A Johnson counter, also known as a twisted ring counter, is a type of digital sequential circuit that consists of a cascade of flip-flops, where each flip-flop output is connected to the next flip-flop's input in a circular fashion. Johnson counters exhibit a unique shifting pattern where the "1" bit moves through the counter in a circular or ring-like manner. This shifting pattern is also sometimes referred to as the "twisted" pattern due to the way the bits move.
In a Johnson counter, each flip-flop is connected to the next one in a way that produces a specific binary counting sequence. The sequence generated by a Johnson counter is a cyclic pattern where only one bit changes its state (from 0 to 1 or from 1 to 0) at each clock pulse. The rest of the bits maintain their state. The pattern repeats after a specific number of clock cycles, depending on the number of flip-flops in the counter.
For example, let's consider a 4-bit Johnson counter with four flip-flops labeled A, B, C, and D. The initial state might be 0000, and the shifting pattern would proceed as follows:
0000
1000
1100
1110
1111
0111
0011
0001
0000 (back to the initial state)
As you can see, the "1" bit moves through the counter in a circular manner, creating the unique shifting pattern. This pattern is often utilized in applications where cyclic or rotating sequences are required, such as in certain types of digital displays, light patterns, or encoding schemes.
It's worth noting that the Johnson counter can be implemented using various types of flip-flops, such as D-type flip-flops or J-K flip-flops, depending on the specific requirements and design considerations.