negative edge

A negative edge is the instant at which a digital signal transitions from a high voltage state (logic 1) to a low voltage state (logic 0). In a timing diagram, it appears as the downward slope of the waveform. This moment matters because many digital circuits and microcontrollers are designed to trigger actions, capture data, or change state specifically when this transition occurs, rather than responding to the steady voltage level itself.

The term "negative" refers to the direction of change: the voltage is moving in the negative direction, from roughly 5 volts (or 3.3 volts in modern logic) down toward ground. The complementary event is the positive edge, when the signal rises from low to high. Some technicians call a negative edge a "falling edge" or "trailing edge", though falling edge is the more standard term in English-language datasheets and specifications.

Negative edges are detected by comparators, flip-flops, interrupt controllers, and counter circuits. A typical microcontroller input pin can be configured to trigger an interrupt on a negative edge, meaning the processor executes a specific routine the moment the signal drops. The speed at which the edge occurs matters: a slow, gradual transition can cause false triggering or timing errors, which is why real-world signals are often cleaned up with Schmitt trigger buffers that enforce sharp transitions.

The timing precision of edge detection is usually measured in nanoseconds. If two signals have negative edges that arrive within a few nanoseconds of each other, synchronization logic must decide which one is processed first. In synchronous digital systems, negative edges from a clock signal are often used to latch new data into shift registers and memory elements, so the timing of these edges directly affects the maximum speed at which the circuit can operate.

Noise and contact bounce are the main practical problems. A mechanical switch or a noisy analog signal may produce multiple false negative edges as it oscillates near the logic threshold. Debouncing circuits, RC filters, or software filtering routines suppress these glitches so the circuit sees only one clean edge per intended transition.