A Johnson counter, also known as a twisted ring counter, is a type of digital sequential circuit that consists of a shift register connected in a feedback configuration to create a cyclic pattern. It is named after the American engineer, Howard Johnson, who developed it in the 1940s.
The unique shifting pattern of a Johnson counter is what sets it apart from other types of counters. In a standard binary counter, the binary value increments by one on each clock cycle, and the output sequence is straightforward. However, a Johnson counter follows a more complex pattern.
A Johnson counter is a n-bit counter with 2^n states, and it produces a single bit of output on each clock cycle. The key feature of the Johnson counter is that it generates a sequence where only one bit changes between consecutive states, either shifting to the right or left. This sequence creates a loop, cycling through all possible states before repeating.
For example, let's consider a 4-bit Johnson counter with the initial state as "0001". The unique shifting pattern of the Johnson counter would be as follows:
0001
0010
0100
1000
0100
0010
0001
0010
... (and the pattern repeats)
As you can see, in each clock cycle, only one bit changes its state, creating a shifting pattern where the '1' bit moves through the counter in a circular fashion. This pattern continues to cycle indefinitely.
Johnson counters have some applications in digital electronics, such as in frequency dividers, ring oscillators, and in generating unique sequences in pattern recognition circuits. They are also used in applications where a cyclic pattern with minimal transitions between states is required.