pir v0.1.0

PIR motion detector signal chain for sampled pyroelectric data.

Application scenarios

• A raw PIR/DOCI front end that outputs digitized samples.
• A battery motion sensor with warm-up and output lock.
• A product that needs a 6-sample trimmed-mean pyroelectric chain.

Contract

void pir_init(pir_t *pir, uint16_t motion_span);
uint8_t pir_step(pir_t *pir, uint16_t sample, uint32_t now_ms);
void pir_set_warm(pir_t *pir, uint8_t warm);
void pir_output_changed(pir_t *pir, uint32_t now_ms);

• step consumes one sample and returns 1 when sustained motion is published.
• Warm-up, jam, and output locks inhibit events but not acquisition/filtering.
• motion_span is the peak-to-peak window threshold in sample units.
• output_changed restarts the 3 s output lock; two consecutive jumps over

the jam delta start the 5 s error lock.

Parameters

• 6 samples per batch; drop min/max, average the middle four.
• 6 batch averages form the window; 16 warm-up batches are skipped.
• Three strict same-direction pairs at window positions (5,4), (3,2), (1,0)

publish an event.

• Defaults: jam delta 400, output lock 3000 ms, error lock 5000 ms.

Tune motion_span for the selected sensitivity.

Memory and timing

• 32 bytes of state; no allocation.
• One bounded 6-entry scan per batch; O(1) per sample.
• Call step at the sensor's sample cadence; do not replay old samples.

Integration

pir_t pir;
pir_init(&pir, 245);
pir_set_warm(&pir, 1);
for (;;) {
    uint16_t sample = read_pir_sample();
    if (pir_step(&pir, sample, millis())) {
        publish_motion();
    }
}

Adapters

• The adapter powers the module, provides the sample frame, and restarts the

output lock when the product output changes.

• Behavior reference: a 6-sample trimmed-mean pyroelectric chain with

three-pair slope detection; this implementation is original.

When not to use

• A digital HC-SR501 output; use a debounce/hold timer instead.
• A sensor without a controlled sample cadence or timestamp.

Verification

make test covers the warm-up skip, a rising window event, a falling window,

the span threshold, and the double-jump error lock.

uint16_t window[PIR_WINDOW_SAMPLES];
uint16_t minimum;
uint16_t maximum;
uint32_t sum;
uint32_t lock_start;
uint16_t lock_duration;
uint16_t motion_span;
uint16_t last_sample;
uint8_t count;
uint8_t window_count;
uint8_t first_batches;
uint8_t warm;
uint8_t jam;
uint8_t locked;