Introduction

In the previous posts we have focused on the design of many combinational circuits. The outputs in those circuits were dependent on the inputs applied at that instant of taking the output. Examples of such circuits were adders like half and full adders, half and full subtractors, BCD adders, Multiplexers, decoders , etc.

But what if we have to add say 5, 10 or even more numbers. Practically we only two numbers at a time, store the partial result in our memory and then add this partial result(memory content) with the next numbers to get next partial sum and so on till all the numbers have been added. To save the partial results we need  memory device. Such types of circuits which employs memory in addition to combinational(or computational) block are the sequential circuits which is describe in the following section.

Sequential Circuits

A sequential circuit is one which can remember its state. This state is stored in a memory element. So a sequential circuit can be defined as the one that store and use the previous state information to determine/compute their next state.

This is exactly how the sequential circuits work. Figure below shows the block diagram of a sequential circuit. The output in a sequential circuit depends on the previous state and the present input. The output state is stored in a memory element for next state operation.

Figure-1: Block diagram of Sequential Circuit

A sequential circuit thus needs the combinational circuit for processing and the memory element for storage of the state or the intermediate result.

The output of the memory element is used to provide feedback to the combinational circuit.

 

Types of Sequential Circuit

1. a)         Synchronous Circuit: In these circuits the synchronizations is achieved by applying a common clock to all the units.

1. b)         Asynchronous circuits: In these circuits, there is no common clock. Clock to all units come from independent or separate sources. in an asynchronous circuit, events are allowed to occur without any synchronization. In such a case, the system becomes unstable which results in difficulties.

Examples of sequential machines:

Flip-Flops, Registers, Counters etc. are the examples of sequential circuits.

Memory element:

Since one of the requirements of a sequential circuit is the memory element, we will now explain a memory element that is used to store 1-bit of information.

1-bit memory element:

Memory stores the binary information. The simplest way of constructing a memory element is to use two inverters connected such that the output of one is connected to the input of the other. Figure-2 shoes such an arrangement.

1-bit Memory

 

If somehow a 1 is inputted at the input of the 1^st NOT gate, Q1 will be ‘0’ and Q= Q1’ will be ‘1’, similarly, If somehow a ‘0’ is inputted at the input of the 1^st NOT gate, Q1 will be ‘1’ and Q= Q1’ will be ‘0’.

It is found that the output attains one of the two stable states. That is, output Q1 if set or reset remain in the same state for infinite time. We can say that the circuit has remembered the state (1-bit value 0 or 1). This is the reason that the above circuit is also called a 1-bit memory element.