Leveraging Size and Processor Diversity of Motherboards and Modules to Provide Superior SWaP in Mil/Aero Embedded Computers

Mil/Aero embedded computers for data acquisition, video control, communications, control systems and many other functions typically require a little tweaking. With so many applications, an all-in-one solution can be a rare commodity. So, design flexibility is paramount. A versatile mil/aero embedded platform starts with the motherboard.

The embedded motherboard form factors of ATX, EATX, Mini-ITX, and MicroATX provide several options as a starting point for the system. Actually, the architecture may be a minor issue to the design engineer. They just need a certain level of processing, IO, expansion, and other key ports to meet the application’s requirements. Of course, the size, weight, power consumption, and reliability are critical issues. The size/form factor of the motherboard is important. Starting with an ideal-sized board can help provide superior SWaP in the system. A smaller motherboard can allow for a smaller enclosure which usually means less weight. Additionally, the small boards can sometimes use less power, another factor in SWaP and possibly may use a smaller and lighter PSU. Figure 1 shows a diagram showing the various motherboard sizes. But there are always tradeoffs.

Figure 1:   A chart showing the various PCIe-based motherboard sizes by form factor.

Figure 1: A chart showing the various PCIe-based motherboard sizes by form factor.

If space is an issue or compactness is desired, there is a tradeoff between size and capability of a motherboard. In particular, Mini-ITX is the most compact motherboard size and comes with at most 1 expansion slot. The processor is typically low power, which can be in the form of Intel® Atom™ or a low TDP (Thermal Design Power) Core i. It will usually have a smaller CPU socket and lower pin count. You won’t see too many desktop class processors, which require a larger socket, more pins and have a higher TDP, at this size. The next step up is MicroATX, which has at most four expansion slots. At this size, there are mostly desktop processors and very few low power versions, because they aren’t capable of supporting the higher slot count. Moving up to ATX, we find that it’s very similar to Micro in terms of performance but the expansion capability has increased to seven slots. The largest size, EATX, keeps the same slot count but now allows dual processors. Not only is this a large increase in processing with double the core count, but other features such as memory capacity and PCIe lane count are greatly increased. EATX is where one will mostly find Intel® Xeon® processors. When it comes to IO, the port count doesn’t vary greatly between different sized motherboards. You’ll still typically find a few display ports, USB, LAN, and COM to support standard peripherals and communicate with a network. If the motherboard is focused on embedded markets, then IO such as LPC, I2C and DIO are made available to communicate with other application specific devices. Storage on the other hand, does vary with size in the form of increased SATA port count. Mini-ITX motherboards will have between 2 to 4 ports, ranging up to 10 on EATX boards.

Figure 1: A chart showing the various PCIe-based motherboard sizes by form factor.

Figure 2: Small fanless systems such as this one utilizing a Mini-ITX motherboard can often meet environmental and shock/vibration requirements for Mil/Aero applications.

Along with the motherboard type, a key consideration for Mil-Aero designers is the processor that provides maximum functionality for their application at typically the lowest power consumption. For Intel-based solutions, the Intel® Core™ i series processors and the Xeon and Atom series provide a distinct set to base a platform upon. For the higher-performance processors, the Intel Core i series is the “race car engine” with very high performance in typically 2 to 4 cores with typically up to 16 PCIe lanes. They offer powerful throughput for systems that are not overly complex in functionality and not a lot of on-board memory is required. With Intel Skylake, the multi-core processing is fast and efficient with speed usually in the 3-4 GHz range. This is beneficial in many Mil/Aero signal processing applications, simulation systems, SIG-INT, and more. The Xeon is the “18-wheeler Semi engine.” It’s not necessarily fast but it can haul a really heavy load. The core count can be massive, numbering into the 20s for some very high end applications. It facilitates a great deal of flexibility with often 32 PCIe lanes and sometimes up to 40 or more. The design is highly versatile for a motherboard manufacturer, giving them a lot of options. Mil/Aero designers utilize the Xeon in many feature rich applications, such as C4ISR or Situational Awareness, or server-type requirements. On the other end are the low power processors, such as the Atom. The Atom is an “compact electric car engine,” built for low power consumption but offers less versatility. These are used in less complex systems with only 1-4 PCIe lanes. They typically have 1-2 cores that are very power efficient — usually around 5W. It should be noted that the processing power continues to expand for the Atom while the wattage remains low. Board designers also enjoy the Atom’s small size which takes up less board space. This is primarily due to the fact that Atoms are now SoCs (System on a Chip) which eliminates the need for a chipset. Overall, it is not only important to start with many open-standard motherboard form factors, it is equally beneficial to have several processor and chipset options or SoC options. Intel’s Atom Series is conducive to Mil/Aero applications that are fanless or conduction-cooled, which is a significant benefit in MTBF, simplicity, lower power, etc.

