Learning Beaglebone Python Programming: Unleash the potential of BeagleBone using Python

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Learning Beaglebone Python Programming

Chapter 1. Before We Begin

Before we start hooking up hardware and writing code, we'll need to have an understanding of what we're working with. This chapter will introduce you to the BeagleBone and highlight the various interfaces it provides to connect to external devices. It will cover:

An overview of the BeagleBone system
An overview of the BeagleBone's peripheral interfaces, and what types of external devices each can connect to
Some additional hardware and tools that you will need if you want to duplicate the examples given throughout the book, and where you can buy them

An overview of BeagleBone

The BeagleBone boards are a series of small, powerful, and affordable Linux computers that are perfect for embedded applications such as home automation, robotics, industrial control, and much more. They are designed by BeagleBoard.org (http://beagleboard.org/) and are fully open source. They are based on the Texas Instruments AM335x 1GHz ARM Cortex-A8 series of microprocessors, and can run a number of different operating systems, including various GNU/Linux distributions, Android, and even Windows Embedded CE. The current BeagleBone model being produced is the BeagleBone Black rev C, which ships with a Debian GNU/Linux distribution. Therefore, this book will focus on using Debian on the BeagleBone Black, though much of the information given will apply to other BeagleBone models and Linux distributions as well.

The following screenshot shows the BeagleBone board:

The BeagleBone Black's AM335x microprocessor contains a number of built-in peripheral interface subsystems, enabling it to accept and generate many different forms of inputs and outputs. The BeagleBone Black includes two 2 x 23 pin rows of female header pins, giving a total of 92 connection points for hardware expansion using the processor's peripheral interface subsystems.

General purpose input/output

The general purpose input/output (GPIO) module handles all the digital input and output. In this context, digital refers to the fact that the signals are binary; they are either 1 or 0, represented by fully on and fully off respectively. In the case of the AM335x, the fully-on level is 3.3V, and the fully-off level is 0V. The GPIO module is used for inputs such as switches and buttons, which are either on or off. Its outputs can be used to control devices, such as LEDs, buzzers, and relays.

Analog-to-digital converter

The analog-to-digital converter (ADC) module is used to measure analog voltages. The AM335x ADC can only measure voltages between 0V and 1.8V (and voltages outside this range may damage your BeagleBone), but, in later chapters, you will learn how to divide larger voltages to be within this range. The ADC can be used to receive inputs from devices such as potentiometers, which can be used to create varying voltages, measure the voltage output of analog sensors for temperature, light, sound, and different types of gases, and with some additional external components it can be used to measure electrical current.

Pulse width modulation

The pulse width modulation (PWM) module is essentially used to generate a square wave signal at a fixed frequency, and then vary its duty cycle. It gives us the ability to accurately generate pulses of a configured duration, repeating at a configured frequency. Like the GPIO module, the PWM module on the BeagleBone Black operates at 3.3V. These PWM signals can be used to control servo motors, vary the speed of DC motors and the brightness of LEDs, and with some additional external components they can be used to generate varying voltages.

Universal asynchronous receiver/transmitter

The universal asynchronous receiver/transmitter (UART) modules are used to transmit and receive RS-232 style serial signals, which is an industry standard for serializing and transferring information between two devices using a pair of unidirectional digital signals. They can be used to communicate with PCs, Bluetooth and Wi-Fi radio modules, and GPS receivers. The BeagleBone Black's UART modules also operate at 3.3V.

Serial peripheral interface

The serial peripheral interface (SPI) module is used to communicate over SPI, which is another industry standard serial protocol. Whereas UARTs are generally used to connect two devices, SPI is made to connect one master device to one or many slave devices. It is commonly used on devices such as small character and graphics LCD screens, external ADCs, and DACs (Digital-to-Analog converters), as well as on many different types of sensor. The BeagleBone Black's SPI modules operate at 3.3V as well.

