Raspberry Pi Computer Architecture Essentials: Explore Raspberry Pi's architecture through innovative and fun projects

The new Raspberry Pi is built on the back of the Broadcom BCM2836. The BCM2836 is a system-on-a-chip processor containing four ARM cores and Broadcom's VideoCore® IV graphics stack.

In contrast to this, previous Raspberry Pi A and B models only contained a single core.

On top of this, several other components make up the device, including USB, RCA, and microSD card storage. The previous Raspberry Pi Model B only contained two USB drives and a microUSB compared to the four USB drives and microUSB of the second version.

You can read a good breakdown of how the two boards standup to each other by visiting the following website: http://www.alphr.com/raspberry-pi-2/1000353/raspberry-pi-2-vs-raspberry-pi-b-a-raspberry-pi-comparison.

So, compared to earlier models, version 2 is a far more capable computer, yet still remains at the same price. The added benefit of having multiple cores allows us to explore different programming techniques for utilizing them.

Next, we shall cover the core components of the Raspberry Pi board in more detail. The following is an image of the board with a description of each component:

Image courtesy of Wikipedia

The Raspberry Pi 2 is a small machine measuring only 85.60 mm x 56 mm x 21 mm and weighing approximately 45g. This small size makes it suitable for embedded projects, home automation devices, arcade machines, or building small multi-device clusters.

The System on Chip (SoC) architecture that the Raspberry Pi 2 implements is the Broadcom BCM2836, which we touched upon earlier in this chapter. This contains a CPU, GPU, SDRAM, and single USB port. Each of these items is discussed in more detail under the appropriate heading.

A central processing unit is the brain of your Raspberry Pi. It is responsible for processing machine instructions, which are the result of your compiled programs.

The BCM2836 implements a 900 MHz quad-core ARM Cortex-A7 processor. This runs on the ARMv7 instruction set.

The ARM architecture reference manual can be downloaded from ARM's website at http://infocenter.arm.com/help/topic/com.arm.doc.ddi0406c/index.html.

The graphics processing unit (GPU) is a specialist chip designed to handle the complex mathematics required to render graphics.

The Broadcom VideoCore Iv 250 MHz supports OpenGL ES 2.0 (24 GFLOPS) Mpeg-2 and VC-1 (with license). It also includes a 1080p30 H.264/MPEG-4 AVC decoded/encoder.

The documentation for the GPU can be found on Broadcom's website at https://www.broadcom.com/docs/support/videocore/VideoCoreIV-AG100-R.pdf.

The Raspberry Pi 2 comes equipped with 1 GB of SDRAM, which is shared between the GPU and CPU.

The previous version of the Raspberry Pi Model B contained only a single microUSB port and a two standard USB ports. The Raspberry Pi 2 has been expanded to include an onboard 5-port USB hub.

This allows you to connect four standard USB cables to the device and a single microUSB cable. The micro USB port can be used to power your Raspberry Pi 2.

The microSD card is the main boot and storage mechanism of the Raspberry Pi. It is upon the microSD card that you will load your operating system and store data. Later in this book we will look at using the microSD purely for booting the Raspberry Pi, and then using a USB hard drive as a storage mechanism. In this chapter, we will delve into how we can setup the SD card with the Raspbian operating system.

One of the benefits of the Raspberry Pi 2 Model B is that it contains an Ethernet port. Many Raspberry Pi packages available on Amazon and similar stores include a wireless USB dongle; however, this results in you having to use up a USB port. If you plan to place your Raspberry Pi near a router or switch or have enough Ethernet cable, then you can connect your Raspberry Pi directly with the Ethernet jack.

The Raspberry Pi 2 supports 10/100 Mbps Ethernet, and the USB adapter in the third/fourth port of USB hub can also be used for Ethernet via a USB to Ethernet adapter.

Ethernet to USB adapters can be purchased from most good electronics stores and you can read more about the technology at https://en.wikipedia.org/wiki/Ethernet_over_USB.

