volatile uint8_t motion_pending = 0; //Generic motion indicator field for low-power waker callback

//...
/* Infinite main loop */
  /* USER CODE BEGIN WHILE */
  while (1)
  {
    /* USER CODE BEGIN 3 */
    // If there is no pending motion and no incoming request, go to STOP mode
    if (!motion_pending)
    {
      // Try to catch a request quickly; if none, sleep.
      if (HAL_UART_Receive(&huart3, rx_buf, 1, 50) != HAL_OK)
      {
        enter_stop_mode();
        continue; // after wake-up, loop again
      }
    }

    //Wait for the window request from the gateway and any motion
    if (HAL_UART_Receive(&huart3, rx_buf, 1, 500) == HAL_OK
    		&& rx_buf[0] == 0x01 && motion_pending)
    {
    	// Clear the motion flag now
        motion_pending = 0;

        // Prepare response window, then send
        tx_buf[0] = 0x00;
        tx_buf[1] = 0x00;
        tx_buf[2] = 0x06;

        if (imu_send_window(&hi2c2, &huart3, tx_buf, LoRa_MAX_DATA_SIZE, 3, 96, 10) == 0)
        {
          HAL_GPIO_TogglePin(GPIOA, USER_LED_Pin);
        }
      }
      // After sending, go back to STOP quickly (next loop iteration will sleep)
    }
//....