Chapter 7. Exploring the Raspberry Pi's GPIO Pins

In this chapter we will explore the next piece of the Raspberry Pi's hardware, the GPIO pins.

GPIO stands for general purpose input/output. The pins can be used to connect external electronic devices to the Raspberry Pi. This also allows us to build our own circuits and control them using software we have written in languages such as C, Assembly, and Python.

In this chapter we will cover:

We will start by walking through the GPIO pins and looking at what each type is.

Introduction to GPIO pins

The GPIO pins can be divided into a number of categories. These are:

Let's begin by looking at the standard GPIO pins and understanding what they do. A diagram with a breakdown of the pins can be downloaded from https://www.raspberrypi.org/documentation/usage/gpio-plus-and-raspi2/images/physical-pin-numbers.png.

A general overview of the GPIO pins can also be found at the Raspberry Pi website at https://www.raspberrypi.org/documentation/hardware/raspberrypi/gpio/README.md.

The standard GPIO pins on your Raspberry Pi 2 provide an interface for other electronic devices you may wish to either control or read data from.

These pins can be configured as output or inputs. As you will in see in the Python and C programs we will write, it is possible to programmatically switch between the two.

One important thing to note is the numbering format. There are two different ways in which we can refer to the pins, via the GPIO number or the physical numbering.

The GPIO numbering (also known as BCM) is the method by which the Broadcom chip sees them. These numbers will appear to be random to you, so it helps to use a reference sheet.

An updated guide to the numbering format can be found at http://pinout.xyz/.

When writing applications, you will need to know these BCM pin numbers in order to switch them between modes.

The second methodology for listing the pins is by their physical position.

You can find the Raspberry Pi 2 physical pin listings at https://www.raspberrypi.org/documentation/usage/gpio-plus-and-raspi2/images/physical-pin-numbers.png. This maps the physical pin location to the pin type.

You will also notice from looking at this diagram that a number of the pins are power pins with a specific voltage. These are important to note, and are discussed in more detail later in this chapter.

Let's now look at some of the specialist pins used on the device.

The I2C standard is used to allow one microchip to talk to another. The Raspberry Pi 2 supports I2C using pins 3 and 5. In Chapter 10, Integrating with Third-Party Microcontrollers, we explore how to use this methodology to communicate between the Raspberry Pi and a microcontroller.

You can read more about I2C at https://learn.sparkfun.com/tutorials/i2c.

With I2C support, we can connect multiple devices to the Raspberry Pi 2 and assign each a unique address. In order to see what devices are hooked up, we can use the i2c-tool application.

To install the toolkit, start by running the following command:

sudo apt-get install python-smbus

This installs a Python module that allows SMBus access via the I2C interface on Raspbian.

A guide to using this module in Python applications can be found at https://pypi.python.org/pypi/smbus-cffi/0.4.1.

Once the installation is complete, you can then run apt-get to install i2c-tools if this wasn't included when you installed python-smbus:

sudo apt-get install i2c-tools

A guide to the tools can be found at http://elinux.org/Interfacing_with_I2C_Devices#i2c-tools.

Let's try checking whether the software installed successfully. Run the i2cdetect command with the –l flag:

i2cdetect –l

The command returns a list of installed buses on the Raspberry Pi 2 and for the moment should return no values.

Tip

Remember, you can use man i2cdetect to read the manual.

Let's now add kernel support for I2C so we can use it in future if we want. Open up the modules file:

sudo vim /etc/modules

Now edit the file so it looks like the following:

# /etc/modules: kernel modules to load at boot time.
#
# This file contains the names of kernel modules that should be loaded
# At boot time, one per line. Lines beginning with "#" are ignored.
# Parameters can be specified after the module name.

snd-bcm2835
i2c-bcm2708
i2c-dev

Here we have added these two lines to the bottom of the original file:

i2c-bcm2708
i2c-dev

Save the file and exit.

Tip

The raspi-config provides a GUI for editing the I2C settings as well. You can access this via sudoraspi-config and then select the advanced options setting.

Next we need to edit the boot config.txt file:

  1. Open this up in your text editor:
    sudo vim /boot/config.txt
  2. Add the following code to the bottom of your file. If you used NOOBS, then add it under the NOOBS autogenerated parameters:
    dtparam=i2c1=on
dtparam=i2c_arm=on
  3. We can now reboot the Raspberry Pi so the changes kick in:
    sudo reboot
  4. Once the device has rebooted you can then log back in.
  5. Now let's try the command again. This time, put sudo in front; this will show more information when you run the command:
    sudo i2cdetect -l
  6. You should see something similar to:
    i2c-1i2c       3f804000.i2c                    I2C adapter
  7. To see any connected devices on the I2C pins, run:
    sudo i2cdetect -y 1

This wraps up the enabling of the I2C port. A huge number of projects are available on the Web that leverage this technology.

