2.2.9. Exercise: Getting Started on the Raspberry Pi

2.2.9.1. Objective

Learn basic usage of a Raspberry Pi: connecting to a laptop or monitor, running programs, updating the system.

The Raspberry Pi exercises assume you have the following baseline materials:

  1. Raspberry Pi B+ or Raspberry Pi 2
  2. power supply cable (USB micro)
  3. microSD card containing course Raspbian image
  4. Pi Cobbler or T-Cobbler breakout board w/ribbon cable
  5. breadboard
  6. ideally: wall-wart power supply for Raspberry Pi (5.0V to 5.25V, min 1 Amp). If not available a laptop can power the Pi, but this is not an ideal solution.
  7. occasional use: powered speakers or earphones

If you have a blank microSD card (minimum 8G) onto which you would like to load a Raspberry Pi operating system, we maintain several customized images in a folder on box.com accessible to the CMU community. Please see the README.txt file for notes on the current versions and links to instructions for writing the image file to a card.

If using from a cluster laptop:

  1. short Ethernet cable
  2. Ethernet adapter (Thunderbolt or USB)

If using with a monitor:

  1. HDMI monitor and HDMI cable
  2. USB keyboard and mouse
  3. USB WiFi adapter

2.2.9.2. Overview

The Raspberry Pi is a powerful microcontroller which runs the Linux operating system. We have both two versions which are very similar physically, but with different computational power: the Raspberry Pi B+ with 512 Mb RAM and a single-core ARM Cortex-A6 CPU, and the newer Raspberry Pi 2 with 1024 Mb RAM and a quad-core ARM Cortex-A7. Both have a 40-pin connector for hardware I/O. A Raspberry Pi boots from a microSD card which acts as its hard drive. It is about as powerful as a low-end cell phone, which puts it orders of magnitude more complex than an Arduino, but much less capable than a current laptop.

The Raspberry Pi is a vastly more powerful computer than the Arduino and has access to a vast array of Linux software, much of which is easily available in packaged form. It makes it possible to easily embed computer vision, audio processing, networking, data logging, 3D graphics, web services, etc. in a small device.

The downside of this is increased complexity. Unlike the Arduino, it takes a few tens of seconds to boot up. Programs can be written using many languages but the programming environment generally lacks the simplicity of the Arduino IDE.

We have tried to bundle a rich set of tools in a standard system to make it as easy as we can to get started. We have the [wiringPi] object for Pd which can directly control hardware I/O pins. For those so inclined, the wiringPi-Python library allows easy programming in Python. If you want to blend the two, the [pdpython] object enables building mixed systems. And it is also possible to program in C, C++, Javascript, Perl, e.g., any major language available for Linux.

2.2.9.3. Interaction with the Pi

This exercise is about becoming familiar with the infrastructure and getting to the point of basic usability. There are a couple of different ways in which people use the Pi:

As a standalone computer. It is possible to plug a monitor, keyboard, mouse, and network port directly into a Pi and just treat it like a desktop computer. The upside is the relative simplicity of getting started and the possibility of generating 3D graphics directly on a monitor. The downside is that the Pi is a slow computer, and burdening it with generating a desktop display makes it feel slow. Plus needing all the accessories defeats some of the point of using a tiny computer. We only recommend this mode as a startup step or for a disconnected system off the network.

As a X11 client. The X11 system dates back to the 1980s but is still a viable means for running graphical user interfaces across the network. The laptop runs an X11 server (e.g. XQuartz.app) which handles drawing on the screen on behalf of a program running on the Pi, communicating over the network via SSH. This method works well for running Pure Data on the Pi from a laptop. This method can work over WiFi, but is much more reliable on a direct Ethernet connection.

As a SSH server. Much of the Pi can be operated from the command line. An ssh client on a laptop can connect to the Pi for an interactive console session. This works over even a slow network connection, and works fine for text editors with console modes (e.g. emacs).

As a remote desktop. It is possible to use a standard remote desktop interface on a laptop to open the Raspberry Pi desktop as a window. This can be familiar, but also can feel slow since the Pi is still generating a full display. The advantage is the minimal amount of hardware, since the only accessory needed is a network connection. This is really only a usable method across a direct Ethernet connection; the WiFi will be unusably slow.

As a Web server. We have installed Node-RED, which can be launched to provide a web-based programming interface. This is not used in the course exercises, but might be viable for advanced students seeking to integrate the Pi hardware with web services.

2.2.9.4. Data Storage on the Pi

We have installed an updatable image of the course materials on the Pi microSD, so it should be possible to work through the exercises as-is. But once you begin copying the templates and generating your own project code and data, you’ll want ways to get your personal files on and off the Pi.

Using shared folders. We have set up a Samba server on the Pi which allows drag and drop file access from a laptop.

Using a USB drive. The Pi can mount filesystems from USB sticks and external hard drives.

Using a SFTP client. Desktop and command-line SFTP clients (e.g. Fugu.app or sftp) can remotely access the Pi filesystem. In many ways, this is more reliable than the Samba system, since there is no persistent connection. Sftp is better for transferring individual files rather than nested folders.

