IntermediateQuestion 5 of 20Source PDF page 39

What's the difference between clock uncertainty and clock jitter?

From PDVerse STA Mentor Guide · pdVerse Mentor Guide

Explanation

Clock jitter is the clock edge itself wobbling cycle to cycle at its source — an inherent property of the clock generator. Clock uncertainty is a margin the timing tool is told to subtract from the setup/hold budget to account for jitter plus other unknowns (like unmodeled skew), and it is set explicitly via set_clock_uncertainty rather than measured directly from the tree.

Timing-path Walkthrough

Pre-CTS, uncertainty is often set conservatively high to cover both jitter and an estimate of tree skew that isn't known yet — e.g., set_clock_uncertainty 120 in the worked example. Post-CTS, once the real tree exists and skew is now propagated and accounted for directly, uncertainty can be tightened to just jitter plus margin — e.g., set_clock_uncertainty 45.

Command

set_clock_uncertainty -setup / -hold

Switch-by-switch

-setup and -hold let uncertainty differ between the setup and hold checks; -from / -to apply uncertainty specifically between a named pair of clocks (inter-clock) rather than globally.

Expected Report Behavior

The uncertainty value directly subtracts from data required time in report_timing; tightening it post-CTS (120 to 45 in the example) recovers slack that was conservatively reserved pre-CTS.

Mentor Note — Common Mistake

Leaving a large pre-CTS uncertainty value in place after CTS, needlessly burning margin that should have been freed up once real skew is propagated.

How To Debug

Compare the uncertainty value applied at different design stages against whether skew is now propagated; if propagated skew is active, verify uncertainty was reduced accordingly, as in the 120 to 45 worked transition.

Visual explanationSTA context: What's the difference between clock uncertainty and clock jitter?
STA context: What's the difference between clock uncertainty and clock jitter?A three-step concept map summarizes the focus, core answer, and practical verification for What's the difference between clock uncertainty and clock jitter?Question focusWhat's the differencebetween clockuncertainty and clockjitter?Core answerClock jitter is theclock edge itselfwobbling cycle to cycleat…Verify in practiceset_clock_uncertainty-setup / -holdUnderstand → explain the mechanism → verify the assumptions

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