In all sequential circuits, the effect of all previous inputs on the outputs is represented by
a state of the circuit. Thus, the output of the circuit at any time depends upon its current
state and the input. These also determine the next state of the circuit. The relationship
that exists among the inputs, outputs, present states and next states can be specified
by either the state table or the state diagram.
State Table
The state table representation
of a sequential circuit consists of three sections labelled present state, next state and output. The present state designates the state of flip-flops before the occurrence of a clock pulse. The next state shows the states of flipflops after the clock pulse, and the output section lists the value of the output variables during the present state.
of a sequential circuit consists of three sections labelled present state, next state and output. The present state designates the state of flip-flops before the occurrence of a clock pulse. The next state shows the states of flipflops after the clock pulse, and the output section lists the value of the output variables during the present state.
State Diagram
In addition to graphical symbols, tables or equations, flip-flops can also be represented
graphically by a state diagram. In this diagram, a state is represented by a circle, and
the transition between states is indicated by directed lines (or arcs) connecting the
circles. An example of a state diagram is shown below.
The binary number inside each circle identifies the state the circle represents. The
directed lines are labelled with two binary numbers separated by a slash (/). The input
value that causes the state transition is labelled first. The number after the slash symbol
/ gives the value of the output. For example, the directed line from state 00 to 01 is
labelled 1/0, meaning that, if the sequential circuit is in a present state and the input is
1, then the next state is 01 and the output is 0. If it is in a present state 00 and the input
is 0, it will remain in that state. A directed line connecting a circle with itself indicates
that no change of state occurs. The state diagram provides exactly the same
information as the state table and is obtained directly from the state table.
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