hysteresis v0.1.0

Keep a binary state while the input stays inside a band.

Application scenarios

• Battery voltage drives a low-battery flag; the flag must not chatter at the threshold.
• A thermostat turns on below one setpoint and off above another.
• A presence detector must not toggle on noise around its threshold.

Contract

void hysteresis_init(hysteresis_t *filter, int32_t low, int32_t high, uint8_t initial);
uint8_t hysteresis_step(hysteresis_t *filter, int32_t value);

• low < high.
• Values at or above high set the state.
• Values at or below low clear it.
• Inside the band the state is unchanged.
• No allocation.

Parameters

• low / high: the band; its width sets noise immunity.
• initial: the safe state at boot.

Memory and timing

• 12 bytes of state; O(1).

Integration

hysteresis_t battery;
hysteresis_init(&battery, 3200, 3400, 0);
uint8_t low = hysteresis_step(&battery, millivolts());

When not to use

• The state must change only after N samples; add debounce.
• The input is analog and must be smoothed; use ema first.

Verification

make test covers both thresholds, the band, and the initial state.

Behavior reference: production control loops; this implementation is original.

int32_t low;
int32_t high;
uint8_t state;