The Raspberry Pi 2 implements the Inter-IC Sound (I2S) serial bus for audio input and output. This allows the device to connect multiple digital audio devices together. A 3.5mm TRRS jack is available and shared with the analog video output. The HDMI component also provides digital audio output.

Further information on configuring the audio output of the Raspberry Pi can be found on the official Raspberry Pi website: https://www.raspberrypi.org/documentation/configuration/audio-config.md.

The main method for interacting with electronic components and expansion boards is through the general purpose input/output (GPIO) pins on the Raspberry Pi.

The Raspberry Pi 2 Model B contains 40 pins in total. Future chapters will also look at how we can program these to control electronic devices.

As the acronym suggests the GPIO pins can accept both input and output commands and can be controlled by programs in a variety of languages running on the Raspberry Pi.

The input for example could be readings from a temperature sensor, and the output a command to another device to switch an LED on or off.

The Raspberry Pinout project provides an interactive guide to each GPIO pin and can be found at http://pinout.xyz/.

As well as providing a digital method for hooking up to a TV or monitor, the Raspberry Pi 2 also comes with analog support. The method of connection is commonly known as a composite or RCA port and earlier models of the Raspberry Pi came specifically with an RCA jack. RCA cables typically come with three connectors, two for audio and one (often yellow) for video.

With the release of the Raspberry Pi 2 the composite video (RCA) and 3.5 mm audio jacks functionality has been merged into a single TRRS hardware component. Therefore, if you wish to use video through this port, you may need to get a 3.5mm Mini AV TRRS to RCA cable instead. These can be found at any good electronics stores or on Amazon.

The Raspberry Pi 2 supports both PAL and NTSC standards.

Also included is a High-Definition Multimedia Interface (HDMI) port. This allows the Raspberry Pi 2 to be hooked up to high definition devices such as televisions and monitors. This port provides a digital alternative to the TRRS jack.

The HDMI port is ideal for streaming video and audio to your TV or monitor.

A number of websites offer microSD cards preloaded with one of the operating systems that are available for the Raspberry Pi 2. An example can be found at the Allied Electronics website at http://www.alliedelec.com/raspberry-pi-8gb-sd-card-raspberry-pi-noobs-1-4/70470344/.

These are a good solution for anybody looking to get up and running quickly or who are not comfortable installing an operating system by themselves from scratch. They are also useful for those who do not have second computer to work with in order to format a new microSD card.

The official Raspberry Pi distributions Element 14 also offer a preinstalled microSD card equipped with NOOBS, a Raspberry Pi 2 operating system boot loader. It can be found at https://www.element14.com/community/community/raspberry-pi.

The second option is to purchase a new blank microSD card and follow the instructions contained in this chapter.

It should be noted that if you do not have a home Mac or PC accessible to format a new blank microSD card, then we would recommend acquiring a preformatted card. This should come loaded with either Debian Jessie Raspbian, or the NOOBS boot loader application.

This book assumes that the reader will be installing the Raspbian operating system himself or herself. The simplest method for doing this is to install the NOOBS operating system installation manager onto your microSD card.

NOOBS makes the setup of your Raspberry Pi 2 easy and also provides you with a mechanism for choosing other operating systems that are compatible with the Raspberry Pi.

The official Raspberry Pi website contains an introduction and guide to NOOBS and can be found at http://www.raspberrypi.org/help/noobs-setup/.

If you already have a blank microSD card, you can download NOOBS from https://www.raspberrypi.org/downloads/noobs/.

When installing Raspbian for the first time via NOOBS you will also be presented with the raspi-config screen. This provides some handy shortcuts that allow you to do the following:

If you choose not to install Raspbian via NOOBS, then the following section will guide you through the process. If you are using NOOBS you can skip to the Raspbian installation wrap-up section.

Your first task will be to download the Raspbian operating system from the official Raspberry Pi website at https://www.raspberrypi.org/downloads/raspbian/.

There are several options for downloading Raspbian including an older version of the OS based on Debian Wheezy. We recommend grabbing the latest version, and it can be obtained over either BitTorrent or via a ZIP file.

Once you have obtained a copy of the operating system you can move onto formatting your microSD card and installing the image.

