A Johnson counter, also known as a twisted ring counter or a walking ring counter, is a type of digital circuit used in electronics and digital systems for generating a sequence of binary values. It is essentially a shift register with a feedback loop that creates a specific shifting pattern of bits. The key characteristic of a Johnson counter is that only one bit changes its state at a time as the counter progresses through its sequence, resulting in a smooth and continuous shifting pattern.
The shifting pattern of a Johnson counter is as follows:
Let's assume we have an n-bit Johnson counter. It consists of a series of flip-flops, typically D-type flip-flops, arranged in a circular fashion with a feedback path from the last flip-flop to the first. The initial state of the counter is all zeros.
As the counter progresses through its sequence, only one bit changes its state (from 0 to 1 or from 1 to 0) in each step. This transition occurs in a circular manner, creating a distinct pattern. The transition of the changing bit is determined by the feedback from the previous flip-flop's state.
For example, let's consider a 4-bit Johnson counter with the initial state 0000:
0000
1000
1100
1110
1111
0111
0011
0001
As you can see, in each step, only one bit changes its state, and this transition follows a clockwise shifting pattern. The specific sequence generated by the counter depends on the number of bits used and the connections between the flip-flops.
Johnson counters find applications in various areas, including frequency division, signal generation, and control systems, where a smooth and balanced shifting pattern is required. The gradual and continuous change of states in the counter can be useful in creating sequences of events or controlling various operations in digital systems.