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Issue getting all data over USART with LL API LL_USART_ReceiveData8

Associate II


   I am using STM32F407VET6 MCU and trying to receive data using LL API LL_USART_ReceiveData8 over USART configured as modbus.

   I confirmed USART transmit (

LL_USART_TransmitData8) is successful through an modbus APP, on rx side of the other device as well as fact  that LL_USART_IsActiveFlag_TC is returning True.

  I also confirmed other receiving device also sending a 8 byte response back. (Confirmed via Modbus app)

  But on the STM32F407VET6 MCU side I am just receiving 1 byte of data. 

  Note - If I don't insert delay (LL_mDelay) I only get first byte, if I add delay I only get 2nd byte. 

After receiving one byte , its just forever waiting on LL_USART_IsActiveFlag_RXNE (never comes out of while loop).

Based on online search I see lots of folks are seeing the same problem (

LL_USART_IsActiveFlag_RXNE getting overrun ?)  but could not find solution yet.
If HAL_UART_Receive is used instead of LL_USART_ReceiveDatta8, MCU is receiving complete response.
Below is my code :-
void LL_USART_Transmit(USART_TypeDef *USARTx, uint8_t *buff, uint32_t len)
    printf("LL_USART_Transmit() rcvd Len= %d\n", len);

   for (int i = 0; i < len; i++)
      LL_USART_TransmitData8(USARTx, buff[i]);
      if (LL_USART_IsActiveFlag_TC(USARTx)) {
        printf("LL_USART_Transmit() successful, data= %d\n", buff[i]);
         printf("LL_USART_Transmit() TC flag False");
uint8_t LL_USART_Receive_Byte(USART_TypeDef *USARTx) {
       // Wait for Read Data Register is not empty
      while (!LL_USART_IsActiveFlag_RXNE(USARTx));
      return LL_USART_ReceiveData8(USARTx);
void LL_USART_Receive(USART_TypeDef *USARTx, uint8_t *buff, uint32_t len)
        printf("LL_USART_Receive() rcvd Len= %d\n", len);

         for (int i = 0; i < len; i++)
                buff[i] = LL_USART_Receive_Byte(USARTx);

               printf("LL_USART_Receive() rx_data= %d\n", buff[i]);

You probably want to pay attention to receive and transmit concurrently and not block, as both things can and do happen at once.

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Can you please elaborate ? Is there any code change recommendations compared to what I posted ?


Consider what is happening on the line. When you send 8 characters, your code is inside LL_USART_Transmit. It is not reading character coming in. So when characters are sent back, and you're still within the sending code, they are missed. UART reception is typically done best in a non-blocking manner such as using HAL_UART_Receive_IT or HAL_UART_Receive_DMA or HAL_UARTEx_ReceiveToIdle_*.


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