A Johnson counter, also known as a rotary counter or a twisted-ring counter, is a type of digital sequential circuit used for counting and generating a specific shifting pattern. It's essentially a shift register with feedback, where the last stage's output is fed back to the first stage's input. This creates a cyclic pattern of bit states as the counter progresses through its sequence.
The unique shifting pattern of a Johnson counter is characterized by the fact that only one bit changes its state at a time during each count cycle. This results in a smooth and consistent transition between the different counter states, which can be useful in applications such as frequency division, LED displays, and waveform generation.
A simple example of a 4-bit Johnson counter is as follows:
Initially, all bits are set to 0: 0000
After the first clock pulse, the pattern shifts to: 0001
Then: 0011
Then: 0111
And finally: 1111
From this point, the pattern wraps back to the initial state (0000) and the cycle continues.
The shifting pattern is "unique" because it's designed to have only one bit changing at a time. This is in contrast to binary counters where multiple bits can change simultaneously, resulting in more abrupt transitions between states.
Johnson counters are particularly useful when you need a gradual and consistent shift in the pattern without sharp edges or rapid changes. They find applications in various areas including digital signal processing, communication systems, and control systems, especially where you want to avoid glitches or sudden changes in signals.