bootloader---15.串口

串口时钟使用了PCLK，所以在初始化串口之前需要先将系统时钟初始化，(除非想一直使用默认时钟)。

一、代码

1. start.S 主要功能是：

     a.关闭看门狗

     b. 初始化系统时钟

     c. 设置堆栈

     d. 跳到main函数

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 .text
 .global _start
 _start:
     /*disable watchdog*/
     ldr r0, =0x53000000
     mov r1, #0x0
     str r1, [r0]

 #define COCKTIME    0x4C000000
 #define MPLLCON        0x4C000004
 #define UPLLCON        0x4C000008
 #define CLKCON        0x4C00000C
 #define CLKSLOW        0x4C000010
 #define CLKDIVN     0x4C000014
 #define CAMDIVN     0x4C000018
     /*FCLK:HCLK:PCLK=1:4:8*/
     ldr r0, =CLKDIVN
     mov r1, #0x05
     str r1, [r0]

     mrc p15, 0, r0, c1, c0, 0
     orr r0, r0, #0xc0000000
     mcr p15, 0, r0, c1, c0,0

     /*MPLL=(2*m*Fin)/(P*(1<<S)), m=(MDIV+8), p=PDIV+2 s=SDIV*/
     ldr r0, =MPLLCON
     ldr r1, =((0x5C<<12)|(0x01<<4)|(0x01))
     str r1, [r0]

     ldr r0, =0x10000
 1:
     sub r0, r0, #1
     bne 1b

     /*UPLL=(m*Fin)/(P*(1<<S)), m=(MDIV+8), p=(PDIV+2), s=SDIV*/
     ldr r0, =UPLLCON
     ldr r1, =((0x10<<12)|(0x01<<4)|(0x01))
     str r1, [r0]

     ldr r0, =0x10000
 1:
     sub r0, r0, #1
     bne 1b

     /* set up the stack    */
     ldr sp, =1024*4
     bl main

 loop:
     b loop

2. main.c 主要功能是：

a. 初始化串口

b. 进入死循环，每次点亮一个led灯，顺便打印一个字母'a'

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 #include "uart.h"
 #define GPBCON (*(volatile unsigned int *) 0x56000010)
 #define GPBDAT (*(volatile unsigned int *) 0x56000014)

 static inline void delay (unsigned long loops)
 {
     __asm__ volatile ("1:\n"
             "subs %0, %1, #1\n"
             "bne 1b":"=r" (loops):"0" (loops));
 }

 void main()
 {
     int i = 1;
     GPBCON = 0x15400;
     init_uart();
     while(1)
     {
         GPBDAT = 0x7FF&(~i<<5);
         i *= 2;
         if(16==i)
             i = 1;
         delay(400000);
         putc('a');
     }
     return ;
 }

3.uart.c 主要功能：

a. 初始化串口

b. 实现发送与接收函数getc与putc

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 #define GPHCON (* (volatile unsigned int *) 0x56000070)
 #define GPHUP (* (volatile unsigned int *) 0x56000078)

 #define ULCON0 (* (volatile unsigned int *) 0x50000000)
 #define UCON0 (* (volatile unsigned int *) 0x50000004)
 #define UFCON0 (* (volatile unsigned int *) 0x50000008)
 #define UMCON0 (* (volatile unsigned int *) 0x5000000C)
 #define UTRSTAT0 (* (volatile unsigned int *) 0x50000010)
 #define UERSTAT0 (* (volatile unsigned int *) 0x50000014)
 #define UFSTAT0 (* (volatile unsigned int *) 0x50000018)
 #define UMSTAT0 (* (volatile unsigned int *) 0x5000001C)
 #define UTXH0 (* (volatile unsigned char *) 0x50000020)
 #define URXH0 (* (volatile unsigned char *) 0x50000024)
 #define UBRDIV0 (* (volatile unsigned int *) 0x50000028)

 #define BAUD_RATE 115200

 void uart_init(void)
 {
     GPHCON = 0x000000A0;
     GPHUP = 0x000007FF;
     ULCON0 = (0x03); //8N1
     UCON0 = (0x01<<2)|(0x01);
     UFCON0 = 0;
     UMCON0 = 0;
     UBRDIV0 = (int)((50*1000*1000)/(BAUD_RATE*16))-1;
 }

 unsigned char uart_getc(void)
 {
     while(!(UTRSTAT0 & 0x01))
         ;
     return (URXH0 & 0xff);
 }

 void uart_putc(char c)
 {
     while (!(UTRSTAT0&0x02))
         ;
     UTXH0 = c;
     if('\n' == c)
         uart_putc('\r');
 }

 void uart_puts(char *s)
 {
     while (*s)
     {
         uart_putc(*s++);
     }
 }

注: uart.c中的uart_putc函数( ascii中:  \n__换行__LF__0x10   \r__回车__CR__0x13  )

1. 当键盘敲下 Enter键时，从串口收到的按键是0x0d(即:\r 回车符:将光标移动到行首，而不换行)。

2. 当调用puts("hello\n");时putc收到\n后，将换行但光标的列不变，然后再输出\r，将光标移动到换行之后的行首，这样 效果 就跟平常的回车一样了。

4. uart.h

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 void uart_init(void);
 unsigned char uart_getc(void);
 void uart_putc(char c);

编译时只需要在Makefile的OBJS中添加uart.o就可以了。

二、测试一下getc函数

2.1 将main中主循环改为接收一个字符然后打印

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 uart_init();
 while(1)
     {
         c=uart_getc();
         uart_putc(c);
     }