Difference between Combinational and Sequential logic circuits.

Combinational and Sequential circuits are the most essential concepts to be understood in digital electronics.

Combinational logic (sometimes also referred to as time-independent logic) is a type of digital logic which is implemented by Boolean circuits, where the output is a pure function of the present input only.




clc


Sequential logic is a type of logic circuit whose output depends not only on the present value of its input signals but on the sequence of past inputs.

slc

Combinational Logic Circuits Sequential Logic Circuits
Output is a function of the present inputs (Time Independent Logic). Output is a function of clock, present inputs and the previous states of the system.
Do not have the ability to store data (state). Have memory to store the present states that is sent as control input (enable) for the next operation.
It does not require any feedback. It simply outputs the input according to the logic designed. It involves feedback from output to input that is stored in the memory for the next operation.
Used mainly for Arithmetic and Boolean operations. Used for storing data (and hence used in RAM).
Logic gates are the elementary building blocks. Flip flops (binary storage device) are the elementary building unit.
Independent of clock and hence does not require triggering to operate. Clocked (Triggered for operation with electronic pulses).
Example: Adder [1+0=1; Dependency only on present inputs i.e., 1 and 0]. Example: Counter [Previous O/P +1=Current O/P; Dependency on present input as well as previous state].

11 comments for “Difference between Combinational and Sequential logic circuits.

  1. elango
    August 17, 2016 at 9:17 am

    thank you for given valuable information

  2. Gautam Gupta
    September 12, 2017 at 6:47 pm

    in combinational ckt we are using gate but in sequential ckt what will we use to designe

    • Sidhartha
      September 19, 2017 at 12:37 pm

      Hey Gautam,
      To design sequential circuit also we use gates. But in case of sequential circuit we use feed back gates (memory elements) along with combinational logic. So that it can work like a latch or flip-flop. If you still have doubt, let us know. We would explain it in a detailed way.

  3. Ruchita
    October 7, 2017 at 3:17 am

    Thankyou! 🙂

    • Sidhartha
      October 13, 2017 at 11:43 am

      Welcome Ruchita 🙂

  4. Sagnik
    October 15, 2017 at 8:22 pm

    In sequential circuit block diagram ,it is said that some outputs are again feedbacked to inputs,after storing them in memory.
    But,where is that memory in sr latch.In sr latch ,it is seen that the output is directly taken to the inputs,and their is no memory.

    • Sidhartha
      October 22, 2017 at 7:34 pm

      Dear Sagnik,

      Memory in the block diagram shows a delay for which the previous output can be hold and can be used as the feedback input in the present clock cycle. In the SR latch the gate delay in the NAND/NOR gates works like the memory block in the block diagram. So, there is no explicit memory block in the SR latch.

  5. tom z prude
    December 31, 2017 at 7:21 am

    thank you,,,,,,but i wanna to ask u examples of each circuits

  6. Raj Bhattacharyya
    January 3, 2018 at 3:18 pm

    thank you for helping the difference between the two circuit

  7. Raj Bhattacharyya
    January 3, 2018 at 3:19 pm

    thank you for giving the information.

  8. Raj Bhattacharyya
    January 3, 2018 at 3:21 pm

    thanks

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