Digital computers consist of the digital logic circuits. The digital logic circuits can be divided into two categories. One is the combinational circuit and another one is the sequential circuit. In this article, we are going to discuss the major difference between combinational and sequential circuits in details.
But before moving on to the difference between combinational and sequential circuits, first, you must know what is combinational and sequential circuits. So, let’s discuss what is combinational and sequential circuits first.
What is combinational and sequential circuits :
The combinational circuit consists of logic gates, where outputs are at any instant determined only by the present combination of inputs without regard to previous inputs or previous state of the output.
On the other hand, a sequential circuit contains logic gates, as memory says their outputs depend on the present input and also on the states of the memory location and elements. That means in the sequential circuit the outputs depends on the inputs as well as previous output or outputs.
This is the basic difference between combinational and sequential circuits. But there are some more important things that we can see. For example, An adder is a combinational circuit but a counter is a sequential circuit. But why?
Why adder is a combinational circuit but a counter is a sequential circuit?
In adder, if you are adding a single bit, say you are adding 1 with 0, then what you are going to get? You are definitely going to get 1. So, is there any use of the previous output in this adder? Definitely not. There is no need for the previous output in this particular addition. So, it is satisfying our condition of combinational circuit i.e. the outputs depend only the present combination of inputs. But what will happen if I say about a counter? Let’s see.
In counter what actually happens in your counting devices like stopwatch? It just increments 1 to the previous output. for example, if it is counting 5, then the previous output was 4. So, it added 1 to 4 to make the output 5. For this type of operation, it requires the information of the previous outputs. Whereas in case of the combinational circuit, it doesn’t require the information of the previous output.
Now the question is how this information of previous output is stored in the sequential circuit? It is done by a memory element called the flip-flop. You can read more about the different types of flip-flops here.
So far we have discussed the basic concept of combinational and sequential circuits and I think you have also got the concept well. Now let’s come to our main topic of discussion i.e. the difference between combinational and sequential circuits.
Difference between combinational and sequential circuits :
|Combinational circuit||Sequential circuit|
|The digital logic circuit whose outputs can be determined using the logic function of the current state inputs.||The digital logic circuits whose outputs can be determined using the logic function of the current state inputs and past state inputs as well.|
|This circuit doesn’t have the capability to store a state inside them.||This circuit is capable to retain the earlier state of the system based on the current inputs and earlier states.|
|The combinational circuit doesn’t contain any memory element.||The sequential circuit contains memory elements which are capable of storing data in a digital circuit.|
|The combinational circuits are implemented using different types of devices such as adders, subtractor, encoders, decoders and multiplexer. This circuits are represented with boolean algebra and simplified using the basic and universal gate.||In the sequential circuit, a latch is considered as the simplest element used to retain the earlier state. Latches can also be called as flip-flops.|
|These circuits are independent of the clock||These circuits use a clock for triggering the flip-flops.|
|The combinational circuits don’t require any feedbacks.||The sequential circuits utilize the feedback from the outputs to inputs.|
|As there are no clocks used, so they don’t need any triggering.||If the flip-flops are triggered, then the circuits are called as synchronous sequential circuit and which are not triggered are called asynchronous sequential circuit.|