How should pre-floorplan clock uncertainty be reviewed?

Short Answer

Clock uncertainty is a margin, and margins are easy to double-count if you're not disciplined about what each piece is actually covering — so the review is really about making sure every dollar of margin is spent exactly once. Break it into its real components: jitter (cycle-to-cycle timing noise from the clock source/PLL), modeling/methodology margin (a deliberate pad added because pre-CTS estimates are inherently imprecise), and the setup vs. hold split — many methodologies apply different uncertainty values for setup checks versus hold checks, since hold margin needs are structurally different.

Technical Explanation

• " If you can't point to why a specific value was chosen, that's the first thing to fix.
• Watch for duplication: if source latency, network latency estimates, AND clock uncertainty are all independently padded with the "same" safety margin, you're not being conservative — you're stacking unnecessary pessimism that can hide real violations behind excess slack, or push you to over-design.
• After CTS, part of this uncertainty (specifically the skew component) typically gets removed since real propagated skew is now measurable — so a pre-floorplan review should also confirm there's a documented plan for updating set_clock_uncertainty once real clock tree data exists, not that the pre-CTS number stays frozen forever.
• A good review artifact: a table per clock showing jitter value, methodology margin value, setup uncertainty, hold uncertainty, and a one-line justification for each — reviewable by someone who wasn't in the room when the numbers were picked.

What To Check

• Warning sign: A large generic value hides weak setup or creates meaningless hold pessimism.
• Inspect: Compare one named object across the related reports; the same object should tell a consistent story.
• Correct: Apply approved per-mode/per-check values and document their source.

Healthy, Suspicious & Hard-stop Results

• Expected: Separate jitter, modelling margin, and setup/hold uncertainty according to methodology; ensure it is not duplicated by other margins.
• 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. ”