18.5 State Tables – Pulse and Digital Circuits


The state table representation of a sequential circuit consists of three sections, labelled as the present state, next state and the output. The present state designates the state of flip-flops before the occurrence of a clock pulse. The next state shows the state of flip-flops after the clock pulse, and the output section lists the value of the output variables during the present state.

18.5.1 State Diagrams

In addition to the graphical symbols, tables or equations, counter can also be represented pictorially by a state diagram. In this diagram, a state is represented by a circle, and the transition between the states is indicated by directed lines (or arcs) connecting the circles. An example of a state diagram is shown in Fig. 18.24.

The binary number inside each circle identifies the state of the counter. 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 as 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.