Flip-flops and related devies

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10
Flip-Flops and Related Devices

Having discussed combinational logic circuits at length in previous chapters, the focus in the present chapter and in Chapter 11 will be on sequential logic circuits. While a logic gate is the most basic building block of combinational logic, its counterpart in sequential logic is the flip-flop. The chapter begins with a brief introduction to different types of multivibrator, including the bistable multivibrator, which is the complete technical name for a flip-flop, the monostable multivibrator and the astable multivibrator. The flip-flop is not only used individually for a variety of applications; it also forms the basis of many more complex logic functions. Counters and registers, to be covered in Chapter 11 are typical examples. There is a large variety of flip-flops having varying functional tables, input clocking requirements and other features. In this chapter, we will discuss all these basic types of flip-flop in terms of their functional aspects, truth tables, salient features and application aspects. The text is suitably illustrated with a large number of solved examples. Application-relevant information, including a comprehensive index of flip-flops and related devices belonging to different logic families, is given towards the end of the chapter. Pin connection diagrams and functional tables are given in the companion website.

10.1 Multivibrator
Multivibrators, like the familiar sinusoidal oscillators, are circuits with regenerative feedback, with the difference that they produce pulsed output. There are three basic types of multivibrator, namely the bistable multivibrator, the monostable multivibrator and the astable multivibrator.

10.1.1 Bistable Multivibrator
A bistable multivibrator circuit is one in which both LOW and HIGH output states are stable.
Irrespective of the logic status of the output, LOW or HIGH, it