Logic circuits

How many states does a Johnson (twisted-ring) counter made of n flip-flops go through?

  1. An
  2. B2n
  3. C2ⁿ
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Answer: B. 2n

A Johnson counter feeds the inverted last output of a shift register back to its input and cycles through 2n states with n flip-flops. Only one bit changes between adjacent states, so decoding is simple and glitches are unlikely.

  • A. n states is a ring counter, which circulates a single 1.
  • B. Correct. A Johnson counter goes through 2n states.
  • C. 2ⁿ states is a binary counter.
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