What is Sequential Circuit & Its Types?

In this tutorial, we are going to learn about Sequential Circuit & Its Types.

Sequential Circuit

  • It is a logical circuit, which is the output depends on the present value of the input as well as the sequence of past inputs/outputs.
  • Sequential Circuit is a combinational circuit with memory.
  • The output of Sequential Circuit depends upon present inputs and present state.
  • As per below diagram of Sequential Circuit can see the information stored in sequential circuit represents present state.
  • The present state and present inputs will define output and input state.

Below is the main point of Sequential Circuit

  • Output Variable at any instant of time depends not only on present input but also on previous outputs.
  • Memory Unit is required for this circuit.
  • It is slow circuit.
  • It is design to difficult.
  • In this circuit, The present output depends on the present input as well as past output/outputs.

Types of Sequential Circuit

  1. Asynchronous
  2. Synchronous
Asynchronous Sequential Circuit   Synchronous Sequential Circuit  
The change in input signal can affect memory element upon activation of the clock signal. The change in input signal can affect memory element at any instant of time.
The maximum operating speed of clocks depends on the time delays involved. Because of the absence of a clock, this circuit can operate faster.
In this circuit memory elements are clocked flip flop. Memory elements are either unclocked flip flop or time delay elements
It is easier to design It is more difficult to design
It is generally ‘ edge triggered’ It is generally ‘level triggered’
Low Cost It has more cost

Below are the table for Synchronous

Present State Next State Output
A                        B A                        B                         Y
0                         0 0                         0                        1
0                         1 1                         1                          0
1                         0 1                         0                          0
1                         1 0                         0                          0

Below are the table for Asynchronous

Present State Next State Output
A                        B A                        B Y                       
0                         0 0                         0 0                        
0                         1 1                         1 0                        
1                         0 1                         0 0                        
1                         1 0                         0 1                        

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