Embedded Open Modular Architecture/EOMA68



= EOMA-68 Introduction =

The purpose of the EOMA68 specification is to bring simple robust mass-produced CPU Cards to end-users. To make end-users lives easier, purchasing decision-making should be made not on technical interface capabilities, neither should they be expected to have significant technological expertise. This is the primary reason why EOMA specifications have no optional interfaces of any kind. For this reason, for the lifetime of the specification (anticipated to be at least a decade) all CPU Cards compliant with the EOMA68 specification will be compatible with all compliant "Chassis" (Note: "chassis" is the term used to describe a product into which a CPU Card is plugged).

The trade-off between choosing a single-board design or even another modular form-factor is as follows:


 * EOMA-68 products will not use all of the integrated functions of single-board designs, automatically resulting in minor cost increases for products.
 * However, single-board designs are typically throw-away products where the lifetime of the product is critically dependent on an extremely fast-changing market.
 * Single-board designs are typically throw-away hermetically-sealed products with neither user-serviceable nor user-replaceable parts
 * EOMA-68 products are user-upgradeable. The Chassis can be kept out of landfill - kept in useful service - for the lifetime of its components.  Only the CPU Card need be upgraded at a much lower cost than a single-board design in order to continuously give the product a new lease of life.
 * EOMA-68 CPU Cards can be shared by the same end-user across multiple products, automatically resulting in a cost saving that far outweighs the minor overhead of a single EOMA68 system when compared to a single hermetically-sealed throw-away product.
 * Old Chassis and CPU Cards can be re-purposed instead of discarded as e-waste.
 * Q-Seven and other similar standards are not realistically user-upgradeable (not for the average person) because products require tools, technical knowledge in the selection of replacement parts, and expert knowledge in the handling of electronics before opening the case.
 * EOMA68 products are upgraded by pushing a button and popping out the module: it literally takes seconds to install a new CPU Card.

So the benefits for end-users are very clear: EOMA-68 is easily understandable long-term investment for its end-users with significant long-term cost savings and reductions in e-waste. The benefits for factories are very clear: CPU Cards aggregated across multiple products means much better bulk purchasing power, and Chassis can also continue to be produced without requiring redesigns pretty much until the components go end-of-life.

No other modular computing standard available today has been designed with these aims in mind.

= EOMA-68 Specification =

This page describes the specification of EOMA-68. The number of pins on the interface is 68; the physical form-factor is the legacy PCMCIA.

Re-purposing of the PCMCIA interface and form-factor has been chosen to create portable mass-volume (100 million units and above) Embedded Computing Modules (Computer on Module). Mass-volume "Lowest Common Denominator" interfaces have been chosen, all of which have existed for over a decade, but are low-power enough to be standard across virtually all mass-produced powerful Embedded CPUs.

The interfaces are:
 * 24-pin RGB/TTL (for LCD Panels and DVI/VGA/HDMI or other display conversion ICs)
 * I2C
 * 1st USB (Low Speed, Full Speed, optionally Hi Speed/480 Mbit/s and optionally USB3.0 or USB3.1)
 * 2nd USB (Low Speed, Full Speed, optionally Hi Speed/480 Mbit/s)
 * 8 pins of General-purpose Digital I/O (GPIO) with multiplexing to SD/MMC and SPI on 6 pins
 * 4 pins "External Interrupt" capable GPIO that are guaranteed to generate a fast hardware interrupt to the SoC
 * SD/MMC (and down-level compatibility to SPI) multiplexed with 6 of the GPIO pins.
 * TTL-compatible UART (Tx and Rx only).

These interfaces are NOT OPTIONAL for CPU Cards. All CPU Cards MUST provide all interfaces (at some level of capability). I/O Boards on the other hand are free to implement whichever interfaces are required for the device. For example: whilst all CPU Cards must have a 2nd USB interface, devices such as tablets or laptops into which CPU Cards are plugged are not required to use it. The only exception is I2C (due to the EOMA-68 identification EEPROM): it is mandatory for all I/O Boards to provide an EOMA-68 identification EEPROM.

Exactly like legacy PCMCIA Cards, EOMA-68 CPU Cards may have absolutely any functions, any additional connectors, peripherals and so on without limitation, except for compliance with the EOMA-68 pinouts and physical size constraints. These additional functions, which may include access ports in the casework, may extend outwards from the user-facing end of the CPU Card to any practical extent, exactly as with legacy PCMCIA.

