The Return of Asynchronous Logic

Asynchronous design Virtually all digital design today is based on a synchronous approach. The total system is designed as the composition of one or more subsystems where each subsystem is a clocked finite state machine; the subsystem changes from one state to the next on the edges of a regular clock. The state is held in a set of flip-flops (registers), and combinatorial logic is used to derive the new state and outputs from the old state and inputs. The new state is copied through the flip-flops on every rising edge of the clock signal. Special techniques are required whenever a signal crosses into the domain of a particular clock (either from outside the system or from the domain of a different clock within the same system), but otherwise the system behaves in a discrete and deterministic way provided a few rules are followed; these rules include managing the delays of the combinatorial logic so that the flip-flop set up and hold times are met under all conditions. Asynchronous design does not follow this methodology; in general there is no clock to govern the timing of state changes. Subsystems exchange information at mutually negotiated times with no external timing regulation.