Taking BeagleBone Black for a walk

Linux powered single-board computers (SBCs) are not quite a dime a dozen, although the price is dropping rapidly enough that such a milestone is not entirely out of the question. While the news in this space over the past two years has been dominated by the Raspberry Pi, there are plenty of other choices. One of the more popular options is the BeagleBone Black, which is more than a match for the Pi in terms of hardware power, and offers a software selection worth exploring, too. Determining which system is right for which task depends on weighing a lot of factors.

Getting around the dog

The BeagleBone Black is the fourth generation SBC from the BeagleBoard project. The first and second devices were called "BeagleBoards;" the BeagleBone moniker referenced the significantly smaller physical dimensions of the third generation product. The BeagleBone Black uses the same form factor as the preceding model. But it also includes updated components and a lower price: $45.

The basic makeup of the board is straightforward. It is powered by a Sitara AM3358 System-on-Chip (SoC) from Texas Instruments (TI), which is built around a Cortex-A8 CPU from the ARMv7 line. There is 512MB of DDR3 RAM on board, plus 2 GB of built-in flash storage and a slot for an additional microSD card. Connectivity includes a wired Ethernet port, one mini USB client port, one full-sized USB host port, and a micro HDMI connector that outputs both audio and video.

But in these modern times, of course, single-board computers need to provide hardware expansion capabilities. This is where the BeagleBone Black first begins to differentiate itself. The long sides of the board include a full spread of pin headers, right up to where the PCB mounting holes get in the way. In grand total there are 92 pins available, which is considerably more than most competing boards. 65 of those pins are available for digital I/O; eight of the 65 can serve as pulse-width modulators (PWMs) for analog output, and four can be configured to serve as timers. Various subsets of these digital pins can also be configured to provide two I2C ports, two SPI ports, and two serial ports (there is also a separate serial debugging header that cannot be used for general purposes).

In addition to the digital I/O pins, there are seven analog input pins, plus an assortment of 3.3V, 5V, and ground pins. If that is not enough, there is also a pad on the back of the board where enterprising users can solder on a JTAG debugging header. Other nice touches include a 5.5mm DC power connector (the device can run off of power supplied by the USB Client port, but having two options is more convenient), hardware power and reset switches, and a built-in array of four programmable LEDs.

The onboard flash storage come pre-loaded with the Ångström distribution—and numerous tools to get started. When the system boots up, it starts a web server running on 192.168.7.2:80 (either over Ethernet or using Linux's USB Gadget API to provide networking if the board is connected to a host computer via USB). This server provides access to the board's documentation (the same documentation and tutorials available at BeagleBoard.org), and there is also a Cloud9-based IDE running on port 3000. The IDE comes preconfigured to make use of BoneScript, a JavaScript library tailored to provide access to the board's hardware expansion features.

Of course, BoneScript is not the only option. One can also SSH into the board and explore it from a terminal like any other Linux system or hook up USB peripherals and an HDMI monitor and use Ångström's lightweight graphical environment. Out of the box, the BeagleBone Black I tested (revision A5C, for the record) runs kernel 3.8.13. Access to the array of expansion pin headers is simple, using sysfs. The traditional first project is to blink an external LED; to do this one should attach the LED's anode to a GPIO pin header, the cathode to a nearby ground header, and export the GPIO pin:

    echo 60 > /sys/class/gpio/export

brings up pin number "12" on the left-side expansion header, so that it is accessible as /sys/class/gpio/gpio60 (the GPIO number for the pin has to be looked up in the documentation). Set it as an output pin with:

    echo out > /sys/class/gpio/gpio60/direction

and the LED can be flashed on and off by echoing 1 or 0 to /sys/class/gpio/gpio60/value. The only real tricky part of the process is figuring out the correct GPIO number; the numbers on the expansion header do not map cleanly to the various GPIO numbers because of all the positive voltage, ground, and other assorted pins mixed in—and because the board tries to spread the pins around evenly to provide some flexibility when attaching components. The reference manual PDF has a table that provides the correct numbers.

Kicking it up a notch

As exhilarating as flashing an LED can be, eventually one needs more from one's hardware, which is where BeagleBone capes come into play. Capes are the equivalent of Arduino shields: pin-compatible circuit boards that fit into the expansion headers on the BeagleBone itself. Most capes have expansion headers on top to match the pins on the bottom that fit into the BeagleBone's headers; that way multiple capes can be stacked, and the pin compatibility means a +5V location on one board does not get turned into a ground pin by a board in between.

Currently the crop of BeagleBone Black capes is somewhat limited because the "Black" model introduced some variations over the original BeagleBone (which was white in color, although it is usually just referred to as "BeagleBone"). The eLinux wiki includes an up-to-date page of available capes, with those that are compatible with the BeagleBone Black marked as such. The simpler capes (such as those providing a breadboard or through-hole prototyping board) are compatible with both models; there are also Black-compatible capes available for attaching small LCD screens, serial ports, GPS receivers, electrical relays, and camera modules.

