Johnson counter or Switch Tail or Twisted Ring Counter is a synchronous counter. An n-bit ring counter circulates a single bit among the flip flops to provide n distinct states. The number of states can be doubled if the shift register is connected as a switch tail ring counter.
A switch tail ring counter is a circular shift register with the complement output of the last flip flop connected to the input of the fist flip flop. The circular connection is made from the complement output of the rightmost flip flop to the input of the leftmost flip flop. The register shifts its contents once to the right with every clock pulse and at the same time, the complement value of flip flop 4 are transferred to flip flop 1.
Starting from all 0’s, each shift operation inserts 1’s from the left until the register is filled with all 1’s. In the following sequence 0’s are inserted from the left until the register is again filled with all 0’s.
Mod of Johnson Counter is 2n, therefore it is known as Divide by 2N Counter.
Frequency of Output = frequency of Clock Pulse / mod
fjohnson = f / 2n
Disadvantage of Johnson Counter
Disadvantage of Johnson Counter is that it doesn’t count in a binary sequence.
Johnson counters can be constructed for any number of timing sequences. The number of flip flops needed is one half the numbers of timing signals.