GPIO and LEDs in Linux and lua

Hi all,

In my previous posts I wrote about my development in Java, but this time I’ll talk about other area I’m working on, robotics. This time I’ll talk about some basics, managing General Purpose Input Output (GPIO) and LEDs in Linux using the terminal, and also using lua, a very lightweight and portable language.


When working with embedded robotics or electronics interfacing with computers, it is (often) a good idea to work with linux for several reasons, just to mention a few:

  • Open source: It’s easy to customize the distro, so you can leave out parts of the OS you won’t use, making it more lightweight to run in an embedded system.
  • Portability: Linux kernel is portable to a huge amount of architectures, making it very attractive for use in embedded robotics.
  • Community: There is a big community out there using linux to develop embedded systems, developing new features to linux, testing, discussing and so on.

GPIO and LED filesystem in Linux

When working in linux there other great advantage is that you can control de LEDs and GPIO just by accessing the file system, without the need of any drivers. For example, if you want to control the built in LEDs you would use the /sys/class/leds/ portion of the file system. For GPIOs you’ll use /sys/class/gpio/
Using the file system it’s possible to control these hardware components from any linux terminal and easily create a program that uses them.
In the next sections I’ll explain how to use them from both alternatives.

managing built-in LEDs with the terminal

It’s very easy to control the built-in LEDs, I’ll show you how in this section.
Open a terminal and go to superuser (superuser permissions are needed in order to work with GPIOs and LEDs by default, although it is possible to change this with some OS rules):

sudo su

when listing the directory /sys/class/leds on my laptop I see these items:

root@rafael-vaio:/sys/class/leds# ls
ath9k-phy0  mmc0::  mmc1::

I’ll show you how to control the LED mmc1:: (on my laptop it’s the one near the memory card slot). First, when listing the contents of the mmc1:: folder I see these items:

root@rafael-vaio:/sys/class/leds# ls /sys/class/leds/mmc1\:\:/
brightness  device  max_brightness  power  subsystem  trigger  uevent

The important files here are brightness and trigger. brightness, as you’ll guess, it’s the file that stores the status of the led. trigger is which trigger will change the status of the led, you can see this using:

root@rafael-vaio:/sys/class/leds# cat /sys/class/leds/mmc1\:\:/trigger 
none ADP1-online BAT0-charging-or-full BAT0-charging BAT0-full BAT0-charging-blink-full-solid mmc0 [mmc1] rfkill0 rfkill1 rfkill2 rfkill3 phy0rx phy0tx phy0assoc phy0radio phy0tpt 

The value between the square brackets is the current trigger for this led (in this case, mmc1). To change this (remember the previous value since you should change it back when you finish), just run

root@rafael-vaio:/sys/class/leds# echo none > /sys/class/leds/mmc1\:\:/trigger 

From now on, there is no system trigger that’ll change this led, so it’s going to be only you changing the value.
To change the value and read it:

root@rafael-vaio:/sys/class/leds# echo 1 > /sys/class/leds/mmc1\:\:/brightness 
root@rafael-vaio:/sys/class/leds# cat /sys/class/leds/mmc1\:\:/brightness 
root@rafael-vaio:/sys/class/leds# echo 0 > /sys/class/leds/mmc1\:\:/brightness 
root@rafael-vaio:/sys/class/leds# cat /sys/class/leds/mmc1\:\:/brightness 

And voila, as simple as that you can start playing with the leds 🙂

managing GPIO using the terminal

The GPIOs are pins that are input/output pins that are connected to the processor, and in Linux it is very straightforward to configure and use them.
This is a bit more complex to test from your pc, since you need to physically access the GPIOs of the processor. In my case, I’ll be using these from a Single Board Computer with linux to read the value of sensors and send commands to actuators on my robotics projects. If you already own one of these, you probably already know how to control them, but if not, there are several options to start with. You can look at the List of single-board computers in wikipedia. In my experiments I used a BeagleBoard rev C4 which has several accessible GPIOs in the expansion bus.

So, shall we?
The steps to control a GPIO in linux are these:

  1. export the GPIO
  2. configure the GPIO as input or output
  3. set the value / read the value

The first step is to export the GPIO, this will tell linux that we’ll be handling the GPIO from the file system. We’ll work in the /sys/class/gpio portion of the fs.
When listing the contents of /sys/class/gpio/ we’ll see only two files: export and unexport. In order to initialize the GPIO with id 56 run:

echo 56 > /sys/class/gpio/export

This will create a folder gpio56 with the following content:

ls /sys/class/gpio/gpio56
direction  edge  power  subsystem  uevent  value

To set the direction (in/out), echo the value ‘in’ or ‘out’ to the direction file:

echo out /sys/class/gpio/gpio56/direction

To set the value, just change the value file, similar as we did with the leds (values 1 or 0):

echo 1 /sys/class/gpio/gpio56/value

That’s it. If you want to use the pin as an input, you should echo ‘in’ to the direction file and read the value file with ‘cat’

LEDs and GPIO using lua

lua is a high level language which has a very small footprint, and runs as fast as a C program. This is desirable when working with embedded systems, since we need to save every last bit of memory.

It is very simple to write a program that uses the led or the gpio filesystem, you just need to execute the commands seen previously and that’s it.
You can check out the code of the simple GPIO lua library I uploaded to Github.

That’s it for now, I’ll be posting more on embedded robotics next week. Let’s talk in the comments!