Embedded Open Modular Architecture

From eLinux.org
Jump to: navigation, search


With recent embedded processors becoming mainstream and powerful enough for general-purpose computing, the Embedded Open Modular Architecture is an initiative to create robust, reliable and interoperable hardware standards for mass-volume systems based around embedded processors, where average users can interchange system modules (containing processor, RAM and storage), even several times a day, without risk of damage, needing any technical knowledge or requiring a technician to assist them.

Products based on EOMA standards should, when sold, be so simple that any salesman can honestly say "Just plug it in: it will work", and where anyone from a small child to an elderly person may be confident in the day-to-day installation, removal and use of EOMA modules in the Electronic Appliances that they own. By complete contrast to existing Industry Standards, there does not exist even a single published open standard which can claim that it is easy for the general public to work with. To explain this puzzling statement further: all other standards require either special technical knowledge, special technical skills, special handling of the device so as not to damage it, and often tools are required. EOMA Standards are designed to require none of these things.

The first initiative is to re-use the old PCMCIA form-factor, in a similar way to Conditional Access Modules. Below, various alternative interfaces are analysed, and thus explain, given the requirements, why PCMCIA was chosen as the first preferred modular format, despite the greatly-reduced pin-count (only 68 pins).

Interface Types

The re-use of industry-standard connectors and sockets is a very common practice, but usually it is for embedded or factory-installation purposes only. Typically in the Embedded world, SO-DIMM is the form-factor of choice to re-purpose, because CPU, RAM, NAND Flash and even some micro-connectors can fit onto the 35mm x 68mm size. Other suitable form-factors include MXM and PCMCIA (CardBus) and potentially ExpressCard, although the limited pincount of ExpressCard makes it much less attractive. Cogcomp have defined their own standard, and have achieved significant backwards-interoperability over a very wide range of CPU types - however this is not re-use of a de-facto standard, but is included for completeness.

Mini PCI

Mini PCI (not to be confused with Mini PCIe) has 3 variants, one of which is a 124-pin standard. Pictures have been seen of removable Mini PCI cards and ejector assemblies (similar to PCMCIA but wider). Tracking down the supplier proved impossible: Mini PCI never really took off into a mass-produced standard. If it still existed, the removable Mini PCI card form-factor would be perfect, because of the extra pins available (124) and the rugged nature of the socket and housing that was seen (once).

SO-DIMM form-factor

This form-factor is suitable for factory-only installation. The 35mm x 68mm size also makes it very tight to fit more than the CPU (up to 25mm sq), NAND Flash (up to 20mm) and more than two RAM ICs (appx 20mmsq each), as well as PMICs. However, some companies such as Gumstix have created a de-facto modular standard for their own products such as the Overo range, using the more expensive POP RAM to make space even for on-board WIFI SIP modules. The majority of SO-DIMM form-factor modules, such as Magniel's OMAPMOD, are double-sided.

All of these options are fantastic for prototyping and for the low-to-mid-range volumes, especially for Hardware enthusiasts and for Industrial purposes, where such modules form-factors are quite common. However, when it comes to placing designs based around SO-DIMM and other similar form-factors of this size into mass-volume production, the space-saving decisions that had to be made immediately become a problem. Many Chinese Factories are simply not equipped to handle the higher number of layers required, nor the smaller PCB track sizes, and simply do not have access to the plasma-based equipment needed to solder Package-on-Package ICs.

PCMCIA form-factor

Use of the PCMCIA 85mm x 55mm form-factor for purposes other than the CardBus standard is not without precedent: [Conditional Access Modules] do not comply with the CardBus electrical and electronic specification. The size and design however is highly suited to hot-swapping and provides convenient carrying and storage. It therefore makes sense to put the entire embedded computer onto the card. The size is large enough to fit CPU (up to 25mm), up to 4 RAM ICs (typically 20mm sq), a NAND Flash IC (up to 20mm) as well as PMICs and other components - even single-sided, thus reducing the cost.

The specification is at Embedded_Open_Modular_Architecture/EOMA-68.

ExpressCard form-factor

The idea to re-use ExpressCard is based on creating something with a smaller form-factor than PCMCIA that resembles the popular "USB-TV" dongles, making it possible to use the exact same processors and board layout in USB-TV dongles, and providing the option (through a little more space) to have a few more outputs or peripherals.

The specification is at Embedded_Open_Modular_Architecture/EOMA-26.

Compact Flash form-factor

Compact Flash Type II Cards are 43mm x 36mm x 5mm, and the connector is 50 pins. There is therefore just enough space to fit a CPU, RAM and NAND ICs. There exists precedent for such tiny processor boards, in the form of the [Enyxos4210 CPU Board] from hardkernel.com. In the case of the Enyxos4210, a Samsung Dual-Core 1ghz ARM Cortex A9 is used, with POP DDR RAM. Unlike the hardkernel CPU board, however, the Compact Flash form-factor would allow the use of the bottom of the board for ICs instead of B2B connectors, because the Compact Flash connector is on the edge. A typical layout would therefore involve CPU, PMIC and one other (small!) IC on the TOP side, and NAND Flash and possibly 2 low-cost DDR RAM ICs on the BOTTOM side.

