What is a Johnson counter and how does its shifting pattern work?
1 Answer
A Johnson counter, also known as a "twisted ring counter," is a type of digital shift register with a feedback mechanism. It is a variation of the ring counter, which is a circular shift register where only one bit is high (1) at any given time and moves from one stage to the next in a cyclic manner. The Johnson counter, however, uses a more complex shifting pattern to create a repeating sequence of 0s and 1s.
A standard Johnson counter is a type of 4-bit shift register, but it can be expanded to have more bits if needed. The basic operation of a 4-bit Johnson counter is as follows:
Initialization: Initially, all the bits in the counter are set to either 0 or 1, depending on the design (e.g., 0000 or 1111).
Shifting: In each clock cycle, the bits shift to the right (or left) in a circular manner. Instead of simply propagating the most significant bit (MSB) through the stages like in a regular ring counter, the Johnson counter has a different shifting pattern.
Shifting pattern: The Johnson counter follows a specific pattern as it shifts. At each clock cycle, one of the bits is complemented (i.e., changes from 0 to 1 or 1 to 0) based on the position of the 1 in the previous state. The specific pattern for a 4-bit Johnson counter is as follows:
0000
1000 (1st bit complemented)
1100 (2nd bit complemented)
1110 (3rd bit complemented)
1111 (4th bit complemented)
0111 (1st bit complemented)
0011 (2nd bit complemented)
0001 (3rd bit complemented)
0000 (4th bit complemented, returning to the initial state)
Repeat: The counter continues to shift in this pattern, creating a repeating sequence of 0s and 1s.
Johnson counters have various applications, including frequency division, pseudo-random sequence generation, and digital pattern generation in testing and simulation. They are also used in certain types of digital clocks and electronic security systems. However, it's worth noting that with the advent of more sophisticated digital devices and technologies, the use of Johnson counters has somewhat diminished in favor of other sequential circuits and more advanced techniques.
A standard Johnson counter is a type of 4-bit shift register, but it can be expanded to have more bits if needed. The basic operation of a 4-bit Johnson counter is as follows:
Initialization: Initially, all the bits in the counter are set to either 0 or 1, depending on the design (e.g., 0000 or 1111).
Shifting: In each clock cycle, the bits shift to the right (or left) in a circular manner. Instead of simply propagating the most significant bit (MSB) through the stages like in a regular ring counter, the Johnson counter has a different shifting pattern.
Shifting pattern: The Johnson counter follows a specific pattern as it shifts. At each clock cycle, one of the bits is complemented (i.e., changes from 0 to 1 or 1 to 0) based on the position of the 1 in the previous state. The specific pattern for a 4-bit Johnson counter is as follows:
0000
1000 (1st bit complemented)
1100 (2nd bit complemented)
1110 (3rd bit complemented)
1111 (4th bit complemented)
0111 (1st bit complemented)
0011 (2nd bit complemented)
0001 (3rd bit complemented)
0000 (4th bit complemented, returning to the initial state)
Repeat: The counter continues to shift in this pattern, creating a repeating sequence of 0s and 1s.
Johnson counters have various applications, including frequency division, pseudo-random sequence generation, and digital pattern generation in testing and simulation. They are also used in certain types of digital clocks and electronic security systems. However, it's worth noting that with the advent of more sophisticated digital devices and technologies, the use of Johnson counters has somewhat diminished in favor of other sequential circuits and more advanced techniques.