UART communication problem between Raspberry Pi 4 and STM32F767ZI

[ChristenTen]

Hello everyone,

I'm trying to debug a problem with the UART communication between the raspberry pi 4 and an STM32.

Indeed, I try to send the number 1 from the raspberry pi to the stm32 but I can't get the right result.

Here is the code I implemented on the raspberry pi 4 :

    import serial 
 
    from time import sleep
 
    
 
    ser = serial.Serial(
 
        port = "/dev/ttyS0",
 
        baudrate = 9600,
 
        parity = serial.PARITY_NONE,
 
        stopbits = serial.STOPBITS_ONE,
 
        bytesize = serial.EIGHTBITS,
 
        timeout = 1
 
        )  
 
data = 1  
 
    while True:
 
        data_str_list = [str(data)]
 
        send_string = ','.join(data_str_list)
 
        ser.write(send_string.encode())
 
        sleep(1)

Here is the STM32 code :

char data_sign[1];
 
 
int main(void)
 
{
 
  
 
  HAL_Init();
 
  SystemClock_Config();
 
  MX_GPIO_Init();
 
  MX_DMA_Init();
 
  MX_USART3_UART_Init();
 
  MX_DAC_Init();
 
  MX_UART4_Init();
 
  MX_TIM6_Init();
 
 
  while (1)
 
  {
 
    /* USER CODE END WHILE */
 
 
    /* USER CODE BEGIN 3 */
 
 
      HAL_UART_Receive(&huart4, data_sign, sizeof(data_sign), 1000);
 
 
  }
 
  /* USER CODE END 3 */
 
}
 
 
static void MX_UART4_Init(void)
 
{
 
 
  huart4.Instance = UART4;
 
  huart4.Init.BaudRate = 9600;
 
  huart4.Init.WordLength = UART_WORDLENGTH_8B;
 
  huart4.Init.StopBits = UART_STOPBITS_1;
 
  huart4.Init.Parity = UART_PARITY_NONE;
 
  huart4.Init.Mode = UART_MODE_TX_RX;
 
  huart4.Init.HwFlowCtl = UART_HWCONTROL_NONE;
 
  huart4.Init.OverSampling = UART_OVERSAMPLING_16;
 
  huart4.Init.OneBitSampling = UART_ONE_BIT_SAMPLE_DISABLE;
 
  huart4.AdvancedInit.AdvFeatureInit = UART_ADVFEATURE_NO_INIT;
 
  if (HAL_UART_Init(&huart4) != HAL_OK)
 
  {
 
    Error_Handler();
 
  }
 
}

When I send 1, I get this with the STM32 debug:

 

Sometimes, the value is 241 but I don't know how to interpret this data. Can you please help me please? I think it's a problem with the data types but I tried to change them but I still can't find the right result.

I also changed the clock frequency of the STM32 but nothing changes. The function HAL_UART returns 0.

[Bruno_ST]

Hello @ChristenTen​ ,

Raspberry PI 4 UART Levels for RX/TX are 0V to 5V and STM32 levels are from 0V to 3.3V.

You need a level shifter to secure communication from STM32 to rasberry PI4. Make sure that you use PIO that are 5V tolerent also.

When PI4 doinx a TX the characters received by STM32 should be undersood.

Try to perform a loopback (shortcut between TX & RX) on STM32 first to be sure the UART is working fine.

BR,

Bruno

[ChristenTen]

Thanks for your answer @Bruno_ST​.

The UART on Raspberry pi 4 is 3V3 and not 5V. I checked this with my scope.

[Muhammed Güler]

Your code on the STM32 side can only read 1 byte. The 1bytes of data you read looked like part of a UTF-8 encoded character.

If you don't know how many bytes the incoming data will be, below is an example I randomly found on google. it will solve your problem.

http://www.bepat.de/2020/12/02/stm32f103c8-uart-with-dma-buffer-and-idle-detection/

[ChristenTen]

@Muhammed G�ler​ 

What I wanted is to receive just 1 byte (0x01 or 0x00). Nevertheless, I get 240 but I can't see the link between 240 and 1 even with the UTF-8 encoding.

[Bruno_ST]

Sorry, you are right !! I mixed-up myself...

Bruno

[Javier1]

I know its a dumb question but is your wiring OK?

Are you wiring your stm32's GND with your raspi GND¿

[ChristenTen]

I already checked the connection : Tx_Raspberry -> Rx_STM32 and Rx_Raspberry ->Tx_STM32 and also same ground.

[Javier1]

i find ftdi's usb-UART modules very usefull to debug this kind of scenarios.

 

**extra: i had some problems back in the day with the raspi's internal bluetooth trying to take over the uart i was using .....