Ring & Johnson Counters#

Ring and Johnson counters count differently from a binary counter: instead of incrementing a number, they circulate a pattern around a shift register. They trade code efficiency for something valuable — outputs that are already decoded and free of glitches.

Ring Counter#

A ring counter is a shift register with its last output fed straight back to its first input. Seed it with a single 1 and that one bit walks around the loop: 1000 → 0100 → 0010 → 0001 → 1000. With N flip-flops it has N states, and exactly one output is active at a time — a one-hot sequence. Because each output is a decoded state, no decoding logic is needed and there are none of the ripple-counter decoding glitches: the outputs are already clean, one-per-step control signals. That makes ring counters ideal for sequencers, stepper-motor phase drives, and LED chasers.

Johnson Counter#

A Johnson counter (also called a twisted-ring or switch-tail counter) feeds back the inverted last output instead. That doubles the sequence: N flip-flops now cycle through 2N states — 000 → 100 → 110 → 111 → 011 → 001 → 000. Its useful property is that only one bit changes per step (a Gray-code-like sequence), so decoding any state takes just a two-input gate and, because no two bits race, the decoded outputs are glitch-free.

The Trade#

The cost is efficiency. A binary counter gets 2ᴺ states from N flip-flops; a ring counter gets only N and a Johnson counter only 2N. Spending flip-flops so lavishly to represent a count would be wasteful — but that is the wrong way to see them. What they buy is decoded, glitch-free outputs with almost no logic and very high speed, which is exactly what a sequencer or phase generator wants. They are the counting family’s link to one-hot state encoding and simple sequencers, where “which step are we on” matters more than “what number is this.”