How do you validate clock period and waveform?

Short Answer

Two clocks can have the exact same period and still be completely different signals — the waveform (where the rising and falling edges actually sit inside that period) is what determines when your active clock edge really happens. Start with the period: compare what's reported [SDC: from create_clock -period] against the interface spec for every mode the chip supports — functional, test, low-power, etc. A clock that's correct in functional mode but wrong in scan mode will pass some checks and silently break others.

Technical Explanation

• Then check the waveform edges: create_clock -waveform {rise fall} defines exactly when the clock goes high and low within that period. If someone fat-fingers the waveform list, you can get a clock with the right period but the wrong duty cycle or edge polarity — and STA will happily analyze against the wrong edge.
• Pay special attention to non-50% duty cycle clocks — some interfaces (DDR-style, certain memory or PHY clocks) deliberately use asymmetric high/low times, and if your waveform assumes 50/50 when the spec says otherwise, every setup/hold check downstream is computed against the wrong edge.
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• Remember this validation happens before CTS — you're checking the constraint's correctness, not the physical clock tree, so errors caught here save you from chasing phantom timing failures later.

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.

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.
• Stop before floorplanning when required logic or timing coverage is missing or unexplained.

What The Interviewer Is Testing

Be ready to explain why this matters before floorplanning.

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. ”