Using rsync or unison. These are programs which perform single- or bi-directional file synchronization between a laptop and the Pi, but which require some careful configuration. These are suitable for nested folders and large sets of files.

2.2.9.5. Maintenance of the Pi

The Linux system on the Pi is a complex infrastructure involving many parts which can be individually updated. Your program is always one of many.

We have provided an update system (the ‘puppet’ system) which can help with keeping your Pi up to date as the course evolves.

2.2.9.6. Steps and observations

We will walk through these steps as a class demonstration, since there are many steps which are hard to document.

Preparation. Gather the materials listed above. Install your microSD card.

Getting connected. If using a cluster laptop, install the Ethernet adapter and attach an Ethernet cable from it to the Pi.

First contact using ssh. If using a cluster laptop, configure as follows:

  1. Open System Preferences/Network and configure the Thunderbolt Ethernet as follows: Configure IPv4 Manually, IP Address 192.168.2.1, Subnet mask 255.255.255.0.
  2. Open System Preferences/Sharing and enable Internet Sharing so the laptop can bridge the Pi to the internet. Enable the “Internet Sharing” setting and set it to share from Wi-Fi to Thunderbolt Ethernet.

This particular configuration takes advantage of the fact that OS X defaults to assigning addresses in the 192.168.2.x subnet when using Internet Sharing. However, if you are using Windows or wish to use a different setup, the network configuration of the Pi is stored in the interfaces.txt file in a FAT32 partition on the microSD card. This can be changed as needed using a normal text editor when the card is attached to a laptop.

Attach power to the Pi and watch the lights flicker as it boots. Try a command-line ssh connection from a Terminal window. The Pi is preconfigured to use address 192.168.2.3, so from a Terminal window try the following: ssh pi@192.168.2.3. If successful, you’ll see a password prompt: the default password is raspberry. If successful, you’ll see a Linux command prompt. Try ls -l to get a listing of files.

First contact using monitor. If using a monitor, connect the HDMI monitor, USB keyboard and mouse, and then attach power to the Pi. You should see a stream of boot messages on the screen culminating in a login prompt. The default username is pi and password is raspberry. If successful, you should see a Linux desktop.

Graphical operation using X11 over ssh. If using a cluster laptop, log out of the console connection, and the log back in with the -X flag: ssh -X pi@192.168.2.3. You’ll again see a Linux command prompt. Try the xterm command; the laptop should automatically launch XQuartz and open a window with a terminal emulator. If that works, try the pd command; it should launch Pure Data with windows appearing on the laptop.

Updating using the puppet system. If using a cluster laptop, try the following command to check your network connectivity: ping www.cmu.edu. If that fails, you’ll need to debug the network connection. The ifconfig command will show you the current network configuration on the Pi. Other useful network commands: hostname -I to check the current IP address, host www.cmu.edu to test name service, and dig www.cmu.edu to test name service in detail. (For details: man dig to read the manual page).

If your Pi is able to connect to the internet, try running an update with sudo pi_update --course. That will update the contents of the /opt/cmuphyscomp/physcomp-f15/ folder to the current course materials. To run all available updates, you may use sudo pi_update, but that can take a few minutes.

Graphical operation using RDP. If using a cluster laptop, launch “Remote Desktop Connection.app” and enter 192.168.2.3 for Computer. You may need to confirm Connect at a security warning. You should then see a Login to xrdp window, enter pi and raspberry as username and password. It should then proceed to show a Linux LXDE desktop within the RDC window. You can log out using the red button at lower right. It is also possible to use “Microsoft Remote Desktop.app”: you’ll need to create a new connection but leave the Credentials section blank.

Getting files on and off the Pi using Samba. If using a cluster laptop, open a Finder window and look under the Shared list on the sidebar. Select raspberry and click Connect As.. and enter the pi/raspberry username and login. It should open a folder representing the pi home folder on the Pi; you can drag and drop to copy files back and forth.

Resizing the filesystem. The course system image is sized to fit on a 4Gb card, but has likely been written to a card larger than 4Gb. You can check whether it has been properly expanded to make use of all available space by running df -h / and seeing whether the size column still says 3.6G or is a larger number commensurate with your actual card. If it needs to be resized, run sudo raspi-config and select 1 Expand Filesystem at the menu, followed by interface item Finish, then confirm an immediate reboot.

Enabling the Pi Camera or SPI ports. The drivers for these devices are not enabled by default. To turn then on, run sudo       raspi-config and select either or both of 5 Enable Camera and 8 Advanced Options followed by A6 SPI.

Installing openFrameworks. The openFrameworks libraries are not included by default in the newer system images, but can be downloaded and compiled directly on your Pi 2. If you would like to work with openFrameworks, a script is provided to build it in /opt/openFrameworks by running sudo setup_openframeworks. It will take some time to run.

Shutting down the Pi. It is best to do a proper shutdown on the Pi rather than just unplugging it to avoid corrupting the memory card. From a command line enter sudo shutdown -h now.