The Raspbian installation process involves two steps:

It is important that we quickly look at what File Allocation Table (FAT) is and why we need it.

FAT is a method for defining which sectors of a disk or microSD card files are stored in and which sectors on the disk are free to have new data written to them.

The standard has its origins in the 1970s for use on floppy disks and was developed by Bill Gates and Marc McDonald.

You can read more about FAT here: https://en.wikipedia.org/wiki/File_Allocation_Table.

Due to its simplicity of implementation and robustness, this standard is still used on SD and microSD cards today. Therefore, it is the format you will need in order to install the Raspberry Pi's operating system onto your microSD card.

Due to its widespread adoption you may find and microSD card you purchase is already formatted to FAT.

We recommend, however, formatting any new cards you purchase to ensure you do not encounter any problems.

The official Raspberry Pi website provides handy how-to guides for the three major operating systems on how to format and install the Raspbian image.

You can read an up-to-date overview of the installation procedure at https://www.raspberrypi.org/documentation/installation/installing-images/README.md.

The following are guides to formatting an SD card for your particular operating system:

Having completed installing the operating system we can now look at some final configuration before exploring some interesting features of Raspbian.

The following section assumes you have your Raspberry Pi connected to a monitor and with a keyboard and mouse available. It also assumes you have your configuration set to boot to desktop and have powered up and logged into your device.

You should at this point connect your device to your home router. If you are planning on using Wi-Fi, read on.

Now that you have successfully installed Raspbian you should see the Linux desktop.

This desktop contains icons in the top menu linking to a number of programs installed by default with the operating system.

One important icon is the link to LXTerminal. This icon launches the Linux terminal window. Click on this icon and you should see the command line load.

The following tasks in this section can all be performed in this window.

As a handy shortcut you can also load the raspi-config application at any time by typing the following command:

sudo raspi-config

If you update settings in this manner you may need to reboot the Raspberry Pi for them to take affect.

In order to connect to our Raspberry Pi 2 from another device via a terminal window we need to ensure that the Secure Shell(SSH) server is up and running. SSH is the default mechanism for secure communication between our Linux machines. If you used NOOBS to install the OS you may have configured the SSH server at this point via the advanced options. We can check that the SSH service is running successfully as follows.

Open up a terminal window from the Raspbian desktop and type the following command:

ps aux | grep sshd

The following sshd process should be displayed. This tells us the services are up and running:

root    2017  0.0  0.3   6228  2892 ?        Ss   15:13   0:00 /usr/sbin/sshd

If the SSH process does not appear, it is simple to start it. Enter the following command into the terminal:

sudo /etc/init.d/ssh start

After you have executed this command try running the following again and check that the sshd process is now running:

ps aux | grep sshd

By default, to login to the Raspberry Pi 2 over SSH you will be prompted for a username and password. If you have not changed this the username is pi and the password is raspberry.

In addition to the username and password method, we can also use an RSA key to authenticate and gain access to the Raspberry Pi over a network. We discuss this process next.

sudo ip addr show

sudo hostname

ssh-keygen -t rsa -b 4096 -C "username"

Generating public/private rsa key pair.

Enter file in which to save the key (/Users/username/.ssh/id_rsa):

Enter passphrase (empty for no passphrase):
Enter same passphrase again:

Your identification has been saved in /Users/username/.ssh/id_rsa.
Your public key has been saved in /Users/username/.ssh/id_rsa.pub.
The key fingerprint is:

ssh-add ~/.ssh/id_rsa

eval "$(ssh-agent –s)"

cat ~/.ssh/id_rsa.pub | ssh user@ip'cat >> .ssh/authorized_keys'

ssh –A username@ip

.ssh/authorized_keys'

Assigning a static IP address to your Raspberry Pi means that when you switch it on or off a new IP will not be assigned. Instead, it will always contain the same IP address, meaning you do not have to hunt down the value assigned to it by the DHCP server each time you reboot.

To start with, check the IP address range on your router and find a free IP address. You will also need the subnet mask and the default gateway.