The website https://learn.adafruit.com/adding-a-real-time-clock-to-raspberry-pi demonstrates how to build a real time clock using I2C support.

Let's now move on and take a look at Rx and Tx pins.

The Rx and Tx pins are responsible for serial communication. Serial communication is the process of sending data one bit at a time in sequence over a communications medium.

Typically, these ports can be used for console input and output. Thus another serial device can be connected to the Raspberry Pi 2 and the serial data read and displayed to the user, allowing them to debug problems.

This is particularly helpful, for example, during the boot process.

You can see whether the console shell is enabled by running the following command from the terminal window:

dmesg | grep tty

You should see the following line in the output:

[    1.266731] console [ttyAMA0] enabled

However, if we wish to use the serial ports to communicate with another serial device such as a modem, then the console login feature would need to be disabled.

This is very simple to do via the raspi-config application.

  1. Start by launching this:
    sudo raspi-config.
  2. From the screen that loads, select Advanced Option.
  3. Then scroll down and select Serial.
  4. You will be presented with a screen that allows you to switch the login shell on and off.
  5. If you wish to disable it select <Yes>.
  6. Exit from raspi-config and then reboot the Raspberry Pi 2.
  7. On logging back in via the command line, run:
    dmesg | grep tty

Your serial pins are now available to experiment with serial devices. Later in this book you will experiment with using serial pins to communicate with an Arduino microcontroller.

To learn more about the subject of serial communication, check out https://learn.sparkfun.com/tutorials/serial-communication.

Serial Peripheral Interface (SPI) is a bus designed for synchronous serial communication.

It is useful for communicating between peripheral devices quickly over short distances.

The Raspberry Pi 2 comes with a single SPI bus that has two chip selects. This bus can be interacted with via the SPI pins on the P1 header.

By default, the SPI master drive is disabled, but we can enable it in a similar fashion to switching the Rx/Tx console off via raspi-config or by manually editing the raspi-blacklist.conf file:

  1. To edit via the GUI, start by loading the raspi-config command.
    sudo raspi-config
  2. Next, select the Advanced Options and locate the SPI option.
  3. From here, follow the options to enable SPI.
  4. Once you have made these changes, reboot the Raspberry Pi and then log back in.
  5. A simple command line test can be run to check everything was enabled correctly:
    echo -ne "\x01\x02\x03"> /dev/spidev0.0
  6. If you wish to manually edit the blacklist in order to enable SPI, this can be done via:
    vim /etc/modporbe.d/raspi-blacklist.conf.
  7. Here you will need to remove the blacklisting for spi-bcm2708.
  8. Once this is done you can reload the driver via:
    sudo modprobe spi-bcm2708.

A guide to interacting with the SPI bus can be found on the official Raspberry Pi website at https://www.raspberrypi.org/documentation/hardware/raspberrypi/spi/README.md

This link also provides information on the nomenclature used when interacting with SPI.

Later in this chapter, we will install the wiringPi library, which also provides a number of tools for interacting with the GPIO pins, including SPI.

Finally, we have a number of standard GPIO pins that can be used for PWM and PPM. PWM stands for Pulse Width Modulation. This methodology can be used to control the amount of power sent to an electrical motor, thus controlling its speed. The principle behind this is the implementation of a square wave.

You can read more about PWM and see a diagram of the square wave at https://en.wikipedia.org/wiki/Pulse-width_modulation.

It is also possible to generate a software-based PWN square wave using any of the standard GPIO pins. The wiringPi library comes equipped with instructions on how to achieve this at http://wiringpi.com/reference/software-pwm-library/. It also includes a number of example projects.

The second acronym, PPM, can also be used to control electrical motors. PPM stands for Pulse Position Modulation and is popularly implemented for servos.

Servos—short for servomechanisms—are a type of electrical component, such as a motor, that uses error sensing negative performance to correct its position.

If you wish to undertake any projects that control R/C devices, such as R/C cars and boats, via your Raspberry Pi, this will be of interest to you as they typically implement servos.

You can find a comparison between PPM and PWM at http://www.endurance-rc.com/ppmtut.php.

You can find example servo projects at https://learn.adafruit.com/adafruits-raspberry-pi-lesson-8-using-a-servo-motor/overview.

Finally, further information and reading on the GPIO pins can be found at https://www.raspberrypi.org/documentation/usage/gpio-plus-and-raspi2/.

Next, we will discuss the power voltages and pins.

With the Raspberry Pi 2, we need to be aware of the voltages used. The GPIO banks all use a voltage of 3.3v. It is important to remember this as applying a voltage higher than this to the pins can seriously damage your device.

The pins can be set to OFF, LOW, and HIGH. You will see this in action when we learn how to switch an LED on and off.

Among the Raspberry Pi GPIO pins, you will also find two 3.3v pins, two 5 volt pins, and several ground pins.

These can be used for powering other devices attached to your Raspberry Pi, either through a breadboard or via the GPIO headers directly.