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Chapter: Digital Logic Circuits - Synchronous Sequential Circuits

Design of synchronous sequential circuit

The design of a sequential circuit is the process of deriving a logic diagram from the specification of the circuits required behaviour.

Design of synchronous sequential circuit

The design of a sequential circuit is the process of deriving a logic diagram from the specification of the circuits required behaviour. The circuit behaviour is often expressed in words. The first step in the design is then to derive an exact specification of the required behaviour in terms of either state diagram or a state table. This is probably the most difficult step in the design, since no definite rules can be established to derive the state diagram or a state table.  The designer intuition and experience are the only guides. Once the description is converted into the state diagram or a state table, the remaining steps become mechanical. We will examine the classical design procedure through the examples in this section. It is not always necessary to follow this classical procedure, as some designs lend themselves to more direct and intuitive design methods. The classical design procedure consist of the following steps:

1.   Deriving the state diagram (and state table) for the circuit from the problem statement.

2.   Deriving the number of flip flops (p) needed for the design from the number of states in the state diagram, by the formula 


3.   Deciding on the types of flip flops to be used. (this often simply depends on the type of flip flops available for the particular design)

4.   Assigning a unique p bit pattern (state vector) to each state.

5.   Deriving the state transition table and the output table.

6.   Separating the transition table into p tables, one for each flip flop.

7.   Deriving an input using the excitation tables.

8.   Deriving input equations for each flip flop input and the circuit output equations.

9.   Drawing the circuit diagram.

 


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