1. 在原理图上找到按键对应的GPIO

在上一篇中，通过原理图已经知道了用户LED1-4，对应了GPM4_0-GPM4_3,再来看下KEY1-4对应的管脚。

用户按键

GPX3

用户按键KEY1、KEY2、KEY3、KEY4分别对应了GPX3_2、GPX3_3、GPX3_4、GPX3_5，并且按键引脚平常是高电平，按键被按下变为低电平。

2. 芯片手册查找GPIO相关寄存器

控制寄存器GPX3CON:

GPX3CON

数据寄存器GPX3DAT:

GPX3DAT

配置GPX3所对应的位为0，即为输入模式。

3. 编写程序

#define GPIOBASE    0x11000000
#define GPM4CON     (*(volatile unsigned long *)(GPIOBASE + 0x02E0))
#define GPM4DAT     (*(volatile unsigned long *)(GPIOBASE + 0x02E4))
#define GPX3CON     (*(volatile unsigned long *)(GPIOBASE + 0x0C60))
#define GPX3DAT     (*(volatile unsigned long *)(GPIOBASE + 0x0C64))

int key(void)
{
    unsigned long value;

    /*设置LED1-4对应GPIO为输出模式*/
    GPM4CON &= ~((0xF<<0) | (0xF<<4) | (0xF<<8) | (0xF<<12));
    GPM4CON |= ((1<<0) | (1<<4) | (1<<8) | (1<<12));

    /*设置KEY1-4对应GPIO为输入模式*/
    GPX3CON &= ~((0xF<<8) | (0xF<<12) | (0xF<<16) | (0xF<<20));

    while(1)
    {
        value = GPX3DAT; //读取GPX3电平状态

        if(value & (1<<2)) //Key1 --> GPX3_2
        {
            GPM4DAT |= (1<<0); //LED1 --> GPM4_0
        }else{
            GPM4DAT &= ~(1<<0);
        }

        if(value & (1<<3)) //Key2 --> GPX3_3
        {
            GPM4DAT |= (1<<1);  //LED2 --> GPM4_1
        }else{
            GPM4DAT &= ~(1<<1);
        }

        if(value & (1<<4)) //Key3 --> GPX3_4
        {
            GPM4DAT |= (1<<2);  //LED3 --> GPM4_2
        }else{
            GPM4DAT &= ~(1<<2);
        }

        if(value & (1<<5)) //Key4 --> GPX3_5
        {
            GPM4DAT |= (1<<3); //LED4 --> GPM4_3
        }else{
            GPM4DAT &= ~(1<<3);
        }
    }
    return 0;
}

Makefile代码如下:

default:
    arm-linux-gcc -c key.c  -o key.o
    arm-linux-ld  -Ttext=0x70003000  key.o  -o key
    arm-linux-objcopy  -O binary   key  key.bin
clean:
    rm -f key.o  key  key.bin   *~

执行make clean，然后make，生成key.bin二进制文件。