Shift Registers

Q1: What is the difference between a ring shift counter and a Johnson shift counter?

A The feedback is reversed.

B A ring is faster.

C The Johnson is faster.

D There is no difference.

ANS:C - The feedback is reversed.

A ring counter is a shift register (a cascade connection of flip-flops) with the output of the last one connected to the input of the first, that is, in a ring. Typically a pattern consisting of a single 1 bit is circulated, so the state repeats every N clock cycles if N flip-flops are used. It can be used as a cycle counter of N states.

Johnson counter

A Johnson counter (or switchtail ring counter, twisted-ring counter, walking-ring counter, or Moebius counter) is a modified ring counter, where the output from the last stage is inverted and fed back as input to the first stage. A pattern of bits equal in length to twice the length of the shift register thus circulates indefinitely. These counters find specialist applications, including those similar to the decade counter, digital to analog conversion, etc.

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