Background to Interface Selection
The interfaces have been specifically chosen because they are either essential or they are multi-purpose buses, and surprisingly they are perfectly adequate despite being Lowest Common Denominator across a wide range of CPUs for at least a decade. The goal here is not to attain ultra-high-speed latest-and-greatest performance but to use proven, long-established interfaces that will be easy to find parts for mass-volume appliances in potentially hundreds of millions of units and above.

Also, some graceful degradation through negotiation at the hardware level is not only desirable but is an essential distinctive and unique feature of the EOMA68 standard, because it dramatically simplifies the "sales pitch" as well as the engineering design, user card selection ("just get one of these, plug it into any product, it will work") and so on whilst at the same time ensuring that the range of SoCs that can be used is significantly diverse and future-proof.


 * I2C - I2C is only two wires, is a global bus that can address multiple devices, and is a long-established proven Industry Standard with thousands of devices available.
 * USB - USB2 is only two wires; USB3.0 is six, USB3.1 is ten. USB, like I2C, is a global bus that can address multiple devices and is a long-established proven Industry Standard.
 * RGB/TTL - 24-pin RGB/TTL was chosen over LVDS or MIPI so as to keep the cost down, and also to keep the signal speed down. Whilst LVDS seems initially to be a good candidate, Single-Channel LVDS is unsuitable for driving 1,920×1,080p60 LCD Panels: most 1,920×1,080 LCD panels require between 2 and 3 LVDS drivers.  MIPI also requires multiple parallel channels to achieve higher data rates.  Any low-cost CPU chosen which did not have LVDS or MIPI would be forced to add a converter chip, potentially on both sides of the interface (CPU card as well as motherboard).  So it makes sense to choose the proven, lower-speed, reliable 24-pin interface, thus making the EOMA-68 Standard suitable for use even with ultra-low-cost 320×240 RGB/TTL LCD Panels, right the way up to HDTV screen sizes.
 * SD/MMC - SD/MMC has a 4-pin, 2-pin, 1-pin and SPI mode. Transfer speed negotiation is possible at the hardware level.  SPI can even be implemented as "bitbanging"

Requirements for USB
All CPU Cards are required to support the full backwards-compatible auto-negotiation USB device capabilities and speeds of all former versions of the USB Interface, up to the maximum speed and capabilities chosen to be provided. Specifically:


 * Providing USB Low Speed (version 1.0 - 1.5 Mbit/s) is acceptable.
 * Providing USB Full Speed (version 1.1 - 12 Mbit/s) is acceptable if Low Speed is also provided.
 * Providing USB Hi Speed (version 2.0 - 480 Mbit/s) is acceptable if Full Speed and Low Speed are also provided.
 * Over the 1st USB port, providing USB Super Speed (version 3.0 - 5 Gbit/s) is acceptable if all lower speeds are also provided.
 * Over the 1st USB port, providing USB Super Speed (version 3.1 - 10 Gbit/s) is acceptable if all lower speeds are also provided.
 * Providing a higher version only and supporting no lower speeds is not acceptable.

To emphasise: providing no USB 3.0 or USB 3.1 over the 1st USB port is acceptable as long as USB2.0, USB1.1 or USB1.0 (and downwards-compatibility) is provided.

Chassis (devices) must support up to a maximum chosen USB specification and all speeds below. This guarantees that any CPU Card will work with any device, with any combination auto-negotiating to the maximum possible speed.

As a counter-example of a SoC that is NOT acceptable, the OMAP 3500 series provides USB 2.0 Hi-speed (480mbit/s) ONLY. Use of this SoC is NOT acceptable.

Requirements for RGB/TTL
The RGB/TTL output is the one point where close attention has to be paid on the part of the CPU Card designers, because of the variance between devices in which the CPU Cards will be plugged. This will need careful monitoring and may warrant a "Certification Programme" to ensure that CPU Cards are compliant with a wide range of devices.


