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What is the reason that JK flip flop is often preferred over SR flip flop?

Expert Answer

  • SR stands for SET / RESET. SR flip flop is considered as one of the most basic sequential logic circuit as possible. It consists of two NAND gates.
  • The reset input will reset the flip flop to 0.
  • The inputs are S and R and the outputs are Q and Q’. The output from each gate is attached to one of the inputs of the other gate as show in the circuit diagram.

In the truth table shown below, if:

  • both the inputs are 0, the output will undergo no change.
  • the SR values are different, Q takes the value of S.
  • both the values are 1, the output is undefined (as both Q and its complement Q’ will take the same value as 0 which is not valid)

With the modified circuit shown below with the extra AND gates we get a JK flip flop. JK stands for Jack Kilby who invented the circuit.


In the truth table shown below we notice that it is similar to SR flip flop except that, when J and K both are 1 at the clock edge, the output will simply toggle instead of being undefined. So if the output was previously 0, it becomes 1 and if it was 1, it becomes 0.

  • In order to avoid an undefined output value in the SR flip flop, the user must eliminate the condition of both inputs being set to 1.
  • This is accomplished in the JK flip flop by adding the extra gates to the existing SR flip flop. This is the reason the JK flip flop is preferred over the SR flip flop.