Building a Pi-based ARM laptop

Becky Stern of Adafruit industries set me on the path of building my very own Raspberry Pi laptop late last year when her blog post describing such a device brought sudden popularity to the idea [1]. Although this was hardly the first mention of such a device, Stern's YouTube video detailing the creation of the necessary "frankencable" virally spread the idea. Many alternative designs followed in what is now a 31-page thread on the Pi Foundation's forum [2], but the core idea remains the same: building an ARM-powered laptop backed by a Raspberry Pi.

Inception

After seeing the blog post on Adafruit's [3] site, I immediately rushed to eBay to secure the key component of the build: an Atrix 4G lapdock (Figure 1). It would be no exaggeration to say I purchased this component before I even watched the video or read the instructions, because I expected that sudden demand would deplete the supply – or at least affect prices. My eagerness paid off, and I secured a brand new unit for the paltry sum of US$  35, shipping included (see Figure  2).

Figure 1: The lapdock designed for Atrix 4G smartphones is the build's key component.

Figure 2: New lapdock unit in its unopened box.

Your luck may vary, but my recommendation is that you take your time procuring this one part if you are budget conscious, because it can easily double the total cost of this project (see the parts list in Table 1 for details).

The Atrix Lapdock is an accessory for a Motorola smartphone that could double as a laptop when connected to this docking station. Because the matching Atrix 4G smartphone has been discontinued, many of these stations have hit the market at deeply discounted prices (eBay still lists more than 200 units as of this writing).

This lapdock is quite a nicely built object, with an original price tag of several hundred dollars. It is effectively a high-resolution 11.6" LCD screen with a resolution of 1366x768 (60Hz refresh rate) incorporating a QWERTY keyboard and a touchpad in a form factor resembling that of today's ultrabooks. Also, the lapdock includes stereo speakers and 36 watt-hour's worth of battery power  – enough for several hours worth of operation  – all in just under 2.4 pounds of weight.

This power is accessible through the lapdock's USB ports and a micro-USB plug on the docking sled, as well as two USB-A sockets located on the back of the device. Additional connectivity includes a barrel plug for the power supply that comes with the lapdock and a micro-HDMI connector directly wired to the screen (Figure 3).

Figure 3: Detail of the connectors on the docking sled (left) and additional connectors on the lapdock body (right).

The keyboard and touchpad are accessible on the micro-USB port, but that is not the case on the Raspberry Pi, where the micro-USB port must be connected to the power source – and only the power source. The full-size USB-A ports are the only place where keyboards and mice will be recognized, because they are the ones wired with data lines.

This is the origin of the "frankencable" approach: By splicing a custom cable taking only data to one side of the board and power only to the other, it is possible to attach the Raspberry Pi to the lapdock with a single USB connection, in addition to the HDMI connection.

First Try

I was quickly able to assemble my variant of a Raspberry Pi laptop by purchasing cables and adapters from multiple eBay sellers to wire up a Raspberry Pi model B to my lapdock. I questioned the durability of the self-made cable approach required by the Raspberry Pi's sub-optimal USB layout and decided to go with two full USB cables: one for power and one for data. I was able to demonstrate this off-the-shelf cabled design at Boston Linux and Unix two weeks after beginning [4].

A drawback of this design was the sheer number of cables and adapters needed, which was not only costly because of shipping fees but also difficult to order and confusing to assemble. A bigger problem, however, is that although the setup is durable, the number of cables involved creates a cable nest behind your Pi laptop and does not provide the user with a reasonable chance to attach the Pi to a consistent location on the combined device. The laptop in effect had to be assembled for every use, a problem that even the frankencable would not address (Figures 4 and 5).

Figure 4: The first all-cable design: a viable stationary setup.

Figure 5: Details of the wiring of the all-cable setup.

Although the ergonomics of the project seem to demand improvement, this first version is fully functional and quite stylish – thanks to the addition of a color-matched stealth black case for the Raspberry Pi board, courtesy of the fine folks at ModMyPi.