JK flip flop

JK flip flop is modified RS flip flop. The inputs R and S are ANDed with Q and Q’ respectively to form the inputs of JK flip flop. Its operation remains same as RS flip flop except when both inputs are 1.The indeterminate state, i.e. when R=1, S=1, is eliminated in JK flip flop and the output toggles or complements its state when J=1, K=1.

Logic diagram

j k flip flop

Working

(i)When J=0, K=0, the output of gate 1& 2 is zero. If present state of output Q is 0, Q’ is 1. So, output of gate 3 is 0. You can notice that there is no change in between present and next state of output Q.

(ii)When J=0, K=1, the output of gate 1 depends on present state of Q. If present state of output Q is 0, Q’ is 1. Output of gate 1 is zero and next state of Q will be 0. Whether present state of Q is 0 or 1, the next state will be set to 0 or in other words, Q resets to 0.

(iii) When J=1, K=0, the output of gate 1 is zero. If present state of output Q is 0, Q’ is 1. So, output of gate 3 is 1. In either case, the next state of Q is set to 1.

(iv) When J=1, K=1, the output of gate 1& 2 will depend on value of Q. If present state of output Q is 0, Q’ is 1. The next state of Q will be 1. The state of Q is toggled or complimented.

The working of JK flip flop can be summarised as following

Present state

Inputs

Next state

Remarks

J

K

Q(t)

0

0

Q(t)

No change

0

1

0

Reset

1

0

1

Set

1

1

Q’(t)

Toggle

 

Characteristic table

Present state

Input

Next state

Q(t)

J

K

Q(t+1)

0

0

0

0

0

0

1

0

0

1

0

1

0

1

1

1

1

0

0

1

1

0

1

0

1

1

0

1

1

1

1

0

Characteristic equation

JK flip flop equation

Q (t+1) = JQ’ + K’Q

Graphical symbol

JK flip flop graphic symbol