key-fsm v0.1.0

Turn a raw key level into debounced short and long events.

Application scenarios

• One power button with short press (on/off) and long press (reset or pairing).
• A single-key menu: short press advances, long press enters setup.
• A device with no room for a key matrix.

Contract

typedef enum { KEY_EVENT_NONE = 0, KEY_EVENT_SHORT = 1, KEY_EVENT_LONG = 2 } key_event_t;
void key_fsm_init(key_fsm_t *fsm, uint16_t debounce_ms, uint16_t long_ms);
key_event_t key_fsm_step(key_fsm_t *fsm, uint8_t raw, uint16_t elapsed_ms);

• Call on a fixed tick with elapsed milliseconds.
• Press is accepted after debounce_ms.
• Long event is emitted once after long_ms while held.
• Release before the long threshold emits one short event.
• Release after a long event emits none.
• No allocation.

Parameters

• debounce_ms: 10..30 ms for a mechanical key.
• long_ms: 600..1500 ms; longer avoids accidental long presses.

Memory and timing

• 12 bytes of state; O(1).

Integration

key_fsm_t key;
key_fsm_init(&key, 20, 1000);
/* every 1 ms: */
switch (key_fsm_step(&key, raw_pin(), 1)) {
case KEY_EVENT_SHORT: toggle_output(); break;
case KEY_EVENT_LONG: enter_setup(); break;
default: break;
}

When not to use

• A key matrix or multi-touch; build a scan layer and use debounce per key.
• The caller cannot provide a stable time base.

Verification

make test covers short, long, release after long, bounce, and zero timing.

Behavior reference: production key modes; this implementation is original.

KEY_EVENT_NONE = 0,
KEY_EVENT_SHORT = 1,
KEY_EVENT_LONG = 2

uint16_t debounce_ms;
uint16_t long_ms;
uint16_t stable_ms;
uint16_t held_ms;
uint8_t raw_last;
uint8_t pressed;
uint8_t long_sent;