1. Ring counter: The ring counter is a application of shift register, in which the output of last flip flop is connected to input of first flip flop. In ring counter if the output of any flip flop is 1, then the output of remaining flip flops is 0. The Ring counters transfers the same output throughout the circuit.

Here, ORI is connected to Preset in FF-0 and it is connected to CLEAR 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 Preset 1 which is used to form the ring in the ring counter.

                                                                                        Truth Table

2. Johnson’s counter or twisted ring counter: The Johnson counter is a modification of ring counter. In this the inverted output of the last stage flip flop is connected to the input of first flip flop. If we use n flip flops to design the Johnson counter, it is known as 2n bit Johnson counter or Mod 2n Johnson counter.

                                            Truth Table