Page 118 - Handout Digital Electronics

LECTURER 15 SEQUENTIAL CIRCUIT EQUATIONS
15.1 Typical sequential circuit diagram

The diagram below shows a typical example of a sequential circuit. The AND gates, OR gates and the
INVERTER (NOT gate) form the combinational logic part of the circuit. The two D flip-flops form the
storage part of the sequential circuit.

The interconnections among the gates in the combinational circuit can be specified by a set of Boolean
expressions, for example DA = Ax + Bx (A & B are the outputs of the two D flip flops and x is the
external input). DB = Ax is derived from the single AND gate whose output is connected to the D input
of flip flop B. There is also an external output Y which is a function of the input variable and the state
of the flip flops, for example Y = Ax + Bx (state equation). The behavior of a sequential circuit is
derived from inputs, outputs and state of the flip flops.

Figure 52: An example of a sequential circuit

The sequential circuit state table

Present State Input Next State Output
A B X A B Y
0 0 0 0 0 0
0 0 1 0 1 0
0 1 0 0 0 1
0 1 1 1 1 0
1 0 0 1 0 1
1 0 1 1 0 0
1 1 0 0 0 1
1 1 1 1 0 1

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