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Board level design: a question of size

Small form factor boards are not a new invention, but the need to reduce size, weight and power has boosted their demand and resulted in several design considerations.

PC/104, with its 3.4 x 3.6in form factor, revolutionised modular industrial computers almost two decades ago. Since then, the small form factor (SFF) module concept has multiplied into an unwieldy range of small factor boards and so called standards.

Generally SFF boards can be classified in three major groups:
• single board computers (sbc), where all functionality is designed into once board with no further extensions.
• stackable computer modules (de facto, the PC/104 family) which can be configured from a bare sbc to large bus systems with arbitrary expansion options realised by standardized modules.
• computer on modules (COM) that concentrate the main computing functions into a standardised module sitting on top of a standard or custom designed baseboard.

SBCs provide the highest level of vertical integration and lowest production costs, especially in volume. If you can't buy a baseboard off the shelf, you need to create a custom board with the required functions. This takes a significant amount of time and qualified engineers, due to the complexity of today's chips – some with 0.4mm pin pitches.

Stackable computer modules are beneficial because almost anything can be purchased off the shelf. This makes hardware development easy and decreases development time by requiring as little planning, purchasing and stacking as possible. So what are the downsides?

PC/104 systems are great for specific applications that need something simple and robust, as long as the demands for computing power, graphics performance and maximum thermal design power (TDP) are not too high. However, the connectors and system overheads are relatively expensive, and most of the available ecosystem is still ISA based. Since technology has been moving away from ISA and parallel busses over the last two decades, the PC/104 world has fractured into to many subsets, including PC/104, PC/104-plus,
PCI/104, PCI/104-Express, PCIe/104 and SUMIT-ISM. This often leads to a 'mix and match' challenge within the ecosystem. What used to be an unbeatable benefit may now be a burden in some projects and many traditional products and components are 'end of life' and are difficult to replace. Cooling maybe also be a little tricky as TDP is limited.

Atom and AMD G series processors are the first, and often only, choice for midrange fanless applications today. For some applications, like panels, the cubical outline may be a problem and the question remains of whether PCIe/104 or SUMIT-ISM finally make the race and will the ecosystem follow?

As far as standards go, COMs are a lot easier, but only four families are actual standards: ETX; XTX; COM Express; and the relatively new Qseven.

Shared COM benefits
COMs are the easiest (and typically most efficient) way to outsource the 'difficult part' of the latest technology, with ultrafine pin grids and emc sensitive high speed signals. COMs, which come preintegrated to ensure short development cycles, are split between general parts (COM) and application specific parts (baseboard). This enables easy management of scaling and upgrades: switching to the latest computing technology, with less power consumption and higher performance, can be as easy as changing the module. COMs are universal and not bound to a specific application. Because there are lot of boards and a range of applications, there are fewer design flaws and higher product quality.

These advantages make COMs the most efficient way to keep pace with trend to higher complexity, smaller size, less weight and lower power consumption, as well as cheaper products and quicker time to market.

ETX, introduced in 1998, was the first true COM standard. With a board size of 95 x 114mm and a maximum TDP of 40W, ETX became an established standard in 2000. It has wide application in the industrial computing and automation markets and can also be found in medical, transportation and gaming applications. Together with PC/104, ETX is the most significant SFF standard with unlimited ISA support. Typically, the need to support ISA is found in industrial control and automation. Since the end of life of Intel's 855 chipset, ETX COMs mainly feature VIA, Geode or Atom processors, although some recent boards use AMD's G-series processors to upgrade graphics performance.

XTX, introduced in 2005, is a fully compatible facelift for ETX, with native SATA support and four PCIe lanes replacing the ISA bus on the fourth connector. This boosts I/O performance to 2.5Gbit/s per lane. If ETX compatibility is required, but not the ISA bus, XTX is the first choice. It provides a low effort upgrade path from ETX and high-performance processors, as well as a gateway to dual core power. XTX can be used as a form factor compatible transition platform on the way from ISA and PCI technology to serial buses such as PCIe.

COM Express was introduced as a standard by PICMG in 2005, with the aim of developing a universal, scalable and legacy free COM standard. While the original specification put board size at 95 x 125mm for basic boards and 110 x 155mm for extended, a group of COM Express manufacturers has agreed on an additional – and popular – size 95 x 95mm, roughly the size of a PC/104 board. Apart from a new connector design and the size variants, there are some significant features, such as: six PCIe-Lanes; a PCI Express graphics (PEG) port with another 16 PCIe lanes; SDVO interface; on-COM Gbit LAN; and a change to a 12V supply. The maximum TDP was raised to 188W for an extended board, enabling more powerful processors and graphics chipsets to be used.

Qseven, the most recent COM standard, was created to support small sized, low power mobile and ultra mobile applications. The 70 x 70mm board features an inexpensive, but reliable, 230pin edge connector which uses the MXM card slot. While TDP is limited to 12W, the specified supply voltage is 5V, so a mobile device can run efficiently from two lithium cells. Qseven offers no legacy support beyond 32bit PCI and LPC, four PCIe lanes and no PEG. On board graphics support two LVDS ports and SDVO (shared with HDMI/Display Port). CAN, SPI and Card SDIO interfaces ensure maximum flexibility.

Qseven is not an X86 only platform; ARM is also specified and supported with the latest version. A useful new feature is a common software API for industrial applications like watchdog timer, i²c bus, display brightness control, bios storage area and reading of system temperatures. This enables safe and easy exchange of boards from different manufacturers without software adaptation.

Finally, remember that when you decide to use a COM, you still need a baseboard and even the best COM is worthless without good support.

Small form factor checklist
• check your legacy and compatibility needs
• if you need ISA, choose ETX
• if you need ETX compatibility, but not ISA, choose XTX
• for a new, legacy free project, consider COM Express or Qseven
• If your TDP exceeds 12W, choose COM Express
• If you need a PEG port to use an external graphics card, choose COM Express
• handheld, mobile or ultra mobile devices suit Qseven
•If you are considering a non X86 platform, consider Qseven
• For batteries or Power over Ethernet, consider Qseven
• If size matters, consider Qseven

Author profile:
Martin Danzer is head of research and development for congatec.

Martin Danzer

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