Our path to libmraa on 96Boards – Part 4

Manivannan Sadhasivam


Welcome to Part - 4 of Our path to libmraa with 96Boards series. In this blog, I’ll give an update on recently added(yet to be merged) support for using onboard LED’s on all platforms in libmraa. This has been one of the support missing in libmraa interms of peripherals for some time, but now we have addressed.


LED (Light Emitting Diode) is one of the most heard electronic component of all times. This little device attracts everyone with its illuminance property. Blinking LED is the first TODO project for all embedded platforms, just like the Hello World! program for all programming languages. Linux has the support for using LEDs through a distinct subsystem.

The control for LED’s in linux is exported to user space through sysfs filesystem under /sys/class/leds. Mostly, each onboard LED’s would appear as an individual soft link. For instance, Dragonboard410c contains the following entries:

lrwxrwxrwx 1 root root 0 Sep  4 12:43 apq8016-sbc:blue:bt -> ../../devices/platform/soc/soc:leds/leds/apq8016-sbc:blue:bt
lrwxrwxrwx 1 root root 0 Sep  4 12:43 apq8016-sbc:green:user1 -> ../../devices/platform/soc/soc:leds/leds/apq8016-sbc:green:user1
lrwxrwxrwx 1 root root 0 Sep  4 12:43 apq8016-sbc:green:user2 -> ../../devices/platform/soc/soc:leds/leds/apq8016-sbc:green:user2
lrwxrwxrwx 1 root root 0 Sep  4 12:43 apq8016-sbc:green:user3 -> ../../devices/platform/soc/soc:leds/leds/apq8016-sbc:green:user3
lrwxrwxrwx 1 root root 0 Sep  4 12:43 apq8016-sbc:green:user4 -> ../../devices/platform/soc/soc:leds/leds/apq8016-sbc:green:user4
lrwxrwxrwx 1 root root 0 Sep  4 12:43 apq8016-sbc:yellow:wlan -> ../../devices/platform/soc/soc:leds/leds/apq8016-sbc:yellow:wlan
lrwxrwxrwx 1 root root 0 Sep  4 12:43 mmc0:: -> ../../devices/platform/soc/7824900.sdhci/leds/mmc0::
lrwxrwxrwx 1 root root 0 Sep  4 12:43 mmc1:: -> ../../devices/platform/soc/7864900.sdhci/leds/mmc1::

And each entry contains the following contents:

-rw-r--r-- 1 root root 4096 Sep  4 13:18 brightness
lrwxrwxrwx 1 root root    0 Sep  4 13:18 device -> ../../../soc:leds
-rw-r--r-- 1 root root 4096 Sep  4 13:18 invert
-r--r--r-- 1 root root 4096 Sep  4 13:18 max_brightness
drwxr-xr-x 2 root root    0 Sep  4 13:18 power
lrwxrwxrwx 1 root root    0 Jan  1  1970 subsystem -> ../../../../../../class/leds
-rw-r--r-- 1 root root 4096 Sep  4 13:18 trigger
-rw-r--r-- 1 root root 4096 Jan  1  1970 uevent

The best place to understand all of the contents is Linux kernel Documentation. But the matter of interest when it comes to controlling LED’s would be the following files:


This file contains value which controls the LED brightness. Maximum value it can accept depends on the value of max_brightness. If your LED has the hardware brightness support, then this file can be used to control the brightness level. In absence of that, any value greater than zero(>0) will turn the LED on and the value of Zero will turn LED off.


This file controls the maximum brightness value in which brightness file can accept. Some platforms may forbid changing this value, resulting the file to be read only.


This is the most interesting part of the Linux LED subsystem. Instead of manually controlling the behaviour of LED, we can make LED blink at a predefined pattern! To make things interesting further, LED’s can also be set to blink at rate proportional to disk activity. The default trigger for LED’s would be mentioned in the device tree of each platform.

Following is the list of triggers available in Dragonboard410c:

none kbd-scrolllock kbd-numlock kbd-capslock kbd-kanalock kbd-shiftlock kbd-altgrlock kbd-ctrllock kbd-altlock kbd-shiftllock kbd-shiftrlock kbd-ctrlllock kbd-ctrlrlock mmc

LED support in libmraa

A Pull Request has been opened for adding LED support to libmraa. It is still under review but looks like will make its way through. Programming interface for accessing LED’s in libmraa are C, C++ and Python. No Node.js & Java support has been added till now.

Source code organization

Source code: src/led/led.c C API: api/mraa/led.h C++ API: api/mraa/led.hpp

Sample Application

Here is the sample C application to access this interface in libmraa:

#include <stdio.h>
#include "mraa.h"

int main(void)
	mraa_led_context led;
	mraa_result_t res = MRAA_SUCCESS;
	int val;

	/* access user1 led */
	led = mraa_led_init("user1");
	if (led == NULL) {
		fprintf(stderr, "unable to initialize requested led\n");

	val = mraa_led_read_max_brightness(led);
	printf("maximum brightness value is: %d\n", val);
	if (val >= 1)
		val = 0;
		/* never reached mostly */
		val = 1;

	res = mraa_led_set_brightness(led, val);
	if (res != MRAA_SUCCESS) {
		fprintf(stderr, "unable to set led brightness\n");


	res = mraa_led_set_trigger("heartbeat");
        if (res != MRAA_SUCCESS) {
                fprintf(stderr, "unable to set led trigger\n");

	printf("led trigger set\n");

	return 0;


That’s it about the LED support in libmraa. Hope this support will help the community to easily access LED interface in libmraa and that being said we are approaching Part-5 of Our path to libmraa with 96Boards. Our goal is to make libmraa a defacto library for accessing Low speed peripherals in linux for all 96Boards.

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