More specialized capes are slowly appearing, too; there is a CAN Bus cape (for automotive usage), an unmanned aerial vehicle (UAV) altitude-and-orientation cape, and a cape designed to run a RepRap 3D printer. As with Arduino, most of the capes are community-design products, so what is available depends mostly on what itches need scratching. However, CircuitCo (who manufactures the BeagleBone original and Black) has been systematically taking Creative Commons–licensed capes from the original BeagleBone and updating the design to match the Black model's pinout, so in many cases a refreshed cape is on the way if it is not already available. Since the original BeagleBone is no longer being manufactured (and was twice the price of the Black), the old capes will probably disappear from production anyway.

On the software side, updates to the Ångström release on the board are available from the beagleboard.org site. Separate kernel sources are available on GitHub for those who cannot wait for a new release, although only minor 3.8 updates are available now. The kernel used is the mainline Linux kernel, but it may be simpler to recompile for the device when starting from the exact sources and configuration used in the official build; the official kernel is also updated periodically with support for new capes.

Perhaps more interesting is the array of other Linux distributions available. Ubuntu, Debian, Arch, Gentoo, Sabayon, and Fedora releases are all available. Although the board is not an officially-supported platform for these distributions, the relatively recent ARM revision and simple hardware of the BeagleBone evidently make an ARM distribution relatively easy to get running, either by overwriting Ångström on the internal storage or by installing an image on the microSD card slot (which is faster to experiment with, since overwriting the onboard storage can take close to an hour). For the more adventurous, there is also an Android port available (from TI), as well as QNX and FreeBSD releases.

One reason installing outside distributions is comparatively simple is that the BeagleBone Black uses standard distribution components like the U-Boot bootloader, and that it does not require binary blobs to boot. The graphics chip is a PowerVR GPU that does come with a binary-only OpenGL ES firmware blob, but the board will run non-OpenGL applications fine without it.

Comparing apples and raspberries

The binary blob issue is one of the more frequently cited issues where BeagleBone supporters feel their hardware has an advantage over the Raspberry Pi, which by size and price is the nearest competitor (and one which gets noticeably more news attention). The Pi, of course, uses a Broadcom BCM2835 SoC with an open source user-space component that speaks to a proprietary firmware blob. More importantly, though, the Pi uses a non-standard boot sequence that actually starts up the board's VideoCore GPU first, and the binary VideoCore OS subsequently loads the Linux operating system.

There are other differences as well, starting with the fact that the Pi uses a generation-older ARMv6 core at a lower clock speed. The Pi also includes expansion headers, but fewer of them than the BeagleBone Black, and some of them can be difficult to access. From a practical standpoint, the BeagleBone Black's DC power plug is another nice touch, since the Pi can only be powered over its micro USB connector. The Pi also lacks a hardware reset switch, which is inconvenient. In addition, although both boards are roughly the same size as an Altoid mint tin (which in recent years has become the standard international unit for circuit-board size), the BeagleBone is an exact fit, while the Pi's slightly larger size and sharp corners make it a mismatch. Finally, BeagleBone capes are designed to be stackable, while Pi expansion boards are generally not stackable due to the asymmetry of the expansion headers (on a side note, it is also odd that Pi expansion boards do not have an official nickname akin to "capes" or "shields;" "toppings" or "meringues" would seem to be the obvious choice).

On the other hand, Pi supporters will be quick to point out that the Pi has nice features of its own, such as a dedicated camera module connector using the standard Camera Serial Interface (CSI), built-in support for Display Serial Interface (DSI) output, full-size HDMI, RCA video out, 3.5mm audio out, and the availability of hardware multimedia decoding.

It is definitely true that the Pi offers an easier multimedia experience than the BeagleBone Black. The Pi has more connector options, and its GPU is capable of handling 1080p video—capable enough that there are several active media center distribution options for the Pi.

That is why it always comes down to matching the product with the project. The BeagleBone Black will likely not be useful as an XBMC or video gaming front end for the home theater, while the Pi is considerably more limited in how many sensors, servos, and hardware relays it can control. For free software advocates, the BeagleBone Black product is not quite devoid of binary blobs, but it is possible to boot and run it with free software.

Not everyone cares about the presence or absence of binary blobs, of course. And no doubt the Raspberry Pi Foundation has already heard a lot about this subject; it seems plausible that in the virtually inevitable Pi follow-up, the result will have less reliance on proprietary firmware. The BeagleBone Black can exert some market pressure on the Pi in that area, demonstrating that an almost-totally-free software SBC system is possible to make today.

That is fair play, though, since the success of Pi clearly pushed the reduction in size and price seen in the Black as compared to earlier BeagleBoard/BeagleBone revisions. Earlier BeagleBoards were noticeably bigger than the Pi, and the first BeagleBone retailed at $89. At $45, the Black is a little more expensive than the Pi, but not by much. There is also a gray area between the "hobbyist" uses targeted by the BeagleBone Black and the "educational" projects envisioned by the Pi, of course. But any cheap Linux-powered SBC is an educational opportunity, which can quickly turn into a hobby—or more.