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A ring counter is a type of counter composed of a type of circular shift register. The output of the last shift register is fed to the input of the first register. The hamming distance of an Overbeck counter is 2, the hamming distance of a Johnson counter is 1. 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 the input of the first register 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... . Johnson counters are often favoured, not just because they offer twice as many count states from the same number of shift registers, but because they are able to self-initialise from the all-zeros state, without requiring the first count bit to be injected externally at start-up. The Johnson counter generates a Gray code, a code in which adjacent states differ by only one bit. == Four-bit ring counter sequences == 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「ring counter」の詳細全文を読む スポンサード リンク
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