BeginnerQuestion 16 of 40Source PDF page 17

What is clock uncertainty?

From PDVerse STA Mentor Guide · pdVerse Mentor Guide

Short Answer

Clock uncertainty is a margin subtracted from the available timing budget to account for imperfections in the clock, chiefly jitter, plus any additional margin or guardband the designer wants to add.

Example

Before clock tree synthesis (CTS), a designer might set set_clock_uncertainty 120 to cover both jitter and an estimated amount of tree skew that doesn't exist as a real, built structure yet; after CTS, since skew is now actually propagated through the real tree, that number can shrink to set_clock_uncertainty 45, covering just jitter and margin.

Why It Matters

Uncertainty directly eats into the setup budget — in the worked example, adding 40 ps of uncertainty turned a +25 ps passing slack into a -15 ps failing slack, showing how a seemingly small margin can flip a path's verdict.

Equation

required (with uncertainty) = period - setup - uncertainty; required (without) = period - setup

Command

set_clock_uncertainty 120 (pre-CTS, covers jitter + estimated tree skew); set_clock_uncertainty 45 (post-CTS, jitter + margin only)

Mentor Note — Common Mistake

Using the same, larger pre-CTS uncertainty value after the real clock tree has been built — this double-counts skew that is now already modeled explicitly, making the analysis overly pessimistic.

Visual explanationSTA context: What is clock uncertainty?
STA context: What is clock uncertainty?A three-step concept map summarizes the focus, core answer, and practical verification for What is clock uncertainty?Question focusWhat is clockuncertainty?Core answerClock uncertainty is amargin subtracted fromthe available timingbudget to…Verify in practiceset_clock_uncertainty120 (pre-CTS, coversjitter + estimated treeskew);…Understand → explain the mechanism → verify the assumptions

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