A Johnson counter, also known as a twisted-ring counter or a switch-tail ring counter, is a type of digital counter circuit used in electronics and digital systems. It's a modification of a standard ring counter, where the output of the last stage is connected back to the input of the first stage, creating a closed loop.
In a Johnson counter, the shifting pattern is such that only one bit changes its state (from 0 to 1 or vice versa) with each clock pulse. This shifting pattern is also known as a "twisted" or "circular" shift.
Let's consider a 4-stage Johnson counter as an example. The counter has four flip-flops labeled A, B, C, and D. The outputs of these flip-flops are connected in a specific way to achieve the shifting pattern:
Initial state: A B C D = 0000
Clock Pulse 1: The first flip-flop (A) toggles its state, while the other flip-flops remain unchanged. A B C D = 1000
Clock Pulse 2: The second flip-flop (B) toggles its state, while the others remain unchanged. A B C D = 1100
Clock Pulse 3: The third flip-flop (C) toggles its state, while the others remain unchanged. A B C D = 1110
Clock Pulse 4: The fourth flip-flop (D) toggles its state, while the others remain unchanged. A B C D = 1111
Clock Pulse 5: The shifting pattern continues, but now the fourth flip-flop (D) toggles its state back to 0. A B C D = 0111
And so on...
The key characteristic of the Johnson counter is that it has a unique property where only one bit changes at a time, and the pattern continuously shifts through all possible states before repeating. This can be useful in various applications, such as generating unique sequences for control signals, generating timing signals, or serving as a pseudo-random number generator.
It's important to note that while a Johnson counter is conceptually simple, implementing it correctly and ensuring stable operation can require careful consideration of clock timing, signal propagation delays, and circuit design.