A Johnson counter, also known as a twisted ring counter or a walking ring counter, is a type of digital circuit that functions as a shift register with a feedback loop. It's designed to produce a cyclic pattern of binary states that continuously shifts through its stages. Each stage in the counter can be thought of as a flip-flop or a binary cell that can be in one of two states: 0 or 1.
The Johnson counter operates by shifting the contents of its stages in a circular fashion. The shifting pattern is such that only one bit changes at a time as the counter progresses, creating a pattern that "walks" through the stages.
Here's a basic explanation of how the shifting pattern works in a 4-bit Johnson counter:
Initially, all the bits (flip-flops) are set to either 0 or 1, depending on the starting configuration.
On each clock pulse, the contents of the counter shift to the right (or left, depending on the design). The last bit becomes the new first bit, and all other bits shift one position down.
However, unlike a standard binary counter where all bits change on each count, in a Johnson counter, only one bit changes its state between consecutive counts. This is achieved by using feedback connections from certain stages back to the first stage.
The feedback connections cause the counter to produce a unique pattern where only one bit changes at a time. This pattern then cycles through all possible combinations before repeating.
For example, let's consider a 4-bit Johnson counter with initial state 0001:
Count 0: 0001
Count 1: 1000
Count 2: 1100
Count 3: 1110
Count 4: 1111
Count 5: 0111
Count 6: 0011
Count 7: 0001 (back to the initial state)
As you can see, only one bit changes its state between each count, and the pattern eventually repeats. This characteristic of the Johnson counter's shifting pattern makes it useful in applications like frequency division, pattern generation, and digital delay lines.
Johnson counters find applications in digital circuits, communication systems, and various control systems where you need a cyclic pattern with minimal transition activity. The ability to produce a steady walking pattern while changing only one bit at a time makes Johnson counters quite unique and versatile in certain scenarios.