A Johnson counter, also known as a "twisted ring counter" or "walking ring counter," is a type of digital counter circuit used in electronics and digital systems. It is designed to cyclically shift a single "1" bit through a series of flip-flops or similar stages, while the rest of the bits remain at "0." The unique characteristic of a Johnson counter is its shifting pattern, which is generated by successively flipping the position of the "1" bit as it progresses through the counter stages.
Here's how a 4-stage Johnson counter works, with each stage represented by a flip-flop (A, B, C, D):
Initial State: A B C D
0 0 0 1
After the first clock pulse: A B C D
0 0 1 0
After the second clock pulse: A B C D
0 1 0 0
After the third clock pulse: A B C D
1 0 0 0
After the fourth clock pulse, the pattern wraps around: A B C D
0 0 0 1
The shifting pattern is such that the "1" bit moves one position to the right with each clock pulse, creating a circular shifting effect. It's important to note that Johnson counters have a maximum sequence length equal to twice the number of stages they have. In the example above, the 4-stage Johnson counter has a sequence length of 8 (4 stages * 2), which means the pattern will repeat after 8 clock pulses.
Johnson counters find applications in areas like frequency division, ring oscillators, and digital signal sequencing. Their unique shifting pattern can be useful for generating sequences or timing signals in various electronic circuits.