BeginnerQuestion 18 of 40Source PDF page 19

What is clock skew?

From PDVerse STA Mentor Guide · pdVerse Mentor Guide

Short Answer

Clock skew is the difference in clock arrival time between two points — specifically, capture arrival time minus launch arrival time.

Example

If the launch clock arrives at a flop at 1.20 ns and the capture clock arrives at another flop at 1.35 ns, the skew is 1.35 - 1.20 = +0.15 ns, meaning the capture clock arrives later than the launch clock.

Why It Matters

The sign of skew matters: positive skew (capture arrives later than launch) helps setup timing but hurts hold timing, while negative skew does the opposite, so skew is a doubleedged factor designers must balance carefully. SOURCE LAUNCH FF CAPTURE FF arrives t=1.20 ns arrives t=1.35 ns skew = capture arrival − launch arrival = 1.35 − 1.20 = +0.15 ns

Equation

skew = capture arrival - launch arrival

Mentor Note — Common Mistake

Assuming skew is always harmless or always harmful — in reality, the same skew value can help one type of check (setup) while simultaneously hurting the other (hold), so you can't ignore either side of the tradeoff.

Visual explanationPositive clock skew
Positive clock skewOn a shared time axis, the capture-clock active edge arrives one hundred fifty picoseconds later than the launch-clock active edge.LAUNCH CLKCAPTURE CLKLaunch: 1.20 nsCapture: 1.35 ns+0.15 ns skewLater capture edge helps setup but hurts hold

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