 * RGB/TTL is a parallel data bus, potentially running at up to 125 or even 150mhz. To ensure that the parallel signals are not skewed, both CPU Cards and I/O Boards MUST ensure that the length of the RGB/TTL tracks (data, HSYNC, VSYNC, CLK and DE) leading to the 68-pin connector - on either side of the 68-pin connector - are all of equal length.  It is recommended that both the source (e.g the CPU) and the sink (e.g an LVDS IC) are placed as close to the 68-pin connector as possible.
 * CPU Cards must provide software-programmable support for anywhere between 190x120 RGB-TTL resolutions all the way up to the maximum that they are capable of, with the maximum resolution being clearly marked on both the CPU Card, as well as the retail packaging in which it is sold.
 * CPU Cards should support up to at least 1920x1080 at at least 50fps. However, some ultra-low-cost SoCs, especially those designed for mobile devices, only support up to XGA or WXGA resolutions.  The use of such SoCs is not entirely recommended.
 * EOMA-68's RGB/TTL interface is 24-bit-wide. If a particular SoC only has e.g. 18-bit or 15-bit RGB/TTL then the LSB (lower) bits MUST be set to logic output level 0 when the LCD interface is enabled: they must NOT be left floating or tri-state.  This ensures that devices which are expecting the full 24-bits do not receive noise on the lower bits of each of the R,G and B 8-bit inputs.

Although there is no reason why individual devices should not have more than one LCD screen, allowing them to be selected, the burden of complexity for screen selection is placed onto the CPU Card software, so any company planning such a multi-screen device should contact the authors of the EOMA-68 specification (lkcl@qimod.com). Realistically, multi-screen devices should consider instead using USB-based screen driver technology such as that from DisplayLink, or place any number of additional Display outputs onto the user-facing end of the CPU Card (most CPU Cards will at least have a Micro-HDMI output).

Requirements for I2C
These are the requirements for provision of I2C on an EOMA-68 interface. The summary is that the I2C bus must not be shared with any peripherals on the CPU Card, and the CPU Card must be able to read an on-board EEPROM (at address 0x51).


 * The I2C bus that is connected to the EOMA-68 interface will expect to have access to an EEPROM that is addressable (readable) at I2C address 0x51.
 * Additionally, there MUST NOT be any devices on the I2C bus on the CPU Card side. The reason is that all other addresses (other than 0x51) must be available for peripherals on the I/O Board.
 * If a CPU Card needs to connect internally to any I2C peripherals on the PCB inside the CPU Card, the CPU Card MUST use a completely separate I2C bus (internally), NOT the one that is connected to the EOMA-68 Interface. i.e. the I2C bus that is connected to the EOMA-68 interface MUST remain completely dedicated to EOMA-68, and MUST NOT be shared with any peripherals on the CPU Card itself.
 * The EEPROM MUST NOT be used for the storage of user data: it is reserved exclusively for EOMA-68.
 * The EEPROM MUST be capable of operating at anywhere from 1.8v to 5.5v, or, alternatively, bi-directional auto-detecting level-shifting MUST be performed on the I2C signals. Generally it is easier to supply the EEPROM with the variable-level voltage VREFTTL.

Please note that there is considerable confusion over the definition of addresses in I2C. The discussion page has some clarification over what consititutes an address (7-bit) and what goes into the first 8 bits (7-bit address plus 1 bit indicating read or write). Adding to the confusion it is extremely common to find datasheets even from respectable companies that directly contradict the I2C specification.

Below is an example circuit showing an AT24C64 with the address set appropriately to 0x51. PLEASE NOTE that the AT24C64 datasheet INCORRECTLY misleads people to believe that the addresses are 0xA2 (for read) and 0xA3 (for write). The address for both read and write is 0x51 (in the top 7 MSBs) with bit 0 (LSB) indicating read (0) or write (1).



Requirements for UART
Strange as it may seem to have requirements for UART this section covers practical issues regarding protection of CPU Cards. When designing I/O Boards it is important to take into consideration that many embedded SoCs do not have proper UART buffering. Typically if the SoC is powered down but the I/O Board continues to be powered up such that it continues to provide a positive voltage to the UART "RX" line this can potentially result in power leakage through the SoC and on to other areas of the PCB. It is therefore critical that I/O Boards ensure that this does not happen.

As this problem is to be taken care of on the I/O Board it is worth observing that CPU Cards do not require UART buffering. They may however require level shifting: the signal levels are, like all other Digital I/O in EOMA68, expected to be relative to VREFTTL.

Below is an example circuit which can be used to protect the UART-RX line, using MOSFETs.



