STM8S MCU serial port debugging

    I recently encountered some problems in debugging on the STM8S microcontroller, so I shared them with you!

    Question: When debugging STM8S with the IAR compiler, the program was paused and found that it could not run normally and was stuck in the serial port receive interrupt function.

    Analysis: It is suspected that the serial port receiving interrupt function did not clear the flag bit, but the function does have a corresponding clear interrupt statement, which is as follows:   

uint8_t tmp;

UART3_ClearFlag(UART1_FLAG_RXNE);

tmp = UART3_ReceiveData8();

    Enter the UART3_ClearFlag(UART1_FLAG_RXNE) function to view the code:

void UART3_ClearFlag(UART3_Flag_TypeDef UART3_FLAG)

{

    /* Check the parameters */

    assert_param(IS_UART3_CLEAR_FLAG_OK(UART3_FLAG));

    /*Clear the Receive Register Not Empty flag */

    if (UART3_FLAG == UART3_FLAG_RXNE)

    {

        UART3->SR = (uint8_t)~(UART3_SR_RXNE);

    }

    /*Clear the LIN Break Detection flag */

    else if (UART3_FLAG == UART3_FLAG_LBDF)

    {

        UART3->CR4 &= (uint8_t)(~UART3_CR4_LBDF);

    }

    /*Clear the LIN Header Detection Flag */

    else if (UART3_FLAG == UART3_FLAG_LHDF)

    {

        UART3->CR6 &= (uint8_t)(~UART3_CR6_LHDF);

    }

    /*Clear the LIN Synch Field flag */

    else

    {

        UART3->CR6 &= (uint8_t)(~UART3_CR6_LSF);

    }

}

    It was found that the receive interrupt flag was indeed cleared, so I went to the STM8S reference manual to look at the description of the UART3->SR register and found that in addition to the receive interrupt, the serial port communication also had an overload error interrupt.

    The receive interrupt is described as follows:

    RXNE: Read data register not empty

    This bit is set by hardware when the data in the RDR shift register is transferred to the UART_DR register.

    If the RXNEIE bit in the register is 1, an interrupt is generated. A read operation on UART_DR can clear this bit. The RXNE bit can also be

    For UART2 and UART3, this bit can also be cleared by writing 0.

    0: data not received;

    1: Data received and can be read.

    The overload error interrupt is described as follows:

    RXNE: Read data register not empty

    This bit is set by hardware when the data in the RDR shift register is transferred to the UART_DR register.

    If the RXNEIE bit in the register is 1, an interrupt is generated. A read operation on UART_DR can clear this bit. The RXNE bit can also be

    For UART2 and UART3, this bit can also be cleared by writing 0.

    0: data not received;

    1: Data received and can be read.

    Therefore, it is found that UART3_ClearFlag (UART1_FLAG_RXNE) only clears the receive interrupt, but does not clear the overload error interrupt, because to clear the overload error interrupt, UART_SR must be read first, and then UART_DR.

    The statement executed in the UART3_ClearFlag(UART1_FLAG_RXNE) function only writes UART->SR but does not read UART->SR.   

 UART3->SR = (uint8_t)~(UART3_SR_RXNE);

    Solution: Therefore, use UART3->SR &= 0xDF instead of UART3_ClearFlag(UART1_FLAG_RXNE), which can clear both the receive interrupt and the overload error interrupt. In this way, if you pause during debugging, the program will not die in the interrupt.   

uint8_t tmp;

UART3->SR &= 0xDF;

tmp = (uint8_t)UART3->DR;

    Question: During STM8S debugging, sometimes some statements cannot hit breakpoints.

    Analysis: I have read articles about optimization before and felt that it was related to optimization. After trying it, it turned out to be true.

    Solution: Because the optimization level in the program configuration is too high, change it to no optimization, and breakpoints can be successfully set during debugging.