I have posted on how to do this exact thing using C++, so the first half is cut-and-pasted from Beaglebone: Controlling the on-board LEDs using C++


One of the first things you would like to do when you connect to the Beaglebone Black is see that you are having an impact on the hardware. In this short post I am going to look at how you can change the behaviour of the Beaglebone on-board LEDs – the four (blue on the BBB) LEDs in the corner near the reset button.

Now, the LEDs are there for a reason, and that reason is to give information about the Beaglebone state (from beaglebone.org):

USR0 is configured at boot to blink in a heartbeat pattern
USR1 is configured at boot to light during microSD card accesses
USR2 is configured at boot to light during CPU activity
USR3 is configured at boot to light during eMMC accesses

We can change the behaviour of these LEDs, but obviously we will temporarily lose this valuable information. The heartbeat tells you that the BBB is alive, which is always useful to know. The others are fairly self explanatory (the eMMC is the solid state memory that you are booting from).

First – At the command line

Sysfs is a virtual file system that is available under Linux that gives information and access to devices and drivers that would otherwise only be accessible in Kernel Space. It gives us a virtual file system in Linux User Space, which is convenient to use; however, it is not highly efficient.

At the command line, we can go to to directory /sys/class/leds:

You can see the four (green!) LEDs, usr0, usr1, usr2 and usr3 and their mappings.

We can go into the directory of the first LED:

Here you see various different sub-directories that give us further information and access to settings. We can find out the current status of the LED by:

You can see that the trigger is set up as a “heartbeat”

We can turn this off by:

And you will see (hopefully) that the LED stops flashing. So, let’s try a few different options:

Turning on the LED

Now that the trigger is off, we can turn on the LED using the brightness setting:

The first LED should go on.

Turning off the LED

We can turn off the LED using the brightness setting:

The first LED should go off.

 Making the LED Flash

We can use the timer trigger to make the LED flash, so do the following:

You have to set up the timer before writing to delay_on and delay_off. So the first line sets up a timer and on the second and third lines we set up the time for the first LED to be on and off (in milliseconds). So this will cause the first LED to flash 10 times per second (i.e. time on + time off =  100ms, so 10 flashes per second).

Setting it Back to a Heartbeat

We can set it back to the way it was before we started:

It should go back to the heartbeat trigger.

Java Implementation

You need to have installed Java on the Beaglebone Black before you try this. Please follow the steps at this Running Java Applications on the Beaglebone Black or watch the following YouTube video:

Now that we have this working at the command line it would be useful if we could write programs that interface to the LEDs in a similar fashion. My Java implementation code is below. It uses the Java.io  libraries to read and write data to the input/outputs using sysfs.

Effectively, this code carries out the exact same steps as what I did in the first part of this post; however, now it is wrapped up as a command that we can add to our $PATH in order that it can be called from any location. Build this code on your desktop computer or use the class file that I have distributed in the bin directory. The way you execute this code on the Beaglebone Black is to go to the directory below ee402 and type:

I have made all of the code available via github, so you can download this by the following steps:

The way we can interact with GPIOs is not much different than this; however, we have complexities to deal with to do with the Device Tree, and we have to use Device Tree Overlays. See my post: GPIOs on the Beaglebone Black using the Device Tree Overlays on this topic.