F407 中printf 的实现

装载  自  http://bbs.21ic.com/icview-860453-1-1.html

配置串口，利用printf函数输出字符，结果输出的还是ASCII码，这是为什么？难道是我的配置问题？
串口配置是没有问题的，那个printf的配置是在网上找的资料，就不清楚了。。

1. #include "stm32f4xx.h"
2. #include <stdio.h>
3. void OpenCom(void)
4. {
5. USART_InitTypeDef USART_InitStructure;
6. NVIC_InitTypeDef NVIC_InitStructure;
7. GPIO_InitTypeDef GPIO_InitStructure;
8. RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA, ENABLE);
9. RCC_APB2PeriphClockCmd(RCC_APB2Periph_USART1, ENABLE);
10. GPIO_PinAFConfig(GPIOA, GPIO_PinSource9, GPIO_AF_USART1);
11. GPIO_PinAFConfig(GPIOA, GPIO_PinSource10, GPIO_AF_USART1);
12. GPIO_InitStructure.GPIO_Pin = GPIO_Pin_9|GPIO_Pin_10;
13. GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
14. GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP;
15. GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
16. GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
17. GPIO_Init(GPIOA, &GPIO_InitStructure);
18. USART_InitStructure.USART_BaudRate = BaudRate;
19. USART_InitStructure.USART_WordLength = USART_WordLength_8b;
20. USART_InitStructure.USART_StopBits = USART_StopBits_1;
21. USART_InitStructure.USART_Parity = USART_Parity_No;
22. USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
23. USART_InitStructure.USART_Mode = USART_Mode_Rx|USART_Mode_Tx;
24. USART_ITConfig(USART1,USART_IT_RXNE,ENABLE);
25. USART_Init(USART1, &USART_InitStructure);        /* USART configuration */
26. USART_Cmd(USART1, ENABLE); /* Enable USART */
27. USART_ClearFlag(USART1, USART_FLAG_TC);
28. NVIC_InitStructure.NVIC_IRQChannel=USART1_IRQn;
29. NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority=0;
30. NVIC_InitStructure.NVIC_IRQChannelSubPriority=0;
31. NVIC_InitStructure.NVIC_IRQChannelCmd=ENABLE;
32. NVIC_Init(&NVIC_InitStructure);
33. }
34. int main(void)
35. {
36. unsigned char i='a';
37. OpenCom();
38. printf("%c\r\n",i);
39. }
40. int fputc(int ch,FILE *f)
41. {
42. USART_SendData(USART1,(unsigned char)ch);
43. while(!(USART1->SR&USART_FLAG_TXE));
44. return (ch);
45. }

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