amba H2系列CPU的GPIO口能作PWM使用的个数有限（GPIO0-GPIO3），从PRM里GPIO: Function Selection章节可以得到如何配置GPIO为PWM功能。
假设想把GPIO0配置成PWM输出，根据上图修改ambarella/boards/h2_xxx/bsp/bsp.h ，如下：
即DEFAULT_IOMUX_REG0_2、DEFAULT_IOMUX_REG0_1、DEFAULT_IOMUX_REG0_0的第0位要分别设为0、1、1。
一、系统层用脚本控制PWM0示例
echo 0 > /sys/class/pwm/pwmchip0/export echo 5000000 > /sys/class/pwm/pwmchip0/pwm0/period （设置周期） echo 2000000 > /sys/class/pwm/pwmchip0/pwm0/duty_cycle （设置有效时间） echo 1 > /sys/class/pwm/pwmchip0/pwm0/enable （使能PWM）
占空比= 有效时间/周期， 占空比越大，LCD背光越亮（有效时间要小于等于周期时间）。
为了方便使用写成脚本set_pwm.sh：
#! /bin/bash
#
# set_pwm.sh <gpio_id> < 1 | 0 > <duty_cycle> <period>
do_cmd()
{
local id=$1
local ena=$2
local d_cycle=$3
local per=$4
local GPIO_ROOT=/sys/class/pwm/pwmchip0
local GPIO_DIR=$GPIO_ROOT/pwm$id
if [[ ! -d $GPIO_DIR ]]; then
echo $id > $GPIO_ROOT/export
fi
echo $ena > $GPIO_DIR/enable
echo $per > $GPIO_DIR/period
echo $d_cycle > $GPIO_DIR/duty_cycle
}
usage()
{
echo " set_pwm.sh <gpio_id> < 1 | 0 > <duty_cycle> <period>"
}
do_main()
{
case "$2" in
1)
do_cmd $1 1 $3 $4
;;
0)
do_cmd $1 0 $3 $4
;;
*)
usage
;;
esac
}
if [[ -z $4 ]]; then
usage
else
do_main $*
fi
二、写一个pwm驱动: pwm.c
#include <linux/init.h>
#include <linux/module.h>
#include <linux/miscdevice.h>
#include <linux/fs.h>
#include <linux/ioctl.h>
#include <linux/pwm.h>
#include <linux/uaccess.h>
#define PWM_MAGIC 'P' //定义一个幻数
#define PWM_ON _IOW(PWM_MAGIC, 0, struct pwm_capture_t) //定义一个ioctl cmd#define PWM_OFF _IOW(PWM_MAGIC, 1, struct pwm_capture_t) //定义另一个ioctl cmd
struct pwm_device *pwm_dev_0;
struct pwm_capture_t {
unsigned int period;
unsigned int duty_cycle;
}pwmm;
static long pwm_ioctl(struct file *file,
unsigned int cmd,
unsigned long arg)
{
int ret;
switch(cmd) {
case PWM_ON:
ret = copy_from_user(&pwmm, (void *)arg, sizeof(struct pwm_capture_t));
if (ret < 0)
return ret;
ret = pwm_config(pwm_dev_0, pwmm.duty_cycle, pwmm.period);
if (ret < 0) {
printk("pwm_dev_0 ioctl fail");
return 0;
}
pwm_enable(pwm_dev_0);
break;
case PWM_OFF:
ret = copy_from_user(&pwmm, (void *)arg, sizeof(struct pwm_capture_t));
if (ret < 0)
return ret;
ret = pwm_config(pwm_dev_0, 0, pwmm.period);
if (ret < 0) {
printk("pwm_dev_2 ioctl fail");
return 0;
}
pwm_disable(pwm_dev_0);
break;
}
return 0;
}
//定义初始化硬件操作方法
static struct file_operations pwm_fops = {
.owner = THIS_MODULE,
.unlocked_ioctl = pwm_ioctl
};
//定义初始化混杂设备对象
static struct miscdevice pwm_misc = {
.minor = MISC_DYNAMIC_MINOR, //动态分配次设备号
.name = "mypwm", //dev/mypwm
.fops = &pwm_fops
};
static int pwm_init(void)
{
int ret;
printk("regisger pwm_misc device\n");
//1.申请pwm资源，设置输出为0
pwm_dev_0 = pwm_request(0, "pwm_0");
if (pwm_dev_0 == NULL) {
printk("pwm_dev_0 register fail\n");
}
ret = pwm_config(pwm_dev_0, 0, 500000);
if (ret < 0) {
printk("pwm_dev_0 config fail\n");
return 0;
}
ret = pwm_enable(pwm_dev_0);
if (ret == 0) {
printk("pwm_dev_0 enable success\n");
} else if (ret < 0 ) {
printk("pwm_dev_0 enable fail\n");
return 0;
}
//2.注册混杂设备
misc_register(&pwm_misc);
return 0;
}
static void pwm_exit(void)
{
printk("unregister pwm_misc device\n");
//1.卸载混杂设备
misc_deregister(&pwm_misc);
//2.释放pwm资源
pwm_config(pwm_dev_0, 0, 500000);
pwm_disable(pwm_dev_0);
pwm_free(pwm_dev_0);
}
module_init(pwm_init);
module_exit(pwm_exit);
MODULE_LICENSE("GPL");
测试程序pwm_test.c：
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#define PWM_MAGIC 'P' //定义一个幻数，要和pwm.c里定义的相同
#define PWM_ON _IOW(PWM_MAGIC, 0, struct pwm_capture_t) //定义一个ioctl cmd， 要和pwm.c里定义的相同
#define PWM_OFF _IOW(PWM_MAGIC, 1, struct pwm_capture_t) //同上
struct pwm_capture_t {
unsigned int period;
unsigned int duty_cycle;
};
int main(int argc, char **argv)
{
int fd;
int a;
int period, duty_cycle;
if (argc < 4) {
printf("usage: %s on/off duty_cycle period\n", argv[0]);
return -1;
}
duty_cycle = strtol(argv[2], NULL, 10);
period = strtol(argv[3], NULL, 10);
if (duty_cycle > period) {
printf("duty_cycle shouldn't larger than period\n");
return -1;
}
struct pwm_capture_t pwmm = {period, duty_cycle};
fd = open("/dev/mypwm", O_RDWR);
if (fd < 0)
return -1;
while(1) {
/* for(a = 0; a <= period; a += 5) {
pwmm.duty_cycle = a;
ioctl(fd, PWM_ON, &pwmm);
usleep(1000);
}
for(a = period; a >= 0; a -= 5) {
pwmm.duty_cycle = a;
ioctl(fd, PWM_ON, &pwmm);
usleep(1000);
}
usleep(1000000);
}
*/
if (strcmp(argv[1], "on") == 0) {
ioctl(fd, PWM_ON, &pwmm);
} else if (strcmp(argv[1], "off") == 0) {
ioctl(fd, PWM_OFF, &pwmm);
} else {
printf("invalid parameter\n");
}
close(fd);
return 0;
}
执行./pwm_test on 200000 500000，会看到LCD背光点亮，增加duty_cycle的值，背光会变得更亮。
pwm除了可以控制屏幕亮度外还可以控制风扇转速，根据cpu负载大小调整散热风扇的转速，也是一个常用的功能。