树莓派

树莓派3b板上有两个LED， pwr （power) 和 act (activity)。是platform_driver gpio-led驱动。

可以通过设备树和gpio-led来额外控制一个LED。如下图在P25和GND之间接入一个发光二极管。 

在设备树bcm2710-rpi-3-b.dts中添加一个LED2来控制外部的LED

&leds {
	act_led: act {
		label = "led0";
		linux,default-trigger = "mmc0";
		gpios = <&virtgpio 0 0>;
	};

	pwr_led: pwr {
		label = "led1";
		linux,default-trigger = "input";
		gpios = <&expgpio 7 0>;
	};

	test_led: test {
		label = "led2";
		linux,default-trigger = "input";
		gpios = <&gpio 26 0>;
	};
};

重新编译dtb,

1. make bcm2709_defconfig  
2. make dtbs  

将生成的bcm2710-rpi-3-b.dtb复制到SD卡boot下， 启动后，可以看到LED2出现

pi@raspberrypi:~ $ cd /sys/class/leds/
pi@raspberrypi:/sys/class/leds $ ls
led0  led1  led2
pi@raspberrypi:/sys/class/leds $ cd led2
pi@raspberrypi:/sys/class/leds/led2 $ ls
brightness  device  max_brightness  power  subsystem  trigger  uevent
pi@raspberrypi:/sys/class/leds/led2 $ cat trigger 
none rc-feedback kbd-scrolllock kbd-numlock kbd-capslock kbd-kanalock kbd-shiftlock kbd-altgrlock kbd-ctrllock kbd-altlock kbd-shiftllock kbd-shiftrlock kbd-ctrlllock kbd-ctrlrlock timer oneshot heartbeat backlight gpio cpu cpu0 cpu1 cpu2 cpu3 default-on [input] panic mmc1 mmc0 rfkill-any rfkill0 rfkill1 

通过设置brightness可以亮灭LED

heartbeat 来触发LED blink

pi@raspberrypi:/sys/class/leds/led2 $ cat brightness 
0
pi@raspberrypi:/sys/class/leds/led2 $ sudo sh -c "echo 1 > brightness"
pi@raspberrypi:/sys/class/leds/led2 $ cat brightness 
0
pi@raspberrypi:/sys/class/leds/led2 $ sudo sh -c "echo heartbeat > trigger"
pi@raspberrypi:/sys/class/leds/led2 $ sudo sh -c "echo none > trigger"
pi@raspberrypi:/sys/class/leds/led2 $ 

Reference

Linux下用文件IO的方式操作GPIO（/sys/class/gpio）

https://blog.csdn.net/lu_embedded/article/details/53061901