A generated clock exists by name but has no valid path from its master. How do you diagnose it?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
The name proves object creation only; inspect master, source, target, transformation, case analysis, disabled arcs, and real netlist 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: Registers downstream remain unclocked or the generated-clock check warns despite a reported clock. • Inspect: Keep at least two possible causes open, then use one named object and the active mode or scenario to separate them. • Correct: Correct the owning generated-clock definition or netlist connection and prove source-to-target reachability.
Command Checks & Actions
• report_clocks: shows clock definitions and relationships
• check_timing -include {generated_clock no_clock}: finds missing or inconsistent timing setup
• report_disable_timing: shows timing arcs that cannot propagate
Run the commands in order. Each line answers a separate part of the check.Healthy, Suspicious & Hard-stop Results
• Expected: The name proves object creation only; inspect master, source, target, transformation, case analysis, disabled arcs, and real netlist connectivity. • Investigate: Registers downstream remain unclocked or the generated-clock check warns despite a reported clock. • Stop before floorplanning when required logic or timing coverage is missing or unexplained. Correct the owning generated-clock definition or netlist connection and prove source-to-target reachability.
Common Mistake
Do not assume the check passed just because ICC2 continued: Registers downstream remain unclocked or the generated-clock check warns despite a reported clock. 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: Registers downstream remain unclocked or the generated-clock check warns despite a reported clock.
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 generated-clock definition or netlist connection and prove source-to-target reachability. Then I would reopen the clean checkpoint and rerun the same evidence.”
Practical Example
Symptom: Registers downstream remain unclocked or the generated-clock check warns despite a reported clock. Corrective path: Correct the owning generated-clock definition or netlist connection and prove source-to-target reachability.
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