Ring Counter and Johnson Counter

A ring counter and a Johnson counter are two shift-register-based counters that circulate a pattern of bits through flip-flops. They are not binary counters — they produce a repeating sequence of one-hot or walking patterns that are easy to decode without extra logic. This guide explains both, their sequences and where each is used.

Ring counter

A ring counter is a shift register whose serial output is fed back to its serial input, with a single 1 circulating through the stages. For a 4-bit ring counter, the sequence is:

An n-bit ring counter has n unique states. Each output is already one-hot, so it can drive a decoded signal (like a stepper-motor phase or a time-slot enable) directly — no decoder needed.

Johnson counter (twisted-ring counter)

A Johnson counter is a ring counter in which the complement of the last flip-flop’s output is fed back to the first. This doubles the state count. For a 4-bit Johnson counter the sequence is:

An n-bit Johnson counter has 2n unique states — twice the ring counter for the same flip-flop count.

Ring vs Johnson counter

Where they are used

Ring counters are used for sequencing and one-hot control (phase generation, time-division multiplexing, state machines where a one-hot encoding is convenient). Johnson counters are used to generate quadrature or multiphase clocks and symmetric timing waveforms. Both avoid the carry-chain delay of a binary counter, so they are fast and glitch-light.

Self-starting concern

A plain ring or Johnson counter can power up in an invalid state (for example all zeros in a ring counter, which then never recovers). Practical designs add reset/decode logic to force a valid starting state — a “self-correcting” counter.

FAQs on ring and Johnson counters

What is the difference between a ring counter and a Johnson counter?

A ring counter feeds the last flip-flop’s true output back to the first and has n states; a Johnson counter feeds back the complement and has 2n states for the same n flip-flops.

How many states does an n-bit Johnson counter have?

2n states — for example a 4-bit Johnson counter cycles through 8 states.

Why use a ring counter instead of a binary counter?

Its outputs are already one-hot, so sequencing signals need no decoder, and it has no carry chain, so it is fast and glitch-free for control applications.

What is a self-starting counter?

One with added logic that forces it out of any invalid/lock-up state into the valid sequence, guaranteeing correct operation after power-up.

Learn sequential design and RTL coding in ChipXpert’s RTL Design course. Related: shift registers, counters in digital electronics.