Ring Counter:-  

• Shift registers can be arranged to form several types of counters. Shift Registers counters are obtained from serial-in serial-out shift registers by providing feedback from the output of the last flip flop to the input of the First flip flop. These devices are called counters because they exhibit a sequence of states. The most widely used shift register counter is the ring counter(also called basic ring counter) and twisted ring counter(also called jhonson ring counter) .

Jhonson Ring Counter:-

Johnson counters are a variation of standard ring counters, with the inverted output of the last stage fed back to the input of the first stage. They are also known as twisted ring counters. An n-stage Johnson counter yields a count sequence of length 2n, so it may be considered to be a mod-2n counter. It is more economical than basic Ring counter but less economical than ripple counter. The circuit below shows a 4-bit Johnson counter. The state sequence for the counter is given in the table.

• The ouput Q of each flip flop is connected to the J input of the next flip flop and the output$\overline Q$of each flip flop is connected to the K input of the next flip flop. However the Q output of the last stage flip flop are connected to the K input and$\overline Q$output of the last stage flip flop are connected to the J input respectively of the first flip flop. Initially let the counter be set to 0. As the first pulse occurs the first flip flop changes its output state from a 0 to a 1. The value 1 shifts from left to right and again when all outputs are 1, the value 0 moves from right to left.
• Again, the apparent disadvantage of this counter is that the maximum available states are not fully utilized. Only eight of the sixteen states are being used. Beware that for both the Ring and the Johnson counter must initially be forced into a valid state in the count sequence because they operate on a subset of the available number of states. Otherwise, the ideal sequence will not be followed.