Ring counter

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 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

See also

• Counter
• Ring oscillator

References

• Mohamed Rafiquzzaman. Fundamentals of digital logic and microcomputer design, John Wiley and Sons, 2005, p.166.
• B. Somanathan Nair, Digital electronics and logic design, PHI Learning Pvt. Ltd., 2004, p. 134.

External links

• Shift Register Counters
• Ring Counters
• The Johnson Counter
• Bálint Pődör "Digital Technics"
• German Wikibooks

de:Ringzähler