should a driving cell be used instead of set_input_transition?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
A driving cell models a library-based source response, while explicit transition provides a direct slew assumption; methodology should choose and avoid conflicting intent.
Technical Explanation
The input driver controls incoming slew, and the output load controls how hard an output must work. Both affect calculated cell delay even before physical implementation begins.
What To Check
• Warning sign: Both are applied inconsistently, or neither represents the real upstream interface. • Inspect: Compare one named object across the related reports; the same object should tell a consistent story. • Correct: Use the approved model, remove conflicting assumptions, and review reported port drive/transition data.
Command Checks & Actions
• report_ports -drive [get_ports *]: shows boundary constraints and electrical assumptions
Run the commands in order. Each line answers a separate part of the check.Healthy, Suspicious & Hard-stop Results
• Expected: A driving cell models a library-based source response, while explicit transition provides a direct slew assumption; methodology should choose and avoid conflicting intent. • Investigate: Both are applied inconsistently, or neither represents the real upstream interface. • Stop before floorplanning when required logic or timing coverage is missing or unexplained. Use the approved model, remove conflicting assumptions, and review reported port drive/transition data.
Common Mistake
Do not assume the check passed just because ICC2 continued: Both are applied inconsistently, or neither represents the real upstream interface. 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: Both are applied inconsistently, or neither represents the real upstream interface.
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: Use the approved model, remove conflicting assumptions, and review reported port drive/transition data. Then I would reopen the clean checkpoint and rerun the same evidence.”
Practical Example
Symptom: Both are applied inconsistently, or neither represents the real upstream interface. Corrective path: Use the approved model, remove conflicting assumptions, and review reported port drive/transition data.
Continue practising