STM32 UART receive method

[danbates (Daniel Bates)]

After a conversation with @dBC about receiving over UART I’ve come up with the following method.

in main.c

/* USER CODE BEGIN 2 */
  // enable IDLE flag interrupt. this will trigger UART interrupt handler in the event the host stop transmitting.
  __HAL_UART_ENABLE_IT(&huart2, UART_IT_IDLE);
  // this initialises the UART Rx DMA. UART is now listening and received data will load directly to buffer.
  HAL_UART_Receive_DMA(&huart2, rx_buff, sizeof(rx_buff));
  /* USER CODE END 2 */

in stm32f3xx_it.c

void USART2_IRQHandler(void)
{
  /* USER CODE BEGIN USART2_IRQn 0 */

  /* USER CODE END USART2_IRQn 0 */
  HAL_UART_IRQHandler(&huart2);
  /* USER CODE BEGIN USART2_IRQn 1 */
  if (__HAL_UART_GET_FLAG(&huart2, UART_FLAG_IDLE) == SET)
  {
    __HAL_UART_CLEAR_FLAG(&huart2, UART_FLAG_IDLE);
    rx_flag = 1;
  /* USER CODE END USART2_IRQn 1 */
}

in main.c

/* USER CODE BEGIN WHILE */
  while (1)
  {
    // if the IDLE flag interrupt has been trigger, this flag should be set to indicate the Rx buffer has data.
    if (rx_flag)
    {
      uint16_t currentMillis1 = HAL_GetTick();
      //
      HAL_UART_DMAStop(&huart2); 
      memset(rx_buff,0,sizeof(rx_buff));
      HAL_UART_Receive_DMA(&huart2, rx_buff, sizeof(rx_buff));
      rx_flag = 0;
      //
      uint16_t currentMillis2 = HAL_GetTick();
      uint16_t millis_taken = currentMillis2 - currentMillis1;
      

      sprintf(log_buffer, "millis_taken: %d\r\n", millis_taken);
      debug_printf(log_buffer);
    }

    // print the millis since boot every interval.
    currentMillis = HAL_GetTick();
    if (currentMillis - previousMillis >= interval[0])
    {
      previousMillis = currentMillis;
      sprintf(log_buffer, "millis: %ld\r\n", currentMillis);
      debug_printf(log_buffer);
    }
    /* USER CODE END WHILE */

just bashing send with a mouse button.

millis_taken: 0
millis: 212000
millis_taken: 0
millis_taken: 0
millis_taken: 0
millis_taken: 0
millis_taken: 0
millis_taken: 0
millis_taken: 0
millis_taken: 0
millis_taken: 0
millis_taken: 0
millis_taken: 0
millis_taken: 0
millis: 214000
millis_taken: 0
millis_taken: 0
millis_taken: 0
millis_taken: 0
millis_taken: 0
millis_taken: 0
millis_taken: 0
millis_taken: 0
millis_taken: 0
millis_taken: 0
millis: 216000
millis: 218000

I’d like to know how many clock cycles is takes. Anyone know how to check?

[dBC]

Just for the record, I use and recommend this tutorial from stm for the uart rx path.

@danbates

uint16_t currentMillis1 = HAL_GetTick();

HAL_GetTick() returns a uint32_t so I think that might be why you’re getting the occasional wrap.

@danbates

I’d like to know how many clock cycles is takes. Anyone know how to check?

Check out the DWT registers, they have a nifty cycle counter. That’s not an stm32 thing, it’s in the ARM core (M3 and M4) so the ARM documentation will tell you how to use them.

[danbates (Daniel Bates)]

@dBC

I use and recommend this tutorial from stm for the uart rx path.

Important to have this logged for sure. I couldn’t get the LL drivers working but could be something to revisit.

There’s a more efficienct way of resetting the dma buffer index too. Instead of HAL_UART_DMAStop() and HAL_UART_Receive_DMA(), there’s something like huart2.hdmarx->Instance->CNDTR = sizeof(rx_buff). Couldn’t get that working.

@dBC

HAL_GetTick() returns a uint32_t

You remind me now, I read it’s a 20-bit counter so needs a 32-bit holder. uint32_t should be used instead aye.

@dBC

Check out the DWT registers

Will do.

[danbates (Daniel Bates)]

@dBC

I think that might be why you’re getting the occasional wrap.

The serial log has millis posted every 2 secs, the millis_taken is a response to a Tx from my computer. The log above shows it’s taking less than 1ms to process a received string of about 50 chars to reset the rx_buffer and get ready for the next packet.
uint32_t wouldn’t have made a difference in this case as the program is early in it’s counting, and GetTick hasn’t gone as far as 65,535.
Does this make sense?

…I’ll stick a deliberate delay in there to get some clearer numbers, making sure the millis_taken is giving something expected. Will report back.

[dBC]

@danbates

I read it’s a 20-bit counter

Not sure what you’re referring to there. Remember the HAL source code is right there in your tree alongside your code. You can see there that HAL_GetTick() does nothing more than return the current contents of a variable called uwTick. It’s declared as uint32_t and gets incremented each systick interrupt (every msec).

You should use uint32_t to store timestamps, then you can do (t2 - t1) type measurements without any fear of wrapping as the wrapping is handled by the unsigned subtraction.

[danbates (Daniel Bates)]

Agreed on using uint32_t, that’s how it’s done.
Correction, it’s a 24-bit decrement timer, the system tick. Doesn’t strictly matter for the purpose of this test, knowing this.

I’ve tested the code again. This time copying the incoming array to a separate array, to create something that could get used futher on in the program.

if (rx_flag)
{
  uint32_t currentMillis1 = HAL_GetTick();
  memcpy(rx_string, rx_buff, sizeof(rx_buff));
  memset(rx_buff,0,sizeof(rx_buff));
  HAL_UART_DMAStop(&huart2); 
  HAL_UART_Receive_DMA(&huart2, rx_buff, sizeof(rx_buff));
  rx_flag = 0;
  uint32_t currentMillis2 = HAL_GetTick();
  uint32_t millis_taken = currentMillis2 - currentMillis1;
  sprintf(log_buffer, "rx_string: %s\r\n", rx_string);
  debug_printf(log_buffer);
  sprintf(log_buffer, "millis_taken: %d\r\n", millis_taken);
  debug_printf(log_buffer);
}

Still takes less than 1ms.
Simplified:

if (rx_flag)
{
  rx_flag = 0;
  memcpy(rx_string, rx_buff, sizeof(rx_buff));
  memset(rx_buff,0,sizeof(rx_buff));
  HAL_UART_DMAStop(&huart2); 
  HAL_UART_Receive_DMA(&huart2, rx_buff, sizeof(rx_buff));
}

sending 50 chars at a time at 460800 baud. 22 seconds worth of button bashing.

millis: 12000
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
millis: 14000
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
millis: 16000
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 1
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
millis: 18000
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 1
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
millis: 20000
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 1
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
millis: 22000
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
millis: 24000
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
millis: 26000
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 1
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
millis: 28000
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
millis: 30000
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 1
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
millis: 32000
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0
millis: 34000
rx_string: 12345678901234567890123456789012345678901234567890
millis_taken: 0

I’m pretty much content with this. It works.
I’d like to directly reset the counter of the DMA register instead of DMA STOP/START, then it’s nearer on par with the efficiency of a DMA ring buffer outlined in the tutorial above. Edit: Got it.
huart2.hdmarx->Instance->CCR &= ~DMA_CCR_EN;
huart2.hdmarx->Instance->CNDTR = sizeof(rx_buff);
huart2.hdmarx->Instance->CCR |= DMA_CCR_EN;