A Johnson counter is a type of digital sequential logic circuit that serves as a shift register with a unique shifting pattern. It is also known as a "twisted ring counter" or "walking ring counter" due to its distinctive sequence of output states. The counter consists of a series of flip-flops (usually D-type flip-flops) connected in a circular manner, forming a loop.
The unique shifting pattern of a Johnson counter involves a cyclic shift of the binary states (0s and 1s) within the counter. In a standard Johnson counter, the states shift to the right or left (depending on the specific implementation), and the last bit is fed back to the first flip-flop, creating a loop.
The shifting pattern for a 4-bit Johnson counter can be illustrated as follows:
Initial State: 0000
Shift Right: 1000
Shift Right: 1100
Shift Right: 1110
Shift Right: 1111
Shift Right: 0111
Shift Right: 0011
Shift Right: 0001
Shift Right: 0000 (back to the initial state)
As you can see, the counter progresses through a sequence of states in which only one bit changes at a time, creating a smooth and balanced transition between the binary values. This unique shifting pattern can be useful in applications like frequency division, pattern generation, and waveform synthesis.