Figure 3:  The 19” rackmount computers can be highly versatile with ATX or EATX motherboards, and a wealth of PSU, airflow configuration, and I/O and expansion options.

Figure 3: The 19” rackmount computers can be highly versatile with ATX or EATX motherboards, and a wealth of PSU, airflow configuration, and I/O and expansion options.

Enclosing the Solution

The ability for a system to be fanless is an important benefit for many Mil/Aero applications. Sometimes there is not significant airflow ventilation or the environment may have sand, dust, or other contaminants. With various-sized motherboards that have low power options, there is a wealth of fanless enclosures configurations that can be utilized across a spectrum of designs. This allows the designer to optimize SWaP and ensure longevity of the system. Additionally, several modifications can be made to fit an application. For example, antennas or WiFi cards, etc, can usually be attached to these SFF computers. See Figure 2 for an example of such a system.

Many mil/aero applications require some expansion options. This is possible in the smaller systems, but typically limited. For example, a Mini ITX motherboard is 170mm x 170mm — which is small. However, there is only room for one PCIe expansion slot. A Mini PCIe slot or two can also be added though. The motherboard systems can provide several slots for adding all types of graphics processing, data acquisition, external expansion/IO, networking, and other cards. Its also beneficial to provide various standard IO options for COM, LAN, serial, VGA/DVI-I, etc.

Rackmount designs provide a lot of expansion capability and rich I/O options for more intensive applications. See Figure 3 for example of an ATX motherboard inside the rackmount computer. This figure shows a typical configuration with front fans, 7 rear slots, a single 700W PSU, and dual hot-swappable fans. For application versatility, a design can provide various Ethernet, VGA, DVI, HDMI, USB, serial, and other ports.

Figure 4:  Where more expansion is required, multi-functional systems provide versatility of mounting options and functionality.

Figure 4: Where more expansion is required, multi-functional systems provide versatility of mounting options and functionality.

A nice hybrid between the rackmount and the small form factor system are the multi-function systems. These typically provide a lot of expansion, but in a more compact system that can be wall-mounted. See Figure 4 for an example of a multi-function system with 4 PCIe/PCI slots, 32 GB of DDR4 memory, an optional optical drive, and multiple graphics, I/O, and communication port options.

Enter SBCs

For a very small form factor, SBC boards can be utilized in many Mil/Aero systems. Note that we are referring to small motherboards as opposed to plugged SBCs that go into backplane-based systems. SBCs add the benefit of multiple board functionality in one board. Often significant processing power can be combined with graphics processing, multiple IO options, graphics and storage ports, and significant DDR3 or DDR4 memory. Using the Xeon, Atom, or Core I processors (along with various chipsets), you can imagine the possibilities for various applications. For a very small and relatively simple solution an Atom processor can be used on a 3.5” SBC. This would provide a stand-alone, compact, low power system for basic computing functionality with display ports and various I/O interfaces.

With their small size, the SBC boards often utilize less power consuming processors (such as Atom-based). As such, they can be a good approach for fanless and conduction-cooled systems. With the SBCs and COM Express modules, very small form factors can be developed around them without adding a lot of size. Fig 5 shows an example of a 3.5” SBC with an enclosure wrapped around it. The SBC is 146mm x 102mm and the enclosure dimensions are 166mm x 157mm as some space is added for fans etc. Note that mounting tabs would add slightly to the size. So, using “exact-sizing” of the enclosures, the size and weight can be optimized.

