A Johnson counter, also known as a "twisted ring counter" or "walking ring counter," is a type of digital sequential circuit used in electronics for counting purposes or generating various patterns. It's a specific implementation of a shift register, a cascade of flip-flops connected in series. The unique characteristic of a Johnson counter is its shifting pattern, which differs from other types of counters.
In a standard binary counter, such as an n-bit binary up-counter, the counting sequence follows a strict binary progression: 000, 001, 010, 011, 100, 101, 110, 111, and back to 000.
A Johnson counter, on the other hand, produces a sequence that "walks" through all possible states, both high and low, in a cyclic manner. For an n-bit Johnson counter, the counting sequence progresses through 2n states before repeating. The distinctive shifting pattern of a Johnson counter is achieved by complementing the output of each flip-flop and feeding it as an input to the next flip-flop in the chain.
For example, let's consider a 4-bit Johnson counter with initial state 0000:
0000
1000
1100
1110
1111
0111
0011
0001
0000 (loop back to the initial state)
In each step, the counter output shifts by one bit, and the bits "walk" in a circular manner. Notice that the sequence goes through all possible binary combinations, both ascending and descending, before returning to the initial state. This unique property makes Johnson counters useful in applications where cyclic patterns or sequences are required.
Johnson counters find applications in shift register-based pattern generation, frequency division, sequence generation for digital display control, and other situations where a cyclic pattern is needed.