Why should input delay not be applied indiscriminately to clock ports?

Short Answer

A clock port isn't ordinary launched data — it's the time reference that every other path in the design is measured against, so treating it like any other input is a category error, not just a minor constraint mistake. set_input_delay says "this signal arrives relative to some clock, with this much delay" — but applying that to the clock signal itself creates a circular, nonsensical statement: you'd be defining the clock's arrival relative to itself (or another clock) as if it were ordinary data.

Technical Explanation

• The classic way this bug slips in is a broad, careless selector — something like set_input_delay ... [get_ports *] — which happily sweeps up your clock ports along with your real data inputs, because the wildcard doesn't know the difference.
• The consequence isn't a clean error; it's nonsensical downstream timing arcs and confusing violations that don't trace back cleanly to a root cause, because the tool is now trying to reconcile a clock definition with a conflicting data-delay constraint on the same port.
• g. [remove_from_collection [all_inputs] [get_ports clk]] or an equivalent filter), and treat any SDC script using unfiltered wildcards on ports as a code-review flag.

What To Check

• Warning sign: A blanket input-delay command constrains clocks and resets as data.
• Inspect: Compare one named object across the related reports; the same object should tell a consistent story.
• Correct: Select intended data ports explicitly and review collection membership before applying delays.

Healthy, Suspicious & Hard-stop Results

• Expected: A clock port defines timing reference behaviour and is not ordinary launched data; bulk get_ports * selectors can impose nonsensical data delay.
• 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. ”