Page 75 - Digital Electronics by harish

UNIT - III
SEQUENTIAL CIRCUITS
Sequential Circuits

In sequential circuits, the output depends not only on the present input conditions but
also on the previous output conditions i.e. the past history of the inputs. The past history is
provided by the feedback from the output back to the input.

3.1 Flip-flops

The basic building block for sequential logic circuits is the flip-flop(FF). Flip-flop is a
bi-stable logic element with one or more inputs and two outputs. The outputs are complement
to each other. A flip-flop can store one bit of binary data „0‟ or „1‟. Flip-flops are used in
counters, shift registers and memory devices. There are six types of flip-flops:

1. SR Flip-flop
2. Clocked SR flip-flop
3. JK flip-flop
4. JK Master Slave flip-flop
5. D flip-flop and
6. T flip-flop

Flip-flop is a one bit memory cell. It can store one bit of information.

3.1.1 SR (or RS) flip-flop

It is also called Set/Reset flip-flop. The logic symbol of SR flip-flop is shown in
figure:

Figure : Symbol of SR flip-flop

The SR flip-flop has two inputs and two outputs. The inputs are S (Set) and R (Reset).
The outputs Q and are complements to each other. i.e. when Q = 0, will be 1 and when Q
= 1, will be 0. The logic diagram of SR flip-flop using NAND gates and the truth table are
shown in figure.

75