在OpenWRT上要控制GPIO有二種方式
1.透過修改/sys/class/gpio/gpioxx/Value來變更，此種方法好處是可以在多種程式語言，
單純透過讀寫檔案的方式就可以改變GPIO，亦不需要知道實際GPIO所對應的記憶體位置。
缺點則是速度較慢。
Ex:切換系統LED
echo "27" > /sys/class/gpio/export
echo "out" > /sys/class/gpio/gpio27/direction
echo "1" > /sys/class/gpio/gpio27/value
or
echo "0" > /sys/class/gpio/gpio27/value
2. Direct Access GPIO Register，這種方式需要知道GPIO所對應的暫存器位置 ，適合以C語言開發，並對切換GPIO腳位速度有需求的使用者，例如RS 485應用。
以AR9331為例，參考它的
DataSheet ，可以得知GPIO的位置為18040000。
讓GPIO 27 閃爍的範例程碼
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <string.h>
#include <time.h>
#define GPIO_ADDR 0x18040000 // base address
#define GPIO_BLOCK 48 // memory block size
volatile unsigned long *gpioAddress;
//Read GPIO Status
//return 0 -->Low
//return >0 -->High
int gpioRead(int gpio)
{
unsigned long value = *(gpioAddress + 1);
return (value & (1 << gpio));
}
// "1" (output) or "0" (input)
void gpioDirection(int gpio, int direction)
{
// obtain current GPIO settings
unsigned long value = *(gpioAddress + 0);
if (direction == 1)
{
value |= (1 << gpio); // set bit to 1 (output)
}
else
{
value &= ~(1 << gpio); // clear bit (input)
}
*(gpioAddress + 0) = value;
}
//Set GPIO
void gpioSet(int gpio, int data)
{
// obtain current GPIO settings
unsigned long value = *(gpioAddress + 2);
if (data == 1)
{
value |= (1 << gpio); // set bit to 1 (High)
}
else
{
value &= ~(1 << gpio); // clear bit (Low)
}
*(gpioAddress + 2) = value;
}
//Init GPIO Address, if /dev/mem not foud, you need to rebuild Image and add this module
int gpioSetup()
{
int m_mfd;
if ((m_mfd = open("/dev/mem", O_RDWR)) < 0)
{
return -1;
}
gpioAddress = (unsigned long*)mmap(NULL, GPIO_BLOCK, PROT_READ|PROT_WRITE, MAP_SHARED, m_mfd, GPIO_ADDR);
close(m_mfd);
if (gpioAddress == MAP_FAILED)
{
return -2;
}
return 0;
}
int main(int argc, char **argv)
{
int pin=27;
int ms=500;
gpioSetup();
sscanf(argv[1], "%d", &pin);
gpioDirection(pin,1);//set pin output
for (int i=0;i<100;i++) //blink system led
{
gpioSet(pin,1); //set syteme led high
usleep(1000*ms); //sleep 500ms
gpioSet(pin,0); //set system led low
usleep(1000*ms); //sleep 500ms
}
}
參考來源：
Working with GPIOs (C/C++)
AR9331 Datasheet