Here is another example that uses schottky diodes. D1 is to reduce the voltage slightly so that it will be below the 3.3v level that is internally supplied to the CPU Card. The same effect could reasonably be achieved using a resistor-divider bridge.



There are also other options such as the use of a MAX2322 RS232 buffer IC. Other options can be found here.

Requirements for SD/MMC and SPI
SD/MMC is a little strange in that it has hardware backwards-compatibility down to SPI in most controllers. However, recently the SD/MMC standard (version 4) has excluded support for SPI. The decision is therefore to "go with the flow" of the SD/MMC standard, but with the stipulation that there is to be *NO* expectation of explicit hardware-accelerated SPI support from the SD/MMC pins. To be absolutely clear: base boards *MUST NOT* expect there to be hardware SPI support from the SD/MMC pins (but that "if pushed", there is a way in software to provide SPI support even if the hardware controller does not provide it).

For where SPI is explicitly needed, there are two available options:


 * An additional and separate SPI interface is provided, on different EOMA68 pins. Use of these pins as precedence is recommended.
 * Even if a hardware SD/MMC controller does not support SPI it is still possible to emulate SPI using "bitbanging". As bit-banging is quite CPU-intensive, and the transfer speed of SPI is 25mhz, designers of CPU Cards as well as designers of I/O Boards need to take this into consideration.

In essence, the SD/MMC committee have caused a bit of trouble, here, but it may be best to trust their experience in that SD/MMC Cards have probably not, for some considerable time, been actually using SPI mode, but have been offering the 2, 4 (and now 8) lane capability for a long, long time. Relative (at the time of writing) to the 80 mega-BYTES per second of the latest SD/MMC 4.0 Ultra-speed cards, 25 mega-BITS per second backwards-compatibility - some 25 times slower - would seem ridiculous to continue to maintain.

On this basis, the SD/MMC interface is therefore offered as primarily being to access SD/MMC devices (cards and peripherals), *NOT* for the explicit primary purpose of accessing SPI devices. However, if SPI hardware interoperability happens to be available, it *MAY* be used, and for all other instances bit-banging *MUST* be provided in software. Board designers *SHOULD* take into account that any SPI interoperability is *LIKELY* to be implemented as bit-banging, and should, as a result, avoid using it except as an absolute last resort, prioritising the dedicated SPI interface available on completely different pins, instead.

CPU Card designers must also take into account that both the SD/MMC and the SPI interfaces are also multiplexed onto bi-directional GPIO. As most SoCs provide exactly this kind of multiplexing by default, the requirement to offer both bi-directional GPIO as well as the other functions is not technically burdensome.

= Pinouts (version 1.0) =

These pinouts make no attempt to be electrically or electronically compatible with the legacy PCMCIA standard. 1 PWM GPIO, 4 explicitly EINT-capable GPIO pins, 4 dedicated GPIO pins, additional GPIO multiplexing pins on other functions, 18-pin RGB/TTL, USB2, USB3.1, I2C, SPI, SD/MMC, UART are included in the Version 1.0 specification.

Four 5.0 V power inputs must be provided: all pins are rated at 0.5 A, so the maximum power dissipation is limited to 10 watts. ''Design consideration: please note that to ensure that thermal dissipation in an enclosed fanless situation is not exceeded, a maximum of 3.5 watts should be respected, or the card must contain its own fan (not recommended). Most systems will not have active cooling.''

All High-speed signals (USB2, USB3) are balanced lines that are still separated using GND or Power pins. All other pins are low frequency, with the exception of the LCD Pixel Clock and Pixel Data pins, which could go as high as 125 MHz for 1,920×1,080 @ 60fps (not recommended). The GPIO pins are available, for general-purpose bi-directional use of digital data only.

The output from the 24-pin LCD RGB/TTL pins must be electrically compatible with a Texas Instruments SN75LVDS83B, where VREFTTL is supplied to IOVCC.

All GPIO should be initialised at start-up as tri-state isolated, or as high-impedance (48kOhm) inputs so as not to interfere with carrier boards. Voltage levels for all GPIO are relative to VREFTTL, and follow CMOS level rules: above 0.7 times VREFTTL for a digital "1", and below 0.3 times VREFTTL for a digital "0". Digital input voltage levels *MUST NOT* exceed VREFTTL, at any time. When the CPU Card is powered off (i.e. when VREFTTL happens to drop to 0V), all Digital IO *MUST* also be powered off.

