Extra character in UART RX buffer

[BentoCopi]

I'm trying to receive data through USART1 using STM32F407. I decided to not use HAL api's to achieve that, so I'm receiving character by character and storing them in an array. Every character I send is received correctly but apart from the actual data I'm receiving an extra character in the first position of the buffer. It sometimes is a 'p' , sometimes a '\n' . I'm attaching some debug screenshots and a code snippet.

I enable the RX DR not empty interrupt:

__HAL_UART_ENABLE_IT(&huart1,UART_IT_RXNE);

My interrupt handler:

uint8_t group_input_str[250];
volatile uint32_t idx = 0;
void USART1_IRQHandler(void)
{
  /* USER CODE BEGIN USART1_IRQn 0 */
	if(__HAL_UART_GET_IT_SOURCE(&huart1,UART_IT_RXNE))
	{
			group_input_str[idx++] = huart1.Instance->DR;	
	}
	if(idx>=250) // prevent overflow
		idx=249;
  /* USER CODE END USART1_IRQn 0 */
  HAL_UART_IRQHandler(&huart1);
  /* USER CODE BEGIN USART1_IRQn 1 */
  /* USER CODE END USART1_IRQn 1 */
}

I sent a buffer containing help from the very begining, from another board(STM32F103),and the logic analiser caught :

But on the debugger I got:

UART configurations in both boards: 115200,8,N,1

I lowered down the baud rate, switched from HSE(8 MHz) to HSI, lowered down SYSCLK from 100MHz to 16MHz, but no success. What could be wrong?

[Andrew Neil]

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@BentoCopi wrote:

I send a buffer containing help from the very beginning, from another board (STM32F103)

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That introduces a level of uncertainty.

So start by just sending individual characters from a terminal.

Examine your group_input_str RX buffer before you send the first character.

Examine it again after sending each character.

[BentoCopi]

Strings sent from the terminal are received correctly, by the way. 

[Andrew Neil]

Which suggests that there's something wrong in your "other board", and/or the way you're using it.

How do you ensure that the receiving board is synchronised to the transmission?

Your LA trace suggests that the "other board" is actually sending a load of NULs - is that right?

If so, how many?

[BentoCopi]

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How do you ensure that the receiving board is synchronised to the transmission?

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As far as I know , once the interrupt is enabled in the receiver board, it just waits for the transmitter to start the streaming in the correct baud rate(asynchronous). 

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Your LA trace suggests that the "other board" is actually sending a load of NULs - is that right?

If so, how many?

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Correct. There are 13 of them. I have a 17 bytes long buffer in the transmitter board. I fill it with help and send it using 

HAL_UART_Transmit(&huart1,str_temp, 17,100);

uint8_t* str_tmp[17] = {'h','e','l','p','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0','\0'};

[Andrew Neil]

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@BentoCopi wrote:

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How do you ensure that the receiving board is synchronised to the transmission?

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As far as I know , once the interrupt is enabled in the receiver board, it just waits for the transmitter to start the streaming in the correct baud rate(asynchronous). 

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So what starts the transmitter board transmitting?

How does  the transmitter board know that the receiver is ready to start receiving before it starts transmitting?

[BentoCopi]

For the sake of tests, for now, I power on both boards and set a breakpoint before HAL_UART_Transmit in the transmitter board. Then I step to send the data.

[Andrew Neil]

So do as suggested for the terminal:

1. Examine your group_input_str RX buffer before you send the first character
2. step your transmitter to send just one character
3. Examine your group_input_str buffer again; check that the correct character was received
4. repeat for the whole string.