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Basic GPIO usage - Ixora Carrier Board - Apalis TK1

 

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Overview

In this lesson, you will learn the basics of GPIO usage on Linux, being able to read or write to a GPIO pin from command-line, as well as by developing a minimal C application.

In this lesson you will:

  • Understand how to translate the hardware pin names to the correspondent Linux sysfs interface numbers.
  • Assemble the additional hardware - an LED and a switch.
  • Use the Toradex GPIO Tool to validate the hardware setup.
  • Configure and use GPIO pins through the Linux sysfs.
  • Debug GPIO configuration.
  • Write minimal sample applications in C.

The information provided in this guide is based in Toradex's knowledge base article GPIO (Linux), as well as other knowledge sources such as kernel documentation and the Linux man-pages project.

Materials (Optional)

Since Ixora Carrier Board doesn't have buttons and LEDs available for debbuging purposes, we present two alternatives to test GPIOs: the first connects two GPIOs configured as input and output respectively; whereas the second alternative uses an external button and LED to achieve a better user experience, however, the following items are required:

  • 1x LED
  • 1x Button
  • 1x Transistor BC548
  • 2x 2k2Ω Resistor
  • 1x 470Ω Resistor
  • 1x Breadboard
  • Jumper wires

Note: On this getting started we are going to use 2x Resistor 2k2Ω, but you can use anyone between 1kΩ and 10kΩ. For Resistor 470Ω, you can use anyone between 100Ω and 1kΩ. For the transistor BC548, you can use any switch component you want, as MOSFET, just change the circuit according to the component.

Step 1

To find out which GPIO number to use in the Linux sysfs interface, you have to know the correspondence between available pins in the carrier board, number of the correspondent pins on the MXM3 connector of the Apalis computer on module and number of the pins on Linux.

Download or open in a web browser the Ixora Carrier Board and the Apalis TK1 datasheets from the respective products pages of the developer website:

Ixora Carrier Board datasheet

Apalis TK1 datasheet

Step 2

For this introduction guide, some pins configured by default as GPIO in the Toradex BSP were chosen. The choice of pins was made based on their availability on all the carrier boards covered by the getting-started guide. This module will not go through the configuration of other pins as GPIO, although it is possible.

First of all, you need to find the correspondence between the MXM3 and the connectors exposed for the developer on the Ixora Carrier Board. Consult the Ixora Carrier Board datasheet and fill the table below based in the example provided:

Note: The notation CONNECTOR.PIN will be employed in this lesson, e.g. X12.5 means pin 5 of the X12 connector.

Ixora Carrier Board (connector.pin) MXM3 pins
X27.14 3
11
13
Table 1 filled

Step 3

Have a look at the table available in the "TK1 Functions List" chapter of the Apalis TK1 datasheet. It provides a list of most of the Apalis pins available on the MXM3 connector.

The MXM3 pins we are interested at are connected to the TK1 SoC and have names defined by the TK1 Ball Name function. Each pin is multiplexed to have a specific function - among them GPIO, therefore the GPIO function is the column that we are interested at.

Having a look at the GPIO Alphanumeric to GPIO Numeric Assignment article, the correspondence between TK1 GPIO name and the Linux numeric representation of the GPIO pins is provided as a table. To find it from the GPIO column it is possible to use the formula below:

GPIO3_P[x].0[y]
Linux numeric representation = 8 X (x - 'A') + y

On the formula above, the x is a character, you need to find the number corresponding to the alphabetic letter, as example A=1, B=2, C=3, AA=27.

Below is an example of how to use the formula, the number inside parentheses are the number corresponding to the alphabetic letter:

GPIO3_PD.03 = 8 X (D(4) - A(1)) + 3 = 27

Either by consulting the table from the article pointed above or calculating it, the previous table with the correspondence between Ixora Carrier Board pins and MXM3 pins can be extended to have the TK1 GPIO name, formed by GPIO controller, always GPIO3, plus pin at SoC level, as the example above PD.03, and the Linux numeric representation. Fill the table below based in the example provided:

Ixora Carrier Board (connector.pin) MXM3 pins TK1 GPIO name Linux GPIO number
X27.14 3 GPIO3_PFF.00 248
X27.17 11
X27.18 13
Table 2 filled

Step 4

Choose two of the GPIO pins from the list above to make a loopback test. This lesson will use the following pins (Linux GPIO number):

  • 248 as Input
  • 237 as Output

Use jumper wires to connect GPIO 147 to GPIO 150.

Step 5

The Toradex Linux pre-built image comes with a tool named Toradex GPIO tool meant for debugging pins configuration. It can also be used to determine the correspondences found in the previous step. We will use it to test the hardware connections.

Note: You need a display and a mouse connected to the system in order to use the GPIO tool. Please go to the beginning of the getting-started guide for more information about assembling the peripherals.

Run the GPIO tool from the target Linux desktop:


  • Starting the GPIO tool

    Starting the GPIO tool


  • GPIO tool initial screen

    GPIO tool initial screen

Step 6

Locate the pins 248 and 237 in the table. Right click the direction of each of them and configure pin 248 as INPUT and pin 237 as OUTPUT. See the changes reflected in the application.


  • Configuring MXM3 pin

    Configuring MXM3 pin

Step 7

Click the Logic checkbox of pin 237 and see the logic checkbox of pin 248 switch on/off.


  • Logic checkbox toggled from GPIO tool

    Logic checkbox toggled from GPIO tool

Step 8

The Linux sysfs interface provides an abstraction to access the GPIO, as well as many other hardware features, from the Linux user-space.

The pin has to be exported first, which guarantees that it is not being used by other kernel drivers nor allow other drivers to use it. It also has to be configured as input or output.

From the Linux terminal, export the pins 248 and 237:

echo 248 > /sys/class/gpio/export
echo 237 > /sys/class/gpio/export

Configure the pins as input and output, respectively:

echo "in" > /sys/class/gpio/gpio248/direction
echo "out" > /sys/class/gpio/gpio237/direction

Step 9

Read the INPUT value as you toggle OUTPUT value:

echo 1 > /sys/class/gpio/gpio237/value
cat /sys/class/gpio/gpio248/value
echo 0 > /sys/class/gpio/gpio237/value
cat /sys/class/gpio/gpio248/value

Steps 10 to 17 (Optional)

The following steps are meant for readers that have the optional items listed in the beginning of this lesson. If you want to go through them, click the dropdown link below:

Steps 10 to 17

FAQ

This lesson only covers the basics of GPIO usage on Linux. Since there are other important topics that were not discussed, this FAQ section is meant as an information complement.

What is the sysfs interface
Where can I find more information regarding GPIO in the Toradex documentation
Should I always use the sysfs interface when I need to use a GPIO
How can I use pins that are not configured as GPIO by default
How can I use more GPIOs than available for a specific module
I dont want to reconfigure pins as GPIO due to Toradex pin compatibility How can I work around this
Linux has a kernel driver for controlling LEDs How to use it
Can I control backlight brightness using GPIO
Is it possible to use a GPIO to shutdown the system
Is it possible to use a GPIO to suspend resume the system
Can I toggle a GPIO earlier than Linux boots
Can I bypass sysfs to have direct access to GPIO
What is the initial state of a GPIO pin
How to set the initial state of a GPIO pin
What happens when a GPIO pin is unexported