A Johnson counter, also known as a circular shift register or a twisted ring counter, is a type of digital counter circuit used for sequential counting or generating various patterns. Unlike regular binary counters that count in a straight binary sequence (00, 01, 10, 11, etc.), a Johnson counter follows a unique shifting pattern, where only one bit changes its state (either 0 to 1 or 1 to 0) at a time as the counter advances.
A Johnson counter consists of a series of flip-flops connected in a ring or a loop configuration, forming a closed path. The output of each flip-flop is connected to the input of the next flip-flop in the sequence, and the output of the last flip-flop is fed back to the input of the first flip-flop, completing the loop. This feedback arrangement enables the circular shifting pattern.
The unique shifting pattern of a Johnson counter can be described as follows:
Initialization: All the flip-flops are set to either 0 or 1, depending on the desired starting state.
Counting: As the counter advances, only one flip-flop changes its state at a time. It moves from one flip-flop to the next in a circular fashion.
Shifting Direction: The direction of shifting can be either to the right (clockwise) or to the left (counterclockwise), depending on the specific implementation.
Repeat: The counter keeps shifting in the circular pattern, and after reaching the last state, it resets back to its initial state, creating a continuous loop of shifting.
The shifting pattern of a Johnson counter allows it to generate various useful sequences, such as Gray codes (where only one bit changes between consecutive values) and pseudo-random sequences. It finds applications in digital communication, control systems, and pattern generation tasks. The number of flip-flops in the Johnson counter determines the number of states it can represent, and the shifting pattern is periodic with a period equal to the number of states in the counter.