A Johnson counter, also known as a switch-tail ring counter, is a type of digital sequential circuit that is used to shift data in a cyclic pattern. It is a modification of the standard ring counter, which is a type of circular shift register. The main characteristic of a Johnson counter is that it has a complementing output at each stage, resulting in more states and a higher count compared to a regular ring counter.
A Johnson counter can be implemented using flip-flops, typically D flip-flops, with feedback connections that form a closed loop. The number of stages in the Johnson counter determines the total number of unique states it can cycle through before repeating. For an n-stage Johnson counter, there are 2n unique states.
The key feature of a Johnson counter is how it shifts data from one state to another. The shift operation in a Johnson counter involves the following steps:
Initially, all flip-flops are set to either 0 or 1, depending on the desired starting state.
At each clock cycle, the counter counts up by one, moving to the next state in the sequence.
As the counter moves to the next state, the output of each flip-flop changes. In a standard ring counter, only one flip-flop changes state at a time, resulting in a single bit shift. However, in a Johnson counter, multiple flip-flops change state simultaneously.
The transition from one state to the next is such that only one bit changes its state, and all other bits toggle (i.e., change from 0 to 1 or 1 to 0). This creates a unique shifting pattern, and hence, the term "switch-tail" is used to describe this counter.
For example, let's consider a 4-stage Johnson counter with initial state 0001. Its states will cycle through the following sequence:
0001 -> 0010 -> 0100 -> 1000 -> 0001 (repeats)
As you can see, only one bit changes its state from 0 to 1 while all other bits toggle.
Johnson counters find various applications in digital electronics, especially in generating unique patterns and sequences, code generation, and in certain types of frequency dividers. Their unique shifting pattern and high count capabilities make them valuable in specific design scenarios.