Digital systems and design

What is sequential and combinational logic

What is sequential and combinational logic

This post answers the question “What is sequential and combinational logic?”. These two circuits are very important and broad categories of circuits defined in digital electronics where the combinational circuits are independent of time and the sequential circuits are dependent on time. 

The combinational logic circuit is a circuit whose output will depend only on the input variables. As the name indicates the output of a combinational circuit will depend only on combinational input variables given. So generally this is a logic circuit that has different logic gates integrated into its circuit.

Combinational circuits features

  • Output is dependent upon the combination of input variables
  • It doesn’t depend upon any previous input or output
  • Doesn’t have any memory
  • It can have n number of inputs and m number of outputs

Examples of combinational circuit

  • Adders and Subtractors
  • Decoders
  • MUX
  • Code Converters
  • Comparators
  • ROM
  • PLA
  • PAL

A sequential circuit is generally a combinational circuit with memory. In a sequential circuit, the present output depends on the present input as well as the present state(past output). Here the present state depends upon past inputs only. That means past inputs are past output and the present state is nothing but past outputs. The information stored in the sequential circuit represents the present state. The present state and present inputs will define output and new state. 

You can see inputs and outputs on the diagram and also at the same time there is a storing process happening that particular output on outputs on the memory block.

This memory will keep the output till the time the it will be going to be used again. Let’s say time comes and it’s time to use the values stored in this memory and these values will be used as the inputs.  So as a result there is a present inputs and the outputs and also the memory element which has already stored the output from the previous inputs. 

So this will become past output and this past output will go to the input and it can be called as the feedback will go as the feedback and the present output will depend upon this present input and the past output. So this is sequential circuit.

In a sequential circuit, everything is combinational except the memory and feedback.

Comparison Between Combinational and Sequential Circuit

Combinational Circuit Sequential Circuit
Output is only depending on present input Output is depending on present input and past output
Has no feedback Has Feedback
Has no memory Has memory
Example: HA/FA, HS/FS, Mux/Demux, etc Example: Counter, register, flip-flop etc