Design of Asynchronous Sequential Machine :
Analysis and design are the two exactly opposite processes. The design procedure for the asynchronous sequential machines is as follows :
(1) Construct a primitive flow table from the given specifications.
(2) Develop the state diagram.
(3) Eliminate the redundant states by state reduction.
(4) Make the state assignment. This is different for asynchronous machines as compared to that for synchronous machines.
(5) Derive a set of excitation and output equations from the reduced flow tables. These equations are then checked to see the presence of static and dynamic hazards, to ensure correct circuit operation.
Deriving the Flow Tables :
• The state tables are used in design of the synchronous sequential circuits. Similarly flow tables are used to design the asynchronous sequential circuits.
• The state change in asynchronous machines occurs independent of clock, based on the logic propagation delay and so causes the states to “flow†from one to the other. Hence the name “flow tablesâ€.
Primary Flow Table :
• It is the special case of the general flow table. In the primary flow table, there is only one stable state for each row in the table.
• Only one input variable should change at any instant of time while constructing the primary flow table.
• Let us discuss the development of primary flow table with the help of following example.