A Johnson counter, also known as a twisted ring counter or Möbius counter, is a type of digital counter circuit used in digital electronics and sequential logic. It is a modification of a traditional ring counter that produces a continuous shifting pattern of ones and zeros.
In a Johnson counter, the bits in the counter shift to the right (or left) with each clock pulse. However, unlike a regular binary counter where each bit toggles between 0 and 1, in a Johnson counter, the shifting pattern follows a specific sequence of 0s and 1s. The sequence is designed in such a way that only one bit changes its state (from 0 to 1 or from 1 to 0) at a time as the counter progresses.
The shifting pattern in a 4-bit Johnson counter is as follows:
0000 -> 1000 -> 1100 -> 1110 -> 1111 -> 0111 -> 0011 -> 0001 -> 0000
Here's how the shifting pattern works:
Start with all bits at 0.
Shift the rightmost bit to 1 while keeping the other bits at 0.
Move the 1 to the next bit position to the left in the next clock cycle.
Repeat step 3 until the leftmost bit becomes 1, forming a sequence of 0s and 1s.
Once the leftmost bit becomes 1, reverse the process, shifting the 1s to the right while keeping the 0s to the left.
This creates a continuous loop of shifting patterns that repeat indefinitely. Johnson counters are useful in applications where a cyclic sequence of states is needed, such as in generating timing signals, LED chasing effects, and various types of pattern generation.
One advantage of Johnson counters is that they produce a more balanced output waveform compared to regular binary counters, which can help reduce glitches and noise in certain applications. Additionally, Johnson counters are relatively easy to implement using simple digital logic gates.
It's worth noting that Johnson counters can have different sizes (number of bits) depending on the desired sequence length and application requirements. The example provided above is for a 4-bit Johnson counter.