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Ring Counter in Digital Logic

  • Difficulty Level : Medium
  • Last Updated : 22 Apr, 2022

Ring counter is a typical application of Shift register. Ring counter is almost same as the shift counter. The only change is that the output of the last flip-flop is connected to the input of the first flip-flop in case of ring counter but in case of shift resister it is taken as output. Except this all the other things are same.

 No. of states in Ring counter = No. of flip-flop used

So, for designing 4-bit Ring counter we need 4 flip-flop. In this diagram, we can see that the clock pulse (CLK) is applied to all the flip-flop simultaneously. Therefore, it is a Synchronous Counter. Also, here we use Overriding input (ORI) to each flip-flop. Preset (PR) and Clear (CLR) are used as ORI. When PR is 0, then the output is 1. And when CLR is 0, then the output is 0. Both PR and CLR are active low signal that is always works in value 0.

PR = 0, Q = 1
CLR = 0, Q = 0

These two values are always fixed. They are independent with the value of input D and the Clock pulse (CLK). Working – Here, ORI is connected to Preset (PR) in FF-0 and it is connected to Clear (CLR) in FF-1, FF-2, and FF-3. Thus, output Q = 1 is generated at FF-0 and rest of the flip-flop generate output Q = 0. This output Q = 1 at FF-0 is known as Pre-set 1 which is used to form the ring in the Ring Counter. This Preseted 1 is generated by making ORI low and that time Clock (CLK) becomes don’t care. After that ORI made to high and apply low clock pulse signal as the Clock (CLK) is negative edge triggered. After that, at each clock pulse the preseted 1 is shifted to the next flip-flop and thus form Ring. From the above table, we can say that there are 4 states in 4-bit Ring Counter.

4 states are:
  1 0 0 0
  0 1 0 0
  0 0 1 0
  0 0 0 1

In this way can design 4-bit Ring Counter using four D flip-flops. Types of Ring Counter – There are two types of Ring Counter:

  1. Straight Ring Counter – It is also known as One hot Counter. In this counter, the output of the last flip-flop is connected to the input of the first flip-flip. The main point of this Counter is that it circulates a single one (or zero) bit around the ring. Here, we use Preset (PR) in the first flip-flop and Clock (CLK) for the last three flip-flop.
  2. Twisted Ring Counter – It is also known as switch-tail ring counter, walking ring counter or Johnson counter. It connects the complement of the output of the last shift register to the input of the first register and circulates a stream of ones followed by zeros around the ring. Here, we use Clock (CLK) for all the flip-flops.
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