Setup time and hold time are timing constraints for flip-flops. Which statement is true?

• A. Hold time is the minimum time data must be stable before the clock edge
• B. Setup time is the minimum time data must be stable before the clock edge
• C. Hold time is the maximum time data can be stable after the clock edge
• D. Setup time is the minimum time data must be stable after the clock edge

Answer: (B)

Timing around a clock edge centers on two requirements: setup time and hold time. Setup time is the minimum period that the data must be stable before the clock edge so the flip-flop can reliably capture it as the clock edge occurs. If data isn’t stable long enough before the edge, the flip-flop may sample an incorrect value. Hold time is the minimum period that the data must stay stable after the clock edge to ensure the captured value doesn’t change during the sampling window. If data changes too soon after the edge, the result can also be unreliable.

So the true statement is that setup time is the minimum time data must be stable before the clock edge. The other options mix up before/after or describe hold time incorrectly (hold time is after the edge, not a maximum time).