The only distinct disadvantage of the reuse of Compact Flash is that it is sufficiently small that re-use of existing casework for CF is not really possible. With PCMCIA, existing cases can be purchased off-the-shelf, leaving just the moulding at the end as the only customised casework required. Additionally, the smaller size may require high-density parts for high-specification devices, increasing the price. However for ultra-low-cost cards, the lower part count would not be a problem.

The specification is being developed at Embedded_Open_Modular_Architecture/CompactFlash.

Custom 200-pin B2B Module

A custom module form-factor is being designed with a housing of 103mm x 68mm x 15mm in size. At each of the 4 corners is a 0.4mm pitch 50-pin B2B connector. On the front fascia plate is the option to put any number of connectors that will fit along the long side. Power is provided in the form of multiple 5V and 3.3V pins, providing a total of up to 32.4 Watts.

This standard is being targetted at end-users for Desktop, SoHo and other mass-volume retail purposes, with the ultimate goal being to reduce the hardware cost for the mid-end Server markets as well as for Industrial, Embedded, Research and Educational purposes by providing these market sectors with access to low-cost off-the-shelf mass-volume high-end PCs as an Open Standardised Modular component with plenty of expansion capability both on and off the module.

The advantages of EOMA-200 over alternatives such as Q-Seven and COM-Express is that there are no optional sizes and no optional interfaces. End-users and Industrial customers can purchase EOMA-200 modules knowing that whatever the price, the only real considerations are whether it has enough on-board interfaces, whether the CPU speed is fast enough and how much memory it can take. Both Q-Seven and COM-Express are fraught with additional technical considerations such as whether the module is the right size and whether the module has the right interface compatibility with the base-board (many of Q-Seven's interfaces are entirely optional and can be entirely missing). As such, Q-Seven and COM-Express are effectively limited to costly Industrial, Embedded and Military purposes; by contrast, EOMA-200 is targetted primarily at the mass-volume sector, to the ultimate financial benefit of other markets as well.

Like all EOMA Standards there are no optional interfaces on EOMA-200.

The specification is being developed at Embedded_Open_Modular_Architecture/EOMA-200.

Alternative Standards

This section covers alternative standards for modular computing.


This is included as a fascinating historical glimpse into the world of computing. It appears to be an early precursor to the XT/AT bus. More information is available on wikipedia at https://en.wikipedia.org/wiki/S-100_bus


Q7 http://www.qseven-standard.org/index.php?id=43 is an embedded 70mm x 70mm standard (also expanded to uQseven which is a 40mm x 70mm size with the same connector format). The Qseven standard is large enough and powerful enough to house x86 CPUs as well as embedded ARM SoCs. However, like the other edge-connector-based standards, it is not suitable for mass-volume user-installable usage, being only suitable for factory-installation due to the risk of ESD damage. Also, as it is a larger form-factor, it rules out usage for smaller devices (it is almost 400% bigger than Compact Flash, for example). Also, the Q7 standard sets quite a high bar for the required interfaces, including PCI-e, MIPI and others, all of which must be placed on the edge-connector, thus requiring some 200+ pins.

Also, due to interfaces being optional (see "Feature Overview" of specification PDF document), fragmentation will occur on both sides of the standard. Special care and attention will be needed by ODMs to ensure that they pick the right module before proceeding to the design stage.

The standard is here: http://www.qseven-standard.org/index.php?id=43. It is mentioned here for completeness.


PC104 and other such small-form-factor embedded boards are based around the assumption that the architecture is based around the x86 PC. In fact, PC104 is a compact form of the legacy PC/AT bus (even PCMCIA is based around the ISA bus). PC-104 is mentioned here for completeness, because due to its age it can be considered as a legacy interface, where the pins, connectors, headers and PCB sizes could potentially be re-used for alternative purposes. The problem with that is that PC104 was designed for Industrial and other embedded purposes, and was never a mass-produced interface in the first place, but is mentioned here merely for contrast and for completeness.


EDM is a new small-form-factor embedded board standard, with sizes of 82 x 60 mm, 82 x 95 mm and 82 x 145 mm. The standard, which is released under a Creative Commons License, uses a 314 pin connector which carries all signals ranging from HDMI to gigabit ethernet to control pins such as GPIO's, I2C, SPI and serial interfaces.

EDM is very similar to the Q-Seven Standard, and as such has the same limitations, including being restricted to factory-installation only (unsuitable for installation by the average non-technical end-user). Additionally, the fact that many of its interfaces are optional means that the marketplace will automatically become fragmented on both sides of the standard.


