串行通讯

   双线   uart    （全双工    异步）

  双线   iic         （半双工   同步）

  三线   spi        （全双工   同步）

   并行通讯

        多根数据线 地址线，如 内存

   硬件连接图：  

寄存器： 

     GPA1CON

    ULCON2 

   UCON2 

   UBRDIV2 

  UFRACVAL2

  UTXH2

  UTRSTAT2 

 UART传输的配置代码：

#define    GPA1CON    (*(volatile unsigned int *)0x11400020)
#define    ULCON2     (*(volatile unsigned int *)0x13820000)
#define    UCON2      (*(volatile unsigned int *)0x13820004)
#define    UBRDIV2    (*(volatile unsigned int *)0x13820028)
#define    UFRACUAL2  (*(volatile unsigned int *)0x1382002c)
#define    UTXH2      (*(volatile unsigned int *)0x13820020)
#define    UTRSTAT2   (*(volatile unsigned int *)0x13820010)

void uart_init(void)
{
    // config pin GPA1_1 GPA1_0  uart mode
    GPA1CON = 0x22;

    // set uart function block
    ULCON2 = 0x03;            //data bit = 8 parity = none stop bit = 1
    UCON2 = 0X05;        //set plling mode
    /*
     * set Band- Rate = 115200
     * DIV_VAL = (100000000/(115200 * 16)) -1 = 53.253
     * UBRDIV2 = 53
     * UFRACAL2 = 0x253 * 16 = 4
     */
    UBRDIV2 = 53;
    UFRACVAL2 = 4;
}

void putc(char c)
{
    while(1)
    {
        if(UTRSTAT2 & 0x02)
        {
            break;
        }
    }
    UTXH2 = c;
}

int main(void)
{
    uart_init();
    while(1)
    {
        delayis();        //汇编调用
        putc('d');
    }
    return 0;
}

Makefile ：

all:
    arm-none-linux-gnueabi-gcc -fno-builtin -nostdinc -c -o start.o start.s
    arm-none-linux-gnueabi-gcc -fno-builtin -nostdinc -c -o main.0 main.c
    arm-none-linux-gnueabi-ld start.o main.o -Tmap.lds -o uart.elf
    arm-none-linux-gnueabi-objcopy -O binary uart.elf uart.bin
    arm-none-linux-gnueabi-objdump -D uart.elf > uart.dis
clean:
    rm -rf *.bak start.0 main.o uart.elf uart.bin uart.dis

链接脚本：

OUTPUT FORMAT("elf32-littlearm","elf32-littlearm","elf32-littlearm")    //指定格式
OUTPUT_ARCH(arm)        //指定输出是ARM
ENTRY(_start)       //链接的第一条指令的地址
SECTIONS
{
    //. 表示当前位置
    . = 0x40008000;    //指定链接的起始地址
    . = ALIGN(4);        //对齐
    .text:            //代码的开始
    {
        start.o(.text)
        *(.text)
    }
    . = ALIGN(4);
    .data:        //数据的开始
    {
        *(.data)
    }
    . = ALIGN(4);
    .bss:        //数据段开始
    {
        *(.bss)
    }
}

汇编程序： 

    .global delayis
    .text
    .global _start

_start:
    b     reset
    ldr     pc,_undefined_instruction
    ldr     pc,_software_interrupt
    ldr     pc,_prefetch_abort
    ldr     pc,_data_abort
    ldr     pc,_not_used
    ldr     pc,_irq
    ldr     pc,_fiq

_undefined_instruction:
    .word    _undefined_instruction

_software_interrupt:
    .word    _software_interrupt

_prefetch_abort:
    .word    _prefetch_abort

_data_abort:
    .word    _data_abort

_not_used:
    .word    _not_used

_irq:
    .word    _irq

_fig:
    .word    _fig

reset:
    ldr r0,=0x40008000    @ 设置异常向量表的启动地址为0x40008000
    mcr p15,0,r0,c12,c0,0        @ Vector Base Address Register

init_stack:
    ldr r0,stacktop        @ get stack top pointer

    @ svc mode stack
    mov sp,r0
    sub r0,#128*4            @ 512 byte for irq mode of stack

    @ irq mode stack
    msr cpsr,#0xd2
    mov sp,r0
    sub r0,#128*4

    @ fiq mode stack
    msr cpsr,#0xd1
    mov sp,r0
    sub r0,#0

    @ abort mode stack
    msr cpsr,#0xd7
    mov sp,r0
    sub r0,#0

    @ undefine mode stack
    msr cpsr,#0xdb
    mov sp,r0
    sub r0,#0

    @ sys mode and usr mode stack
    msr cpsr,#0x10
    mov sp,r0            @  1024 byte for user mode of stack
    
    b main

delayis:
    ldr r4,=0x1ffffff

delayis_loop:
    sub r4,r4,#1
    cmp r4,#0
    bne delayis_loop
    mov pc,lr

    .aliqn 4
    @ swi_interrupt handler

stacktop:
    .word    stack+4*512

.data

stack:    
    .space 4*512

    .end