A jhonson counter is a digital circuit made of flip flops as show in the figure below. It is also called a twisted ring counter as in a jhonson counter the negation of the last flip flops is connected as the serial input to the first flip flop.
A johson counter is designed with N flip flops and will hae 2N unique states. The Q' output of the last flip flop i fed as input to the first flip flop. Thus if we start with all the flip flops in the clear state, then at the first clock pulse the negation of "0" that is "1" is fed as input to the first flip flop. Thus output of first flipflop becomes 1, this operation continues for N clock pulses, making all the outputs as 1. Then again the process is repeated for N clock pulses for it to reach the initial state again as depicted in the truth table below. The highlighted cells in each row indicate the unique combination to recognize that state.
A jhonson counter can generate 2N unique states for N flip flops which is double the number generated by a ring counter.
The videos below shows the working of a jhonson counter.
The circuit and simulation was created using the software "DEEDS"