UART4 RX Signal level

[Kim.Andy]

I use the UART4 like the following initialization.

void uart4_configuration(void)
{
    GPIO_InitTypeDef GPIO_InitStructure;
    USART_InitTypeDef USART_InitStructure;
    /* enable usart clock */
    RCC_APB1PeriphClockCmd(RCC_APB1Periph_UART4, ENABLE);
 
 
    RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOC, ENABLE);
 
 
    // Configure USART Tx as alternate function
    GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
    GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP;            //GPIO_InitStructure.GPIO_PuPd  = GPIO_PuPd_NOPULL;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
    GPIO_InitStructure.GPIO_Pin = RS485AXM_TX_PIN;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
    GPIO_Init(RS485AXM_COM_PORT, &GPIO_InitStructure);
 
    // Configure USART Rx as alternate function
    GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
    GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_DOWN;            //GPIO_InitStructure.GPIO_PuPd  = GPIO_PuPd_NOPULL;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
    GPIO_InitStructure.GPIO_Pin = RS485AXM_RX_PIN;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
    GPIO_Init(RS485AXM_COM_PORT, &GPIO_InitStructure);
 
    GPIO_PinAFConfig(GPIOC, GPIO_PinSource10, GPIO_AF_UART4);
    GPIO_PinAFConfig(GPIOC, GPIO_PinSource11, GPIO_AF_UART4);
 
 
    USART_StructInit(&USART_InitStructure);
    USART_InitStructure.USART_BaudRate = 9600;
    USART_Init(UART4, &USART_InitStructure);
 
    USART_Cmd(UART4, ENABLE);
 
    NVIC_EnableIRQ(UART4_IRQn);
}

The Rx signal level is abnormal like the following.

The low level is not the zero(ground) level.

But the Tx level is ok like the followings.

Although I detach the RX Pin from the PCB Board, and tie with the TX Pin,

RX/TX Pin level is like the abnormal level(the first image)

Is the initialization is wrong?

Thanks

[Olivier GALLIEN]

Hi @AK.12.26im​ 

In order to reach faster the relevant community could you please add further information about your context ?

chip, board, application ...

Thanks

Olivier

[PatrickF]

Sound like your usage of GPIO_Init() is wrong, you miss the GPIO_InitStruct.Alternate setting and GPIO_PinAFConfig() does not exist in CubeMP1 drivers.

Here is an example of UART AF mux setting generated by CubeMx:

    /**UART4 GPIO Configuration    
    PD1     ------> UART4_TX
    PH14     ------> UART4_RX 
    */
    GPIO_InitStruct.Pin = GPIO_PIN_1;
    GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
    GPIO_InitStruct.Pull = GPIO_NOPULL;
    GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
    GPIO_InitStruct.Alternate = GPIO_AF8_UART4;
    HAL_GPIO_Init(GPIOD, &GPIO_InitStruct);
 
    GPIO_InitStruct.Pin = GPIO_PIN_14;
    GPIO_InitStruct.Mode = GPIO_MODE_AF;
    GPIO_InitStruct.Pull = GPIO_NOPULL;
    GPIO_InitStruct.Alternate = GPIO_AF8_UART4;
    HAL_GPIO_Init(GPIOH, &GPIO_InitStruct);