[STM32F4] stm32f407利用printf函数输出的还是ASCII码，不知道怎么回事？

#include "stm32f4xx.h"

#include <stdio.h>





void OpenCom(void)

{

  USART_InitTypeDef USART_InitStructure;

  NVIC_InitTypeDef NVIC_InitStructure; 

  GPIO_InitTypeDef GPIO_InitStructure;



  RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA, ENABLE);        

  RCC_APB2PeriphClockCmd(RCC_APB2Periph_USART1, ENABLE);

 

  GPIO_PinAFConfig(GPIOA, GPIO_PinSource9, GPIO_AF_USART1);        

  GPIO_PinAFConfig(GPIOA, GPIO_PinSource10, GPIO_AF_USART1);



  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_9|GPIO_Pin_10;

  GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;

  GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP;

  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;

  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;

  GPIO_Init(GPIOA, &GPIO_InitStructure);



  USART_InitStructure.USART_BaudRate = BaudRate;

  USART_InitStructure.USART_WordLength = USART_WordLength_8b;

  USART_InitStructure.USART_StopBits = USART_StopBits_1;

  USART_InitStructure.USART_Parity = USART_Parity_No;

  USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None;

  USART_InitStructure.USART_Mode = USART_Mode_Rx|USART_Mode_Tx;



  USART_ITConfig(USART1,USART_IT_RXNE,ENABLE);

  USART_Init(USART1, &USART_InitStructure);        /* USART configuration */

  USART_Cmd(USART1, ENABLE); /* Enable USART */

  USART_ClearFlag(USART1, USART_FLAG_TC);



  NVIC_InitStructure.NVIC_IRQChannel=USART1_IRQn;

  NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority=0;

  NVIC_InitStructure.NVIC_IRQChannelSubPriority=0;

  NVIC_InitStructure.NVIC_IRQChannelCmd=ENABLE;

  NVIC_Init(&NVIC_InitStructure);  

}



int main(void)

{  



   unsigned char i='a';     

   OpenCom();

   printf("%c\r\n",i);

            

 }

 

int fputc(int ch,FILE *f)

{

  USART_SendData(USART1,(unsigned char)ch);

  while(!(USART1->SR&USART_FLAG_TXE));

  return (ch);

}