Inter-Integrated Circuit

Inter-Integrated Circuit (I2C) is yet another industry standard serial protocol. It also allows a master device to communicate with a bus of many slave devices, but it requires fewer pins than SPI. It is commonly used by real-time clocks (RTCs), as well as in many types of sensors, including Micro-Electro-Mechanical Systems (MEMS) devices, such as accelerometers, magnetometers, and gyroscopes. The BeagleBone's I2C modules operate at 3.3V.

Tools and additional hardware

The majority of the demo programs in this book use external hardware that must be purchased separately. Each time a demo program is given, which requires additional parts, they will be listed by part number and/or description. We will do our best to use the most readily available and lowest cost parts. All of the parts used can be purchased from one or more of the following resources:

SparkFun: https://www.sparkfun.com/
Adafruit Industries: http://www.adafruit.com/
Digi-Key: http://www.digikey.com/
Mouser: http://www.mouser.com/
Farnell / Newark / Element14: http://www.farnell.com/

The circuits in each demo will be assembled using solderless breadboard and jumper wires. Both come in many different shapes and sizes.

Breadboards and jumper wires can be purchased from any of the preceding links, and you'll probably want to start out with one standard-sized breadboard and a jumper wire kit, such as that from Adafruit:

Breadboard: http://www.adafruit.com/products/239
Jumper wires: http://www.adafruit.com/products/153

That should provide enough breadboard space and jumper wires to assemble most, if not all, of the demo circuits in this book.

Just like with software, it is inevitable when assembling hardware that things won't always work the first time. There are many tools that can greatly reduce the time it takes to fix these problems. The most useful for the circuits in this book will be a multimeter, which is a tool that measures voltage and current, and often additional properties such as resistance, capacitance, and frequency. Both SparkFun and Adafruit carry very affordable digital multimeters. While these are not high quality measurement tools, they are certainly suitable for these circuits. Though not essential, I would highly recommend having some sort of multimeter on hand when building the circuits in this book.

More helpful than a multimeter for debugging tools such as PWM and serial protocols is an oscilloscope, which shows you a plot of voltage over time to visualize many different signals in a circuit. This is a more expensive tool, and will be less necessary for these circuits. Throughout the book, however, you will see screen captures of an oscilloscope to show various signals, and it should become evident just how helpful they can be. Again, Adafruit and SparkFun carry affordable oscilloscopes.

Board comparison

There are a number of low-cost single-board GNU/Linux computers on the market these days, so let's take a look at how the BeagleBone Black stacks up against a couple of its most popular competitors.

	BeagleBone Black	Intel Edison	Raspberry Pi 2 B
CPU	1 GHz single-core ARM Cortex-A8	500 MHz dual-core Intel Atom	900 MHz quad-core ARM Cortex-A7
Flash	4 GB eMMC, expandable with uSD	4 GB eMMC	uSD card
RAM	512 MB	1 GB	1 GB
Video	microHDMI	N/A	HDMI, Composite
Network	10/100 Mbit Ethernet	Dual-band a/b/g/n Wi-Fi, Bluetooth 4.0	10/100 Mbit Ethernet
GPIO pins	65	20	40
ADC channels	7	6	N/A
PWM channels	8	4	2
UARTs	4	1	1
SPI ports	2	1	1
I2C ports	2	1	2
Coprocessor	2x 200 MHz 32-bit PRU microcontrollers	100 MHz 32-bit Intel Quark	N/A
Price (USD)	$49	$49.95	$39.95

The BeagleBone Black offers great performance and far more hardware expansion capabilities at about the same cost as the Edison and Raspberry Pi 2 B. That combined with its active open source community makes it a great choice for a huge variety of projects.

Another important feature of the BeagleBone is the two built-in PRU (programmable real-time unit) microcontrollers. These are built right into the AM335x CPU and are on the ARM interconnect, so they can share memory with the ARM processor as well as provide direct access to the peripherals. This means high-speed, real-time tasks can be executed on the PRUs asynchronously without any interruption from the Linux kernel. With growing kernel driver support and documentation to compile and load firmware to PRUs, and for communicating with the code running on them from GNU/Linux user space, they really set the BeagleBone apart from much of its competition. The PRUs are outside the scope of this book, but there are plenty of tutorials and examples to be found on the Web.