Choosing an embedded single board computer

An Embedded Single Board Computer is a device that contains all the components of an Embedded Computer on a single circuit board. This means a microprocessor / central processing unit (CPU), random access memory (RAM), storage and various forms of I/O. This is none too dissimilar to Single Board Computers aimed at the hobbyist / maker markets such as the Raspberry Pi, Arduino or the Beagleboard. There are some subtle differences however, which can make a huge difference to performance and suitability when applied to industrial or embedded computing applications.

The first thing to consider when choosing an Embedded Single Board Computer is whether you need one at all. This may sound like a strange statement, but the deployment environment and intended use is the single most important factor in choosing the right product for your application. To illustrate this quite simply, ask the following question:

“Is the device going to be used in a benign environment like my home or office and for a single noncritical project where system failure doesn’t put mine or my businesses reputation at risk?”

If the answer is yes, then that maker board you were considering might well do the job, but for any other application you are going to need an Embedded option. Read on.

Modern Embedded Computing applications demand increasingly more compute power as we develop innovative ways to solve problems or automate processes that were previously only possible with pen, paper, and the requisite grey matter. Thinking about the compute power required, whether that’s CPU, GPU, or both, is the first step in designing an Embedded System, as this generally has an impact in many other areas such as thermal design, mechanical footprint, data handling and connectivity.

Embedded Single Board Computers are available with many different processing options, from the very basic microprocessor to multicore, multithread and GPU accelerated beasts capable of near Realtime HD image analysis and rapid processing of high bandwidth sensor data. Having a good handle on the volume and type of data processing your application demands is a key starting point in selecting the right platform.

Embedded Single Board Computers have much wider temperature tolerances than their commercial cousins — typically between -40°C and +85°C — and this is a hugely important benefit.

Colder environmental extremes are generally easier to manage; realistically a board rated at -20°C should run reliably installed in -20°C ambient temperatures. But where the ambient temperature in the installation environment is at the upper end, say 50°C on a factory production line, you cannot simply assume that an SBC rated to 50°C would be sufficient.

Embedded Single Board Computers at the very least are likely to be placed inside a control cabinet, but what is more common is for them to be embedded deep inside a larger subsystem with no airflow, and where the ambient temperature will rise way above that on the production line itself. Computers generate heat; therefore, it is always important to build in some overhead, and it is for this reason that some Embedded Single Board Computers are rated right up to 85°C, and occasionally even higher.

You are perhaps familiar with a heat sink design; flat or moulded on one side to mate to the item generating heat, and fins on the other side designed to maximise its surface area allowing optimal Passive Convection Cooling. Heatsinks are used in both Embedded Single Board Computers and Fanless Embedded PCs to keep them cool and within operating tolerances. The problem with heatsinks however, is you need to provide adequate space around the heatsink so it can do its job effectively. Put an Embedded Single Board Computer with a heatsink in a small enough box, and you simply end up heating the surrounding air and subsequently the over-heating problem reappears. This is where a heat spreader comes into its own.

Heat spreaders replace the fins with a flat piece of aluminium. Using conduction, this piece of aluminium ‘spreads’, or rather conducts, the heat onto whatever surface the heat spreader is mated to. Take the small box example above, place the heat spreader on inside of the box itself and the whole box now becomes a heat sink. For most, the concept of a heat spreader is not new, but even so, design engineers are always pleasantly surprised when they find out that Embedded Single Board Computers often have a heat spreader option available, as it can make designing for thermal management a much simpler process.

There are some instances where Single Board Computers need to be paired with a display of some kind. Fundamentally there are two types of display interface available — ‘embedded’ display connectors and those found on the I/O plate of the board. It might be tempting to use the standard I/O plate options; DVI, DisplayPort, HDMI and good old VGA are standard connectors, and most engineers have a draw full of these types of interconnecting cables. They are fine where there is ample space for the connectors, or where the display will be a standard off-the-shelf style monitor. However, when designing a machine or an OEM product which embeds the Single Board Computer inside, the display of choice is almost invariably a TFT LCD panel.

Standard TFT LCD panels do not have these types of common display connections. Instead, they replace them with ‘embedded’ display connectors such as LVDS and eDP. Low Voltage Differential Signalling (LVDS) has been one the most common types of TFT LCD video input since its introduction in the 1990s and all of the major LCD panel vendors support it. Embedded DisplayPort (eDP) is a more recent addition and has some advantages over LVDS, such as lower power consumption and less complexity in cabling. Whichever you choose, ensuring your Embedded Single Board Computer supports the correct format, and you have a reliable cables supplier, will be an important step in your selection process.

Depending on your space envelope, you have some choices to make in terms of Embedded SBC form factor. Common form factors such as 2.5”, 3.5”, PICO-ITX and PC/104 are widely available, and depending on which side of the fence you sit, you might even consider some of the newer offerings such as 4x4 or Industrial NUC when choosing an Embedded SBC. What will certainly influence your choice however is the I/O required for your project, as I/O is typically fixed along with the source of your data and peripherals. It then follows that the level of system cabling and integration you are comfortable with will also play a part in your decision making. Let us explain:

Like the display options, I/O on an Embedded SBC is broken out via the I/O plate, by varying types of connectors on the board or even with smaller daughter boards on high density connectors. The smaller the board, the less common types of connections will be available such as DB9 for Serial, USB and even RJ45 for Ethernet. Increasing your I/O density on smaller boards will force you to either engineer your own cable kit, or have one made up by your Embedded SBC supplier. This of course is not a real showstopper, but it is important to consider this when looking at datasheets, especially in your development stage as custom cable kits can create added complexity and lead-in time when fixing your bill of materials.

It is always worth a look at the expansion options on an Embedded SBC, especially when the I/O options on the datasheet don’t quite fit your requirements. Very much like with I/O, as Embedded SBCs get bigger, expansion options increase. The trick with expansion and storage is to remember that with modern boards the two often use the same Bus, and this can be used as an advantage keeping the systems flexible and the overall footprint small.

For example, mPCIe expansion slots can be used for both storage in the form of mSATA SSDs, and to add connectivity expansion cards such as Wi-Fi, Cellular or any number of other options only limited by imagination. This is also true of the newer M.2 format, and whilst there might not yet be such a large range of M.2 expansion cards, new variants are appearing on the market all the time.

There are many factors to consider when choosing an Embedded Single Board Computer for your project. We have covered of the headlines here, but there are so many industry and application specific nuances it’s difficult to cover everything. The key is in the data you wish to capture and process — in what environment are your sensors, how much data do they capture, and how do you want to interface with your data sources and finally where are you going to display and process the data you capture? Having a solid understanding of these key points will make choosing an Embedded Single Board Computer a much easier process, and with a little knowledge of the market, or a solid hardware partner to rely on, you should not go far wrong in your next project.