If you tell someone that you’re working on a Raspberry Pi project, they are increasingly likely to ask “3 or 4?” rather than “one crust or two?” Almost 40 million of the hackable Pi single board computers (SBCs) have sold since the credit-card sized product debuted in 2012.
Beyond the Raspberry Pi, the SBC market is booming as embedded computing continues to spread throughout our infrastructure. The pandemic did little to stem growth, as demand increased for healthcare and connectivity solutions and people sought out personal hacking projects to stay sane during the shutdowns. An Allied Market Research report valued the SBC market at $2.86 billion in 2019 and projected an increase to $3.80 billion by 2027.
SBCs are often found under the hood of home automation devices, industrial equipment, signage computers, consumer electronics, robots, smart cameras, gaming systems, healthcare devices, defense systems, and a wide variety of sensor-connected Internet of Things devices and gateways. They are increasingly built into planes, trains, buses, and cars for infotainment and assisted navigation. They also power a new class of edge AI devices that perform tasks ranging from license plate detection to product inspection.
Even if you focus your gaze on the mostly Arm-based and community-backed SBCs under $200 that run Linux, there are at least 150 different models to choose from. There is also a larger market in commercial boards sold in bulk to industry, which are more likely to be x86-based.
SBCs vs. Motherboards vs. Phone Boards
The SBC is a close sibling to the motherboard in your desktop computer, which shares most of the same memories, ports, and components. SBCs tend to be smaller, less powerful, and more power efficient, although the gap is closing with the rise of SBC-based mini-PCs. As the Raspberry Pi grows more powerful, people are popping them into enclosures for desktop replacements.
SBCs also share much in common with the PCB in your phone, which tends to be smaller, but more powerful. The main difference between an SBC and either a desktop computer or phone is the software that runs on them. PCs and phones tend to run OSes – including Linux or Linux variants such as Android – that host numerous multitasked applications.
By contrast, SBCs are usually intended for single tasks. Although Raspberry Pi users in particular have more and more apps to switch between, SBCs usually run embedded firmware that is optimized for a single application. Especially in industry, SBCs often run in the background unattended.
SBCs vs. SOMs
SBCs can be considered as larger and more accessible alternatives to compute modules, also known as system-on-modules (SOMs). SOMs are often smaller than a Raspberry Pi – the Raspberry Pi Compute Module 4 (CM4) measures only about 2.2x1.6 inches – and they usually lack the real-world ports found on SBCs, such as USB, Ethernet, serial, and HDMI.
SOMs often adhere to form-factor standards such as COM Express and SMARC that enable a degree of interoperability and easier processor updates. They are rarely sold in individual units and are even more rarely open source.
Some SBCs are deployed as finished products such as home automation equipment or robots, typically in smaller volume projects. However, they are more typically used for prototyping. Engineers often use an SBC to try out different configurations, features, and software before designing a custom production board that uses the same system-on-chip, often deployed on a SOM.
SOMs are often sold with carrier boards that are much like SBCs, but without the processor and memory. Even in the under $200 segment, there are some sandwich-style boards such as the HummingBoard that combine a carrier board with a SOM and are sold as SBCs. This approach can make it easier to transition from prototyping to a final custom mainboard that includes the SOM.
Like SOMs, SBCs can be divided into different form-factor standards, but the standards are mostly limited to size, and only partially enable interoperability. In the market for commercial SBCs based on Intel and AMD x86 chips, there is wide adoption of form factors such as Mini-ITX (170x170mm), 3.5-inch (about 146x101mm), and Pico-ITX (100x72mm).
Few Arm-based SBCs adopt these form factors. In the Arm world, the biggest standard is the Raspberry Pi itself. There are many dozens of SBC brands that approximately mimic the Pi’s 87x56mm footprint while offering mostly similar port selections and layouts. Many more SBCs adopt the Pi’s 40-pin GPIO header, enabling at least theoretical use of hundreds of Raspberry Pi HAT add-on boards.
Other board standards with much smaller adoption include the BeagleBone and its pseudo-clones, which share a Cape add-on standard. There is also 96Boards.org, which certifies Linux-driven SBCs with common sizes and expansion headers for Mezzanine add-ons.
Processors for SBCs
The main decision in selecting an SBC is choosing the right processor. Many of the Intel-based SBCs use the lower-power Intel Atom class, including the latest Elkhart Lake family. Intel also offers lower-power versions of its Core processors that are more suitable for embedded, such as its 8th Gen Whiskey Lake and the more recent 11th Gen Tiger Lake. There are also high-end boards that use more desktop- and server-oriented Core CPUs such as 9th Gen Coffee Lake or 10th Gen Comet Lake.
AMD plays a lesser role in embedded with its more media-focused Ryzen Embedded SoCs. Although these are increasingly popular, there are very few SBCs with higher-end Ryzen desktop processors.
In the Arm segment that dominates low-cost SBCs, there are many more vendors in the mix. If Raspberry Pi Trading had not cut a sweet deal with Broadcom to receive exclusive use of processors such as the RPi 4’s 1.5GHz, quad-core, Cortex-A72 based Broadcom BCM2711, Broadcom would likely be much more dominant in the segment. There is nothing particularly special about the Broadcom SoCs, but SBC vendors are always looking to piggyback the leader to improve compatibility.
Among SBCs aimed at consumers, Arm platforms such as Allwinner and Rockchip have dominated. Lately, Amlogic SoCs have seen wider adoption on SBC brands, such as Khadas Vim, and Odroid. As you move more into the higher-end media and industrial boards under $200, you will see more NXP i.MX8 family processors along with Intel chips and Arm brands such as TI Sitara, Samsung, Xilinx Zynq, Qualcomm, and others.
SBC Software and Support
Most of the under-$200 SBCs offer varying degrees of open hardware support. Schematics and other open hardware files are typically available, and most offer community resources like forums and online tech support.
Customized Linux and Android images are typically available, which are increasingly tied to mainline Linux. There is a lot of sloppy work out there in OS support, especially on the lower-cost models, but OS reliability is improving.
The most popular OS choices for Linux SBCs are Ubuntu and Yocto Project, which is not a distribution but a standardized collection of components and libraries. Other desktop distros such as Fedora are sometimes supported on higher-end x86 SBCs.
Linux continues to dominate the Arm SBC world and increasingly competes with Windows 10 in the larger market for commercial x86 SBCs. Low-power microcontroller-driven boards that run real-time operating systems such as FreeRTOS, tend to be more like SOMs (see the new Raspberry Pi Pico). Many Arduino boards, however, are more like SBCs.
In both the Linux and RTOS MCU board markets, the open source RISC-V architecture has begun to make inroads, with new Linux-driven SBCs such as SiFive’s HiFive Unmatched. We will examine RISC-V in a future article.
Despite the increasing dominance of the Raspberry Pi, there is still a vast selection of different vendors, products, processors, and feature mixes to choose from. Although consolidation has begun, the many new use cases for embedded computing should keep the segment lively for some time.