Latches and flip-flops

Abstract

The power of digital electronics stems from its ability to provide and use memory elements. Instead of output signals being dependent only on the present state of the inputs they can be made dependent on the history of inputs applied to the circuit. A simple example shows the difference between these two operating conditions. With combinational logic the only possible outputs from one input (X) are 0, 1, X and X, and only X requires the intervention of any electronics. One possibility for a circuit with memory is for the outputs to register the number of times a single input has changed from 0 to 1, i.e. count the number of input pulses. With 10 outputs 1024 (2¹⁰) different output states are possible, and, if a binary representation is chosen, the circuit can count from 0 to 1023. To obtain a larger counting range, the circuit needs changing to provide more outputs but no change is required to the number of inputs. Circuits containing combinational and memory elements are known as sequential circuits.