A falling edge (or negative edge) is a 1 → 0 transition: the instant a signal drops from high to low. It is the mirror image of the rising edge.
A negative-edge-triggered flip flop samples its inputs at each falling clock edge and ignores the rising ones. Its symbol has a small bubble on the clock input, next to the triangle. Without the bubble, assume positive-edge triggering.
Finding falling edges on a slotted waveform:
- Look for a slot where the signal is 0 and the slot before it is 1.
- A long low stretch holds only one falling edge, at its start.
- A signal that starts at 0 has no falling edge in its first slot.
Where they appear: some designs clock half their flip-flops on each edge to do work twice per cycle. In a ripple counter, each stage is clocked by the previous stage's output, and whether it counts up or down depends on which edge the stages use. Inverting a clock turns its falling edges into rising ones, so a negative-edge flip-flop behaves like a positive-edge one with an inverted clock.