The Johnson Ring Counter, also known as “Twisted Ring Counters,” has feedback that exactly mimics the standard Ring Counter mentioned earlier. However, in this case, the inverted output Q of the last flip-flop is connected back to the input D of the first flip-flop, as shown below. This type of ring counter offers a significant advantage as it requires only half the number of flip-flops compared to the standard ring counter, effectively halving its modulo number. Consequently, an “n-stage” Johnson counter will circulate a single data bit, resulting in a sequence of 2n different states. Therefore, it can be regarded as a “mod-2n counter.”
Advantages of the Johnson counter
- The Johnson counter can count twice the number of states compared to the ring counter, despite having the same number of flip-flops.
- It can be implemented using D and JK flip-flops.
- The Johnson ring counter is designed to count data in a continuous loop.
- It serves as a self-decoding circuit.
Disadvantages of the Johnson counter
- The Johnson counter does not count in a binary sequence.
- A larger number of states remain unutilized in the Johnson counter compared to the number of states being utilized.
- The number of flip-flops required is half the number of timing signals.
- It can be constructed for any desired number of timing sequences.
Applications of the Johnson counter
- The Johnson counter is used as a synchronous decade counter or divider circuit.
- It finds application in hardware logic design for creating complex Finite State Machines, such as ASIC and FPGA designs.
- The 3-stage Johnson counter is utilized as a 3-phase square wave generator that produces a 120-degree phase shift.
- It can be employed to divide the frequency of a clock signal by altering its feedback.
What is the difference between the ring counter and Johnson counter?
The ring counter has a MOD of ‘n’ for an n-bit counter, whereas the Johnson counter has a MOD of ‘2n’ for an n-bit counter.
What is a D flip-flop?
A D flip-flop, also known as a clocked flip-flop or delay flip-flop, tracks the input and transitions its output to match the input value.
What is an asynchronous counter?
An asynchronous counter has control over its outputs regardless of the input clock pulses applied. It relies on flip-flops to register the input and produce the desired outputs.
What is the full form of a D flip-flop?
The full form of a D flip-flop is a Data flip-flop, as it stores the value present on the data line.
- Johnson Ring Counter
- Ring Counter in Digital Logic
- Difference between Ring Counter and Johnson Counter
- Ripple Counter in Digital Logic
- Bi-Directional Counter
- Difference between Straight and Twisted Ring Counter
- Types of Sequential Circuits
- Clock Signal and Triggering
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