//....
static void enter_stop_mode(void)
{
  // Clear pending EXTI to avoid immediate wake-up
  __HAL_GPIO_EXTI_CLEAR_IT(MPU_INT_Pin);

  HAL_SuspendTick();
  HAL_PWR_EnterSTOPMode(PWR_LOWPOWERREGULATOR_ON, PWR_STOPENTRY_WFI);
  HAL_ResumeTick();

  //After STOP, clocks must be restored
  SystemClock_Config();

  //Re enable the interfaces being used
  MX_USART3_UART_Init();
  MX_I2C2_Init();
}

//....