Figure 5:  With the compact size of 2.5-4” SBC motherboards, “exact-sized” enclosures tied to the I/O configuration of the board can optimize SWaP (Size, Weight, and Power).

Figure 5: With the compact size of 2.5-4” SBC motherboards, “exact-sized” enclosures tied to the I/O configuration of the board can optimize SWaP (Size, Weight, and Power).

COM Express

COM Express is a module and carrier board pair that allows the application to dictate the size and shape of the motherboard. The module handles all of the processing related functions, while the carrier handles of the IO and expansion. In our “car engine examples”, the COM module incorporates the engine, transmission and drivetrain, whereas the carrier is the chassis of the vehicle. Essentially, the module handles the processor, memory and provides an interface in the form of a standard connector to the carrier. With this two-board approach, there is tremendous flexibility for Mil/Aero designers for COM Express to work in a wide range of applications. The carrier board can easily be customized to various I/O and expansion requirements while the module board remains consistent. Over time, the performance of the module can be upgraded and will continue to interface with the carrier board. With a standard interface between the carrier and module, a wide range of COM Express boards and manufacturers can be used over time. This scheme also allows the design of the carrier to remain secret.

The COM Express modules are very versatile, with several standard size interfaces including Basic, Extended, Mini, and Compact.


Mini: 55 × 84 mm (2.2 × 3.3 in)
Compact: 95 × 95 mm (3.7 × 3.7 in)
Basic: 95 × 125 mm (3.7 × 4.9 in)
Extended: 110 × 155 mm (4.3 × 6.1 in)

As the Basic and Extended versions are larger, they often will have the hotter, more powerful Intel Core i processors. To offer more versatility, the COM Express designer can utilize various generations for optimal price/performance. With smaller versions like the Compact and Mini, typically SoC-based Core I or Atom processors will be used. Of course, a key factor in the capability of COM Express are the various Types (1-6, 10). The Type 6 version of COM Express offers up to 24 PCIe lanes, which is the most common today. Each Type of COM Express module defines the number of PCIe lanes available I/O ports that can be used.

Enclosure Versatility

Depending on the application, often commercial or industrial-grade rackmount enclosures are suitable for Defense computing needs. However, sometimes more rugged designs are required. To make a commercial industrial rackmount chassis more rugged, there are some modifications can be made. Designers need to choose the proper operating/storage temperature of components and to withstand environmental effects (moisture, salt fog, sand, etc). Plus, conformal coating can be used to protect the components in the system. By going from cold rolled steel to aluminum material and thick reinforcement points, you can both strengthen and lighten the enclosure. Hold-down bars to secure the modules into the slots and other fasteners/dampers can be utilized to meet higher levels of shock and vibration. System redundancy is another concern for high-reliability systems. Power redundancy with voltage/health monitoring options can also be implemented.

Having various chassis depth options can help the customer optimize the size and weight in the enclosure. With special mounting provisions and a versatile design methodology this computing platform, for example, can be significantly changed with dual redundant PSUs and the drive bays in the rear via cabling or customization.

Aside from design, another key factor in providing computing platforms for Defense & Government applications is long life cycles. This would be influenced by the selection of a consumer vs embedded motherboard. The consequence of a consumer board is its limited availability over the course of years or decades to deal with support issues. Having to choose a new board means costly requalification of hardware as well as lost time. Not to mention that this might be completely unacceptable for mission critical application. Embedded motherboards come with longer lifecycle support. Revision control is also a consideration as consumer motherboards will change components based on whatever is available and the most cost effective at the time of manufacture. An embedded motherboard can lock the BOM and stock parts based on the life of the project. Offering extended CPU and other component lifecycle support is important for Defense requirements.

Finding the Right Size

In Mil/Aero systems, optimizing SWaP is a critical issue. By having versatile motherboard and module options, a system engineer can find a vast array of options to maximize performance while minimizing space for their design application.

Kenny Kuo is Systems Specialist, DFI Technologies.

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