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The meaning of word asynchronous is without synchronization. The asynchronous sequential circuits are the circuits without a synchronizing clock. The asynchronous sequential circuits do not use the clock. The synchronous machines need to take into consideration internal time delays caused by Flip Flops, and other logic, before deciding the clock speed. Due to the absence of the clock signal, there is no such restrictions on the asynchronous machines. However due to lack of synchronizing clock, the asynchronous sequential circuits are more difficult to design. The asynchronous sequential circuits are faster than the synchronous circuits due to the lack of clock signal. In the synchronous sequential circuits, the clocked flip-flops are used as memory elements. But in the asynchronous sequential circuits, the unclocked flip-flops or delay elements are used as memory elements. The comparison of synchronous and asynchronous sequential circuits is illustrated in Table below.

Practical Area of Applications :
• The asynchronous sequential systems are preferred in those digital systems which have several subsystems or modules.
• Asynchronous sequential circuits are also preferred for very fast digital systems.
• For example consider transmission of data from one computer to the other. If the distance between the two computer systems is too large for the parallel synchronous transmission, then asynchronous serial transmission is preferred.
• The LSI devices used to accomplish this task is called as UART (universal asynchronous receiver transmitter).
• In UART the internal control is synchronous but the data transfer is asynchronous.

Advantages of Asynchronous Machines :
1. They are fast.
2. Synchronization is not needed.
3. Clock signal is not used.

Disadvantages :
1. These circuits are difficult to design.
2. Care must be taken when selecting the proper state assignment, so that the delays are predictable and unwanted state transitions are avoided