A Johnson counter, also known as a twisted-ring counter or a walking-ring counter, is a type of digital counter circuit used in digital electronics and sequential logic circuits. It is designed to cycle through a sequence of binary states in a continuous loop, creating a shifting pattern.
The Johnson counter is formed by a series of flip-flops (usually D-type flip-flops) connected in a ring, where the output of each flip-flop is connected to the input of the next one. The last flip-flop's output is fed back to the input of the first flip-flop, completing the loop. The number of flip-flops in the ring determines the length of the counter sequence. A Johnson counter with 'n' flip-flops can produce a sequence of 2^n distinct states.
The shifting pattern of a Johnson counter is unique and follows a particular sequence. Each time the counter advances by one state, only one bit changes its value, either from 0 to 1 or from 1 to 0. This change "shifts" through the counter from one end to the other, hence the name "Johnson counter" or "shifting-ring counter." The pattern is designed in such a way that the transition between states occurs in a Gray code sequence. In Gray code, only one bit changes between consecutive values, which reduces the chance of errors due to multiple bit changes happening simultaneously.
For example, let's take a 3-bit Johnson counter:
State 0: 000
State 1: 001
State 2: 011
State 3: 010
State 4: 110
State 5: 111
State 6: 101
State 7: 100
As you can see, in each state transition, only one bit changes its value, ensuring a smooth and controlled shifting pattern.
Johnson counters have various applications, such as in shift register designs, frequency dividers, and applications where cyclic patterns are required, such as rotating displays and stepping motor control.