/* imu.c - LIS3DH FIFO burst → Edge Impulse → BLE packet * Running on Nordic nRF52 (64 KB RAM, no OS) */ static void twi_callback_acceleration(ret_code_t result, void *p_user_data) { /* Burst-read 32 FIFO samples (196 bytes) over I²C */ for (uint8_t rx_bit = 0; (rx_bit + 5) < IMU_RX_BUFFER_SIZE; rx_bit += 6) { /* Axis-flip compensation: B7/B8/W7 mount differently */ float acc[3] = { (float)(int8_t)(m_rx_buffer[rx_bit + 1]) * 32 * FLIP_X, (float)(int8_t)(m_rx_buffer[rx_bit + 3]) * 32 * FLIP_Y, (float)(int8_t)(m_rx_buffer[rx_bit + 5]) * 32 * FLIP_Z }; ei_wrapper_add_data(acc, 3); /* Feed ring buffer */ acceleration_data_count++; activity_detection_data_ready = true; } } void activity_detection(void) { if (!activity_detection_data_ready) return; activity_detection_data_ready = false; ei_impulse_result_t result = { 0 }; ei_wrapper_get_data(&result); /* DSP + TFLite inference */ /* Pick highest-probability class */ uint8_t main_activity = 0; uint8_t strongest = 0; for (size_t i = 0; i < EI_CLASSIFIER_LABEL_COUNT; i++) { uint8_t p = (uint8_t)(result.classification[i].value * 100); if (p > strongest) { strongest = p; main_activity = i + 1; } } /* Discard low-confidence or anomalous results */ if (strongest < CONFIDENCE_THRESHOLD_PERCENTAGE || result.anomaly > ANOMALY_THRESHOLD) main_activity = ACTIVITY_ANOMALY; /* Transitions >2 s → enqueue BLE packet to gateway */ if (current_activity != main_activity && (get_uptime() - previous_activity_change_time) > MINIMUM_ACTIVITY_LENGTH) activity_queue_packet(); ei_wrapper_start_prediction(0, acceleration_data_count); acceleration_data_count = 0; }