初次使用stm32的串口，参照网上找到的f1的例程改了改用在f4上。可是从超级终端上看就是没有收到数据。是我配置的有问题么？用的是usart3，配置文件如下：

void USART_RCC_Configuration(void)
{
  RCC_APB1PeriphClockCmd(RCC_APB1Periph_USART3, ENABLE);
  RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOD, ENABLE);
}

void USART_GPIO_Configuration(void)
{
  GPIO_InitTypeDef GPIO_InitStructure;
 
  GPIO_InitStructure.GPIO_Mode  = GPIO_Mode_AF;
  GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
  GPIO_InitStructure.GPIO_PuPd  = GPIO_PuPd_UP;
  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_100MHz;
  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_8|GPIO_Pin_9;
  GPIO_Init(GPIOD, &GPIO_InitStructure);
 
  GPIO_PinAFConfig(GPIOD, GPIO_PinSource8, GPIO_AF_USART3);
  GPIO_PinAFConfig(GPIOD, GPIO_PinSource9, GPIO_AF_USART3);
}

void USART_Configuration(void)
{
  USART_InitTypeDef USART_InitStruct;
 
  USART_RCC_Configuration();
 
  USART_InitStruct.USART_BaudRate = 115200;
  USART_InitStruct.USART_StopBits = USART_StopBits_1;
  USART_InitStruct.USART_WordLength = USART_WordLength_8b;
  USART_InitStruct.USART_Parity = USART_Parity_No;
  USART_InitStruct.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
  USART_InitStruct.USART_Mode = USART_Mode_Tx | USART_Mode_Rx;
 
  USART_Init(USART3, &USART_InitStruct);
  USART_ITConfig(USART3,USART_IT_RXNE,ENABLE);
  USART_Cmd(USART3, ENABLE);
 
  USART_GPIO_Configuration();
}

主程序：
int main(void)
{
    SystemInit();
    USART_Configuration();
    printf("x0c");printf("x0c");
    printf("33[1;40;32m");
    printf(" *******************************************************************************");
    printf(" ************************ Copyright 2009-2012, ViewTool ************************");
    printf(" *************************** http://www.viewtool.com ***************************");
    printf(" ***************************** All Rights Reserved *****************************");
    printf(" *******************************************************************************");
    printf(" ");
    while(1)
    {
           
        if(USART_GetFlagStatus(USART3, USART_IT_RXNE) != RESET)
        {
        printf("%c",USART_ReceiveData(USART3));
        }

    }
}
还请各位指导！