Next, we need to assign the free address to the Raspberry Pi. Editing the interfaces file can do this:

sudo nano /etc/network/interfaces

In the open file you will need to locate the line that specifies eth0 or wlan0 depending on whether you are wired or wireless. For example:

iface eth0 inet dhcp

Change the value dhcp to static:

iface eth0 inet static

Once this is done we need to add three lines directly below it specifying the IP address we wish to assign, netmask, and gateway. You should have these values from checking your router earlier.

Paste these in below the interface. An example is show here:

address 192.168.1.132
netmask 255.255.255.0
gateway 192.168.1.1

Save the file using Ctrl + X and press Y to save.

We do not need to reboot the Raspberry Pi to apply these changes, but can stop and start the network interface using the following commands:

sudo /etc/init.d/networking stop
sudo /etc/init.d/networking start

If you now run the command from earlier to check the IP address, you should see it is the new value you assigned:

sudo ip addr show

In the preceding example we used the nano text editing tool. We shall now look at some other options for editing files.

In addition to the text editors installed by default with Raspbian, you may also wish to install Vim, a powerful text-editing tool. You will see this tool referenced later in this book, so you may find it easier to follow along if you install this.

To install it via our package manager run the following command:

sudo apt-get install vim

Vim is a complex tool but if you persist with it, you will find it rewarding. A guide can be found here: http://vimhelp.appspot.com/.

Finally, there are a number of other text editors worth exploring if you wish. You can find a list at the official Raspberry Pi website here: https://www.raspberrypi.org/documentation/linux/usage/text-editors.md.

The following diagnostic tests provide basic information on your Raspberry Pi. This just provides a taster and many more are available. A more comprehensive list of commands is available via the links at the end of this section.

You should, however, run these tests to get a basic idea of what is possible.

The system information of your Raspberry Pi can be run via the following command:

cat /proc/cpuinfo

Version information can be seen via the following command:

cat /proc/version

Memory information can be accessed using the following command:

cat /proc/meminfo

The microSD cards partitions via the following command:

cat /proc/partitions

To check the temperature of the device we can use the vcgencmd command:

vcgencmd measure_temp

We can also use this command with a different parameter to see the voltages. The basic command is as follows:

vcgencmd measure_volts id

In this command, id is one of the following items:

You can find more commands at elinux.org where a guide to vcgencmd can be found (http://elinux.org/RPI_vcgencmd_usage).

You may want to tweak the performance of your Raspberry Pi 2. This can be achieved by overclocking the device.

Overclocking is the process of forcing the CPU or other component, for example the GPU, to operate faster than its advertised or OS configured clock frequency. In the process of overclocking it is also possible to change the operating voltage to increase the device's speed.

There is a risk associated with overclocking a device, such as instability of its operation or faster degradation of components.

The raspi-config menu provides a set of screens to guide you through this process.

You can access raspi-config from the command line by typing this command:

sudo raspi-config

Then you select an overclock option from the menu that is presented.

Alternatively, you can modify the boot configuration file directly from inside the terminal window.

You will need to edit the /boot/config.txt file.

Once you have this open you will see a number of commented out values, for example, #arm_freq=800.

In the case of our Raspberry Pi 2 the processor runs at 700MHz. We could uncomment this line and up the speed of the processor to 800MHz.

An in-depth guide to overclocking the Raspberry Pi 2 can be found at Hayden James' website: http://haydenjames.io/raspberry-pi-2-overclock/.

For those interested in exploring diagnostic tools further there is the option of downloading the pigpio GPIO pin test. As you start to work more with the pins this test will come in handy for debugging problems, and allow you to check if you have accidentally damaged a pin.

You can download the pigpio library directly to your Raspberry Pi from http://abyz.co.uk/rpi/pigpio/download.html.

An overview and instructions on use can be found at http://abyz.co.uk/rpi/pigpio/index.html.

Example applications and tests can be found at http://abyz.co.uk/rpi/pigpio/examples.html.

For those looking for a digital waveform view for the Raspberry Pi's GPIO pins you can install piscope from http://abyz.co.uk/rpi/pigpio/piscope.html.