Introduction to Sequential Circuits

UNIT: II 

Chapter 2: Synchronous Sequential Logic

Syllabus

Introduction to Sequential Circuits - Flip-Flops - operation and excitation tables, Triggering of FF.

Introduction to Sequential Circuits 

•Fig. 2.1.1 shows the block diagram of sequential circuit/Finite State Machine (FSM).

• Memory elements are connected to the combinational circuit as a feedback path.

• The information stored in the memory elements at any given time defines the present state of the sequential circuit.

• The present state and the external inputs determine the outputs and the next state of the sequential circuit.

• Thus we can specify the sequential circuit by a time sequence of external inputs, internal states (present states and next states) and outputs.

• The counters and registers are the common examples of sequential circuits.

The memory element used in sequential circuits is a flip-flop which is capable of storing 1-bit binary information.

Comparison between Combinational and Sequential Logic Circuits

Clock

• A clock signal is a particular type of signal that oscillates between a high and a low state and is utilized to co-ordinate actions of circuits.

• It is produced by a clock generator.

• The most common clock signal is in the form of a square wave with a 50% duty cycle, usually with a fixed, constant frequency as shown in Fig. 2.1.2.

• Circuits using the clock signal for synchronization may become active at either the rising edge, falling edge or in the case of double data rate, both in the rising and in the falling edges of the clock cycle.

• The time required to complete one cycle is called 'clock period' or 'clock cycle'.

• Ideally, the clock signal should have sharp transitions from one level to other as shown in Fig. 2.1.2.

Review Questions

1. Define sequential logic circuit.

2. What is flip-flop?

3. Give the comparison between combinational and sequential logic circuits.

4. What is clock? State its use.