The option for a CPU Card to provide USB3.0 or USB 3.1 is also available, if a given system has it. If, however, a particular system does not have USB3, the pins must not be used for other purposes, and must be left unconnected (floating). Additionally, I/O Boards must not use the unused pins for any other purpose and must leave them unconnected (floating). This is to ensure that automatic down-negotiation of USB2 occurs correctly and that damage does not occur to USB3-capable CPU Cards when plugged into I/O Boards with only USB2 capability.

Pin 43 is used for Card-specific / implementation-specific boot, power-up or reset purposes. It is to be connected to an external switch that pulls directly down to ground to indicate "boot / power-up / reset", and otherwise is floating. The length of time or the number of times that the switch is pressed is entirely implementation-specific for any given Card, and is entirely unlimited and unrestricted in scope by this specification. Potential examples which are not the full scope of possibilities include "brief press" for "bring up an on-screen shutdown dialog" or "press and hold" for "emergency power-off" and "press and hold to power on" and "press to bring out of sleep mode".

Note also: for factory-install purposes, cards are of course permitted to use all and any pins, ports or methods required to carry out factory-installs and testing, as long as after factory-install the 68 pins are entirely EOMA-68 compliant. Examples of such uses would include a test-bench with an SD/MMC interface for first firmware boot, a JTAG interface and other diagnostics.

Table of EOMA-68 pinouts
= Start-up procedure =

It is required that all pins be disabled (floating tri-state) with the exception of the I2C Bus, the 5.0v Power and the Ground Pins. I2C Bus Master is then enabled, to search for an I2C EEPROM at address 0xA2. This EEPROM contains Linux Kernel "Device Tree" data, which specifies the devices available on the motherboard, as well as the actual pin-outs. The exact format of the EEPROM data is yet to be decided.

One important aspect of reading the I2C EEPROM is that the CPU card can then correctly access and initialise on-board devices. It also defines the purpose and use of the GPIO pins (if any are required). Also, the format of the LCD data is specified. For example, the pinout diagram on this page assumes 24-pin RGB TTL, but some motherboards may have LCD panels which only have an 18-pin RGB/TTL interface. The data in the I2C EEPROM therefore provides clear specifications on all the motherboard's peripherals.

Discussion of the startup procedure is here on arm-netbooks

= Future Versions =

All LCD and GPIO pins must be tri-state floating in order that future versions of this standard can provide faster (or merely alternative) interfaces. At the time of writing (2011), the interfaces in the 1.0 Specification are "Lowest Common Denominator" yet are still present across the majority of 2011's powerful embedded SoCs (OMAP4440, Enyxos4210, Tegra 3, iMX53, iMX6, Allwinner A10, A20 etc.) However, in the future, the "Lowest Common Denominator" could well comprise MIPI instead of RGB/TTL, 2 lane PCI-express (or its successor), and USB-3 instead of USB-2 (perhaps even a faster version of ULPI).

As of 2011 however, the total number of Embedded CPUs supporting all these newer interfaces and still keeping to a 1.5 watt budget is precisely zero. Support for these high-speed interfaces will therefore be re-evaluated in 2 to 3 years time, and a future version of this standard created when a large proportion of available embedded CPUs have these or other high-speed interfaces that are available at the time.

= Deliberate Mechanical Non-interoperability =

The re-use of the PCMCIA standard pinouts with no electrical or electronic compatibility requires mechanical means to ensure that newer cards cannot be inserted into legacy sockets. The proposed solution is therefore to deploy a fascia plate on the EOMA-68 card that is both larger in width than the standard 55 mm as well as recessed by some 8 mm, along the length of one of the 85 mm edges. The exact dimensions are yet to be determined, as specific PCMCIA housings need to be examined to ensure that one side can take the recessed "edge stop". The following part, PCMCIA Ejector Assembly from Tyco Electronics, is ideally suited: slimline and nothing at the left side.

= Physical Dimensions =

There are two sets of acceptable dimensions: as with the legacy PCMCIA interface, these must be backwards-compatible.

