Asynchronous circuit

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Asynchronous circuits are electrical circuits that, unlike synchronous circuits, operate without a global clock . In a purely synchronous circuit, the clock signal is applied to all flip-flops in parallel, ie all flip-flops switch at the same time. In asynchronous circuits, not all flip-flops are directly connected to the same clock signal. The clock signal of a flip-flop can be, for example, the output signal of a preceding flip-flop or it can be the output signal of a logic gate.

This distinction is not relevant for arithmetic units ( datapath ) - they generally do not need a clock signal - but only for control units ( controlpath ).

Control units must be able to save their internal status. In asynchronous circuits, this is guaranteed either by integrated signal memories in the form of flip-flops or by the signal level of the incoming and outgoing signal lines from external signal memories. A distinction is made between external signals that are controlled by the environment and local signals that are controlled by the circuit itself.

Changes to the signal state can be made independently of one another or in parallel with asynchronous switching . At the theoretical level, this can be described by Signal Transition Graphs (STGs), a special form of Petri nets .

In the past , the use of asynchronous circuit technology has increasingly lost its importance in practical circuit development . With a synchronous circuit design, the time for the signal delay and the switching of the flip-flops can usually be determined or estimated quite well. With asynchronous circuit technology, this is usually only possible with a great deal of effort, since subsequent switching processes can each depend on the exact switching time of the preceding component. Furthermore, with the purely asynchronous circuits, there is a risk of design errors and switching errors, since only a minimum switching duration and a maximum switching duration are usually specified for the components in real operation and the actual switching time is somewhere within the specified range. Today, highly complex circuits are always designed as purely synchronous circuits.


An example of an asynchronous processor architecture is the AMULET processor, based on the ARM architecture .


  • J. Cortadella, M. Kishinevsky, A. Kondratyev, L. Lavagno and A. Yakovlev: Logic Synthesis for Asynchronous Controllers and Interfaces . Springer, 2001, ISBN 3-540-43152-7 .