What is slew (transition time), and why does it matter?

Short Answer

Slew, also called transition time, is how long a signal takes to swing from a low logic level to a high one, or back again. A slower slew adds extra delay to every gate downstream, wastes power, and makes a wire more vulnerable to noise from its neighbors.

Technical Explanation

Every signal in the design is really a sloped ramp, not a clean instant flip between 0 and 1, and how steep that ramp is turns out to matter almost everywhere in STA.

• How it is measured: the tool measures the time between two voltage thresholds on the ramp, commonly 10% and 90% of the supply voltage, or 20% and 80% on some modern libraries.
• Why a slow slew adds delay: a gate downstream cannot switch fully until its input crosses its own logic threshold, so a lazily rising input pushes that crossing point later, adding delay.
• Why it costs power: while a slow-rising input crosses the middle region, both halves of the receiving gate can be partially on at once, drawing extra current that a sharp edge would avoid.
• Why it matters for noise: a slow-moving victim wire sitting near a fast-switching aggressor wire is far easier to nudge off course by capacitive coupling than a wire that is already snapping through its transition.
• Why the tool enforces a limit: set_max_transition (PT) caps how slow any signal in the design is allowed to get, since delay tables lose accuracy and power blows up well past that limit.

Follow-up Question & Model Response

Why does a max-transition violation on one net sometimes cause the tool to report incorrect delay on a completely different net?

Candidate Model Response: Delay tables in the Liberty library are only characterized across a defined range of input slews, so once a signal's slew runs past that range, the tool has to extrapolate rather than look up a real value. That extrapolated delay can be inaccurate, and because that net's output slew feeds the next gate's input, the error can propagate downstream. This is exactly why max-transition is treated as a design rule check the tool enforces everywhere, not only on paths that already show a timing violation.