ULP-COM is a new small-form-factor embedded board standard, with sizes of 82 x 80mm and 82 x 50mm, and a maximum height limit of 4mm. It re-uses the x86-based MXM form-factor and sockets, just as EDM does.

ULP-COM includes many industrial interfaces (SPI, 4x CAN, 12x Serial) as well as requiring 3 PCIe x1 ports, and offering Single-channel LVDS, 24-pin RGB/TTL, and HDMI (with an option for DisplayPort), 2 CSI Camera Interfaces (one 2-lane, one 4-lane). This basically raises the bar to an extremely high level, eliminating many low-cost ARM SoCs from participating with the standard, whilst still leaving it highly suitable for many Industrial and other Factory-install purposes. It should become more suitable as a general-purpose computing standard once more SoCs have PCIe, CSI and so on.

However, like EDM, the inclusion of optional (multiplexed) functionality leaves some concern over the viability of the standard: should motherboards support both alternative options? What if one module does not support the alternative function that is required by a motherboard? In the cases where the Module is to be effectively permanently installed in its motherboard, this question is not of such concern, but then why use that standard at all; why not just make a Single Board computer (for large volumes, at least). This issue applies equally across all of the edge-connector-style Modules.

By contrast - and also learning from the lesson of ULP-COM, the multiplexing on EOMA standards is restricted firstly to Digital I/O pins, secondly to I/O that is the same TTL voltage level (so that there is no issue over voltage-level conversion) and thirdly to being two functions only: General-purpose bi-directional I/O and one other function. The reason for this is quite simple: even if a designer creates a module based around a CPU which does not support GPIO / function multiplexing (as is common within the embedded SoC industry), a two-way multiplexing IC could conceivably be used as a last resort to perform simple multiplexing externally. Contrast this with a situation where pins are used for example for high-speed LVDS or MIPI and other different functions with different speeds and voltages, and the complexity (and cost) of external multiplexing goes up often to the extent where the total BOM makes the use of that particular SoC cost-ineffective and thus completely pointless.


96boards is another small-form-factor standard, which has released its first standard as a consumer-grade board. Variants are either 85 x 54 x 12mm high or 85 x 100mm x 12mm. The specification is unusual when compared to other hardware standards in that it also sets minimum software support requirements.

Just like virtually every single one of the alternative standards reviewed on this page, the 96boards consumer standard contains "optional" features that make it extremely hard for average end-users to pick a board, when there is no guarantee in the future that alternative upgraded boards will have the same availble functionality. Additionally, the choice of a mandatory 1.8v GPIO voltage level makes for additional cost on both the board design side as well as mezzanine connector side, unless by pure chance the parts available on either side of the standard happen to be available at exactly 1.8v voltage levels.

Additionally, at the time of release (January 2015), despite the standard being a 1.0 release, it is an extremely unclear document where many significant and important factors are left undocumented and unspecified.


M2.com is an extremely well-written, extremely simple and extremely clear "nothing is optional" arduino-style standard that has clearly had a lot of thought put into it by extremely competent electrical engineers. Just like EOMA standards, any multi-functionality pins offered are to the most absolute basic choice: one single clearly-defined function multiplexed with simple GPIO. Nothing in the standard implies mutually-exclusively incompatible choices or (unlike 96boards) imposes hardware design criteria onto processor manufacturers in order to comply with the standard: everything is nailed down and very straightforward. The only exception to this rule is that UART is multiplexed with CANbus. On a high-speed interface such as SPI or SD/MMC, ordinarily this complex double-functionality could potentially be a severe design flaw, however both CAN and UART are sufficiently low-speed (even when ultra-low-power embedded processors are used) that bit-banging of either UART or CAN could be considered, should any one processor simply not have both functions available to multiplex onto the same pins. All digital signal levels are CMOS, with voltage tolerances clearly and unambiguously specified as either 1.8v or 3.3v. Strangely there is no VREF, presumably because VCC is extremely low (and in effect implicitly acts as VREF?). The key differences between EOMA68 and M2.com are that EOMA68 does not have any ADC pins, and M2.com does not have any kind of video output. Current EOMA standards are targetted at general-purpose computing, whilst M2.com is definitely targetted towards the ultra-low-power market that has recently been bandwagon-rebranded with the title "Internet of Things" but is in essence a mass-commercialisation of the concepts first brought to mainstream attention with the "Arduino" series of controller boards. Even though it is in no way a competitor to current EOMA standards due to the completely different target market, M2.com is included here for reference and completeness due to the extremely competent way in which the standard is presented. M2.com is therefore worthwhile reading and studying for this reason alone.

Contact and Discussion

For questions, comments and general discussion, please use arm-netbook mailing list. Further information is available at http://rhombus-tech.net.