Ring counter
Encyclopedia
A ring counter is a type of counter composed of a circular shift register
Shift register
In digital circuits, a shift register is a cascade of flip flops, sharing the same clock, which has the output of any one but the last flip-flop connected to the "data" input of the next one in the chain, resulting in a circuit that shifts by one position the one-dimensional "bit array" stored in...

. The output of the last shift register is fed to the input of the first register.

There are two types of ring counters:
  • A straight ring counter or Overbeck counter connects the output of the last shift register to the first shift register input and circulates a single one (or zero) bit around the ring. For example, in a 4-register one-hot counter, with initial register values of 1000, the repeating pattern is: 1000, 0100, 0010, 0001, 1000... . Note that one of the registers must be pre-loaded with a 1 (or 0) in order to operate properly.

  • A twisted ring counter, also called Johnson counter or Möbius counter (also Moebius), connects the complement of the output of the last shift register to its input and circulates a stream of ones followed by zeros around the ring. For example, in a 4-register counter, with initial register values of 0000, the repeating pattern is: 0000, 1000, 1100, 1110, 1111, 0111, 0011, 0001, 0000... .

Four-bit ring counter sequences

Straight ring/Overbeck counter Twisted ring/Johnson counter
State Q0 Q1 Q2 Q3 |State Q0 Q1 Q2 Q3
0 1 0 0 0 0 0 0 0 0
1 0 1 0 0 1 1 0 0 0
2 0 0 1 0 2 1 1 0 0
3 0 0 0 1 3 1 1 1 0
0 1 0 0 0 4 1 1 1 1
1 0 1 0 0 5 0 1 1 1
2 0 0 1 0 6 0 0 1 1
3 0 0 0 1 7 0 0 0 1
0 1 0 0 0 0 0 0 0 0

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