BeginnerQuestion 11 of 40Source PDF page 14

What is slack?

From PDVerse STA Mentor Guide · pdVerse Mentor Guide

Short Answer

Slack is the margin by which a timing check passes or fails — the difference between the required time and the arrival time.

Example

If the required time at an endpoint is 930 ps and the data arrival time is 870 ps, the slack is 930 - 870 = +60 ps, meaning the path has 60 ps of margin to spare.

Why It Matters

Slack is the single number engineers scan across a whole design to find the worst (most negative) paths, making it the primary metric for judging whether a chip meets timing and where to focus fixing effort.

Equation

slack = required - arrival

Mentor Note — Common Mistake

Forgetting which direction the subtraction goes for setup versus hold — setup slack is required minus arrival, while hold slack is arrival minus required — mixing these up flips your pass/fail conclusion.

Visual explanationSlack compares required time with arrival time
Slack compares required time with arrival timeA horizontal timeline shows data arriving at 870 picoseconds, required at 930 picoseconds, leaving positive sixty picoseconds of slack.Arrival = 870 psRequired = 930 psSlack = 930 − 870 = +60 ps

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