A Johnson counter is a type of digital sequential logic circuit that acts as a shift register with a feedback mechanism. It is designed to produce a repeating sequence of binary values. It is also known as a twisted ring counter or a switch-tail ring counter.
The Johnson counter consists of a chain of flip-flops connected in a circular or ring configuration. Each flip-flop in the chain is triggered by the output of the previous flip-flop, creating a circular shift of data within the circuit. The interesting aspect of a Johnson counter is that it produces a specific pattern of 1s and 0s as it cycles through its states.
The shift operation in a Johnson counter is achieved through the feedback mechanism, where the output of the last flip-flop is connected to the input of the first flip-flop. When a clock signal is applied, the data is shifted from one flip-flop to the next, creating the desired sequence. This shifting process continues with each clock pulse, causing the pattern to cycle through all its possible states.
The specific pattern generated by a Johnson counter depends on its design and the number of flip-flops used. For example, a 4-bit Johnson counter would have 4 flip-flops and produce a repeating sequence of 4 bits. The pattern can be configured to be either the straight binary count (0, 1, 2, 3, 0, 1, 2, 3, ...) or the complement of the binary count (15, 14, 13, 12, 15, 14, 13, 12, ...), depending on the application and desired output.
Johnson counters have various applications in digital electronics, such as frequency division, pattern generation, and control circuitry. They are also used in applications where cyclic or rotating patterns are required, such as in mechanical systems and LED display drivers.