An edge-triggered device acts only at the instant its clock changes. It ignores its inputs at every other moment.
- A positive-edge-triggered device acts on the rising edge, where the clock goes 0 → 1.
- A negative-edge-triggered device acts on the falling edge, where it goes 1 → 0. Its symbol has a bubble on the clock pin.
The opposite is level sensitive: a latch responds for the whole time its enable is at the active level.
Why edge-triggering matters: in a real circuit, flip-flops feed each other. If they were level-sensitive, a value could ripple through several stages during one long high clock level, so how far data moved would depend on how long the clock stayed high. With edge-triggering, every flip flop samples at the same instant, using values from just before the edge, then updates. Data moves exactly one stage per clock. That is what lets shift registers, counters and pipelines work.
The price is a timing window. The input must be steady for a short setup time before the edge and a hold time after it.
A symbol hint: on a schematic, a small triangle on the clock pin marks an edge-triggered input.