Mod 2 Ring Counter (with D flip-flop)
Ring counters are one of the most important applications of shift registers. They are created by connecting multiple flip-flops to one another (such that the output of one flip-flop is the input for another), and by connecting the output of the last flip-flop to the input of the first flip-flop.
For a mod 2-ring counter, two flip-flops will be required. Recall that the number of states that can be counted by a ring counter is equal to the number of flip-flops being used. Since a mod 2-ring counter can count up to 2 states, 2 flip-flops will be required. Since a ring counter is a synchronous counter, the clock needs to be in an “ON” state so that state transitions can happen and the ring counter can function normally.
From the diagram, we can see that D1 = Q0 and D0 = Q1
Table: Combining the excitation table and the state table here for convenience.