This is a special type of register in which 1 moves in the output in the ring i.e. initially output of 1st FF is 1. On next edge this 1 is transferred to output of 2nd FF while previous output becomes 0. Similarly on next clock output of 3rd FF becomes 1. Similarly it continuous till last FF goes 1. After this 1st FF goes 1 goes again and whole procedure is repeated. This way 1 is moved in a ring as:
Clock Q4 Q3 Q2 Q1
1st tick 0010
And so on
Hence we use only 4 states out of 16 states possible in Ring counter.
Or we can say there are 12 unused states in Ring counter.
Circuit diagram to achieve Ring Counter is as:
To start the Ring counter, we firstly give START=0 and then rightmost FF is set and all others are reset and hence initial output is 0001
We can also realize Ring counter using JK flip-flop as:
Application: We can use Ring counter in the system where we have to perform different operations sequentially and repeatedly. Suppose we have to do operations A, B, C & D. Firstly we have to do A, then B, then C, and then D. after performing all operations we have to perform operation A and so on. In this case we can use Ring counter to initiate these operations sequentially.