A Johnson counter is a type of digital sequential logic circuit that functions as a shift register with feedback. It is used to generate a cyclic sequence of binary values, where only one bit changes at a time, creating a rotating or shifting pattern. Johnson counters are often used in various applications, such as frequency division, pattern generation, and control systems.
A Johnson counter typically consists of a chain of flip-flops (usually D-type flip-flops) connected in a closed loop configuration. The output of one flip-flop is connected to the input of the next flip-flop, and the output of the last flip-flop is fed back to the input of the first flip-flop. This feedback loop creates the shifting pattern characteristic of a Johnson counter.
The shifting pattern of a Johnson counter is achieved through careful arrangement of the feedback connections. Let's consider an n-bit Johnson counter. In this case, the shifting pattern consists of 2^n unique states, and each state represents a binary value.
Here's how the shifting pattern works:
Initialization: All flip-flops are initially set to the same state (either all 0s or all 1s) to start the shifting pattern.
Shifting: On each clock cycle, the binary value stored in the flip-flops shifts by one position. The feedback connections determine which bit changes its value. Specifically, the feedback taps are chosen such that only one bit changes its state in each clock cycle, and the rest remain the same.
Closed Loop: The feedback loop ensures that the last flip-flop's output is fed back to the first flip-flop's input, completing the loop. This creates a cyclic pattern, as the shifted-out value from the last flip-flop becomes the new input for the first flip-flop.
The specific arrangement of feedback connections depends on the number of bits in the Johnson counter. The goal is to create a sequence where only one bit changes at a time, and this is achieved by carefully designing the connections.
Johnson counters find applications in various areas, including:
Frequency Division: Johnson counters can be used to divide an input clock frequency by a factor of 2^n, where n is the number of flip-flops in the counter.
Pattern Generation: The shifting pattern of a Johnson counter can be used to generate specific repeating patterns, which can be useful in applications such as LED displays, digital signage, and sequence generation.
Control Systems: Johnson counters can be employed as part of control systems to generate timing or sequencing signals for various operations.
It's important to note that while Johnson counters offer simplicity and ease of implementation, they may not be the most efficient solution for all applications, especially those requiring complex sequences or higher speed operations.