Type II
The physical dimensions are a maximum of "Type II" (i.e. 5mm maximum height). Cards should typically have all user-facing connectors "flush" with the standard PCMCIA size. This will ensure that when a Card is inserted into a device, the connectors of the CPU Card appear to be part of the actual device. However: devices should cater for the possibility that an EOMA-68 Card may have connectors sticking out of the end, to any practical height.

As the EOMA-68 pinouts have been designed to avoid matching the power lines of PCMCIA, there is no need for mechanical blocking.

Type III
Type III Cards have a maximum height of 8mm: this is typically reserved for x86-based CPUs which require up to 10 watts to operate. Motherboards that take the Type III cards must also accept the Type II lower-power cards.

TBC: Type III Cards should not assume that there will be fans available in the devices in which the cards are inserted, and should make arrangements for the removal of heat.

= Thermal Considerations =

In order to reduce the cost of Motherboards and system design, Type II Cards should not exceed an average of 3.5 watts power consumption for prolonged periods of time, despite there being provision for up to 10 watts on the EOMA-68 connector.

CPU Cards and Motherboards that support the Type III 8mm-high cards must be designed with a Thermal Dissipation capability that takes the 10 watt TDP into consideration, as well as taking into consideration the power consumption and heat generation of the devices used on the Motherboard as well. Whilst fan-based solutions are acceptable, the use of thermally-conductive copolymer plastics (some of which have thermal dissipation capabilities exceeding that of Aluminium) are recommended.

= Header Connectors =

Within the physical dimensions, there is absolutely no restriction on the number of connectors, interfaces, headers, expansion headers or antenna protruding from the end of the device. For example: a PCMCIA CPU card may typically have, for best useability, a Micro-HDMI, a USB-OTG, a 5-pin Audio Jack and a Micro-SD Card Slot. These four interfaces fit neatly within the 54 mm × 5.5 mm fascia plate size limit, as long as mid-mount connectors are used.

Also, on the actual EOMA-68 CPU Card PCB itself, there is no restriction on the number of expansion headers (populated or unpopulated) - the only consideration being that the EOMA-68 CPU card clearly cannot have expansion headers except for Engineers and Embedded Device Designers, and also have a metal shield installed around the EOMA-68 CPU card at the same time. However, there is no reason why the expansion headers should be unpopulated, supplied without a metal shield to Embedded Engineers, yet the exact same device shipped in mass-volume with a metal shield installed, for the average user.

The only issue to note is that there is a maximum power budget of about 10 watts (although there are four 5.0V 0.5A pins) but also that there is a practical maximum power dissipation of EOMA-68 cards of about 4 watts. There is no provision in the standard for air-cooling (fans) in the cases: most devices will be a passive-cooled layout.

If however the EOMA-68 card is designed to operate "stand-alone", for example by being provided with a Power Connector on its user-facing edge or by making use of USB-OTG, then of course the designers are free to disregard these constraints. If however the CPU card is also expected to operate inside a conformant device, then it must adjust accordingly and stick within the 4 watt heat dissipation budget.

= Example Motherboards =

Here is a list of example designs which conform the EOMA-68 Standard:


 * Mini Engineering Board - suitable for Free Software Developers, ODM Development, SoC "Board Support Packages", Experimentation, Prototyping, Electrical Engineers, Training and R&D purposes.
 * Monster Engineering Board - suitable for ODM Development "Demonstration" Purposes: designed to be "cut down to size", requiring the minimum amount of CAD/CAM Development effort and maximising return on investment.
 * The Obligatory Tablet - a simple tablet motherboard which could potentially be developed as a very low cost single-sided 2-layer PCB. Components are chosen to reduce development cost and risk, as well as reduce manufacturing cost.
 * Laptop - a laptop motherboard which could potentially be developed as a very low cost single-sided 2-layer PCB, through the use of modular and optional components for WIFI and 3G.
 * LCD (TV) - an LCD Monitor (or Picture Frame) which can be upgraded into a TV or an All-in-One Computer or an Internet TV or Personal Video Recorder or Media Centre. very versatile yet simple to do.
 * Passthrough or "Blank" Card - a special type of card which simply passes through the connectors, with little or no signal conversion.

= Contact and Discussion =

For questions, comments and general discussion, please use arm-netbook mailing list

= Slides =

[[Media:eoma68.pdf | Introduction of EOMA]]

= FAQ =

The FAQ is now on its own page.