Introduction of Sequential Circuits

Digital Principles and Computer Organization:

Chapter 5: Sequential Circuits - Flip-Flops

Introduction

• There are many applications in which digital outputs are required to be generated in accordance with the sequence in which the input signals are received. This requirement cannot be satisfied using a combinational logic system. These applications require outputs to be generated that are not only dependent on the present input conditions but they also depend upon the past history of these inputs. The past history is provided by feedback from the output back to the input.

• Fig. 5.1.1 shows the block diagram of sequential Inputs circuit/Finite State Machine (FSM). As shown in Fig. 5.1.1, 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

Combinational circuits

1. In combinational circuits, the output variables are at all times dependent on the combination of input variables.

2. Memory unit is not required in combinational circuits.

3. Combinational circuits are faster in speed because the delay between input and output is due to propagation delay of gates.

4. Combinational circuits are easy to design.

5. Parallel adder is a combinational circuit.

 Sequential circuits

1. In sequential circuits, the output variables depend not only on the present input variables but they also depend upon the past history of these input variables.

2. Memory unit is required to store the past history of input variables in the sequential circuit.

3. Sequential Circuits are slower than the combinational circuits.

4. Sequential circuits are comparatively harder to design.

5. Serial adder is a sequential circuit.

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. 5.1.2. 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. 5.1.2.

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.