/* USER CODE BEGIN 0 */
uint8_t receiveBuffer[6];
uint8_t cmdReceived = 0;
/* USER CODE END 0 */

...............

  MX_USART1_UART_Init();
  MX_TIM1_Init();
  /* USER CODE BEGIN 2 */
  LL_USART_EnableIT_RXNE(USART1);
  LL_USART_EnableIT_ERROR(USART1);
  /* USER CODE END 2 */

/* USER CODE BEGIN 4 */

void USART_CharReception_Callback(void){
  static int bytes = 0;
  uint8_t received_char;
  
  received_char = LL_USART_ReceiveData8(USART1);
  if(received_char == '@'){
      bytes = 0;
      cmdReceived = 1;
  }
  else{
    receiveBuffer[bytes] = received_char;
    bytes++;
  }
    
  
  if (bytes == sizeof(receiveBuffer)){
    bytes = 0;
    cmdReceived = 1;
  }
 
    

}

/* USER CODE END 4 */

/**
  * @brief This function handles USART1 global interrupt / USART1 wake-up interrupt through EXTI line 25.
  */
void USART1_IRQHandler(void)
{
  /* USER CODE BEGIN USART1_IRQn 0 */
  USART_CharReception_Callback();
  /* USER CODE END USART1_IRQn 0 */
  /* USER CODE BEGIN USART1_IRQn 1 */

  /* USER CODE END USART1_IRQn 1 */
}