Ring counter

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A ring counter is a type of counter composed of a circular shift register. 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 counter, with initial register values of 1000, the repeating pattern is: 1000, 0100, 0010, 0001. Note that one of the registers must be pre-loaded with a 1 (or 0) in order to operate properly.

• A twisted ring counter or Johnson counter 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.

[edit] Four-bit ring counter sequences

Straight ring/Overbeck counter						Twisted ring/Johnson counter
Clock pulse	Cycle	Q0	Q1	Q2	Q3	Clock pulse	Cycle	Q0	Q1	Q2	Q3
0	I	1	0	0	0	0	I	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
4	II	1	0	0	0	4		1	1	1	1
5		0	1	0	0	5		0	1	1	1
6		0	0	1	0	6		0	0	1	1
7		0	0	0	1	7		0	0	0	1
8	III	1	0	0	0	8	II	0	0	0	0

[edit] External links

• Shift Register Counters
• Ring Counters