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
References
External links