rotary-encoder v0.1.0

Quadrature decoder for a mechanical rotary encoder.

Application scenarios

• A menu knob on an instrument.
• A setpoint or volume control.
• A position readout on a manual stage.

Contract

typedef struct {
    uint8_t state;
    int16_t position;
} encoder_t;

void encoder_init(encoder_t *encoder);
int8_t encoder_update(encoder_t *encoder, uint8_t a, uint8_t b);
int16_t encoder_position(const encoder_t *encoder);

• a and b are the two switch levels, 0 or 1.
• update returns +1 or -1 for a valid edge, 0 for no change or an invalid jump.
• position accumulates every valid edge; divide by four for one detent.
• Call at a fixed rate or from a pin-change interrupt.

Parameters

• Mechanical encoders often produce four edges per detent.
• Add an RC filter or sample debounce when the encoder is noisy.
• Reverse the sign in the adapter if the direction is inverted.

Memory and timing

• 4 bytes; O(1); no allocation.

Integration

encoder_t knob;
encoder_init(&knob);
int8_t step = encoder_update(&knob, read_a(), read_b());
if (step != 0) {
    menu_move(step);
}

Adapters

• Read the two GPIO pins with pull-ups.
• Reference: PaulStoffregen/Encoder (MIT) state-table behavior.
• This implementation is original.

When not to use

• A motor encoder with an index or high count; use a hardware QEP.
• A noisy line without filtering; debounce first.

Verification

make test covers a full clockwise cycle, counter-clockwise, invalid jumps,

and a bounce sequence.

uint8_t state;
int16_t position;