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 a variation of the ring counter, but with a distinct shifting pattern that provides unique advantages in certain applications.
The Johnson counter consists of a series of flip-flops connected in a "ring" configuration, where the output of one flip-flop feeds into the input of the next flip-flop, and the last flip-flop's output is connected back to the first flip-flop's input, completing the loop. This circular arrangement creates a continuous cycle of states.
The shifting pattern of a Johnson counter is different from a standard binary counter. In a binary counter, the count progresses in a binary sequence, where each bit toggles from 0 to 1, and the least significant bit (LSB) flips more frequently than the more significant bits.
In contrast, a Johnson counter follows a "twisted" sequence, where only one bit changes state at a time, creating a "1" that moves along the ring of flip-flops. This pattern effectively eliminates the problem of multiple bits changing simultaneously, which can cause glitches and unwanted intermediate states in certain applications.
For example, let's consider a 4-bit Johnson counter. The four flip-flops are labeled as A, B, C, and D. Initially, all the flip-flops are in the state 0, represented as "0000."
The shifting pattern of a 4-bit Johnson counter can be described as follows:
All bits shift right: "0000" -> "0001" -> "0010" -> "0100" -> "1000"
All bits shift left: "1000" -> "0100" -> "0010" -> "0001" -> "0000"
As you can see, the 1-bit moves through the counter in a cyclic pattern, providing a unique shifting behavior compared to standard binary counters.
Johnson counters are particularly useful in applications where a simple and glitch-free sequence is required, such as in frequency dividers, LED chasers, and some types of sequencers. Additionally, they find applications in shift register designs and pseudo-random number generators, among other digital circuit implementations.