Johnson counter

The Johnson counter, also called a twisted ring counter, switch-tail ring, walking ring counter or Mobius counter, is a type of ring counter built from flip-flops connected into a shift register, with the complemented output of the last register fed to the input of the first so that a stream of ones followed by zeros circulates around the ring. It is named for Robert Royce Johnson, who developed a number of different shift-register-based counters aimed at producing different numbers of states with the simplest possible feedback logic, and filed for a patent in 1953; the Johnson counter is the simplest of these. A straight ring counter with N flip-flops has N states while a Johnson counter has 2N states, it can self-initialize from the all-zeros state, and it generates a Gray code in which adjacent states differ by only one bit, so its pattern can be sampled asynchronously without glitches. It is commonly used to generate cycles with an even number of states.