Embedded Open Modular Architecture/EOMA68

= 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 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)
 * I2C
 * USB (Low Speed, Full Speed, optionally Hi Speed/480 Mbit/s and optionally USB3)
 * 10/100 Ethernet (optionally 1,000 ethernet)
 * SATA-II (optionally SATA-III)
 * 8 pins of General-purpose Digital I/O (GPIO).

These interfaces are NOT OPTIONAL for CPU Cards. All CPU Cards MUST provide all interfaces. 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 an SATA interface, devices such as tablets or laptops into which CPU Cards are plugged are not required to have an SATA hard drive.

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.


 * SATA - The only interface which is not particularly common on mass-produced powerful Embedded CPUs is SATA-II: this can be constructed from a USB-to-SATA converter IC such as the Genesys Logic GL831A or the JMicron JM20337.
 * I2C and USB - The reason for choosing I2C and USB2 is because they are both only two wires, are global buses that can address multiple devices, and they have been long-established proven Industry Standards in their fields.
 * Ethernet - 10/100/1,000 Ethernet was chosen because it is prevalent on the majority of computing devices. In the case where chosen CPUs do not have Ethernet, a USB-to-Ethernet converter IC such as the SMSC LAN9500 can be deployed.
 * 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.

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 (1.5 Mbit/s) is acceptable.
 * Providing USB Full Speed (12 Mbit/s) is acceptable if USB Low Speed is also provided
 * Providing USB Hi Speed is acceptable provided that Full Speed and Low Speed is also provided
 * Providing USB Hi Speed (480 Mbit/s) only and supporting no other speeds is not acceptable.

Requirements for Ethernet
All CPU Cards are required to support the full auto-negotiation capabilities of Ethernet, up to the maximum speed chosen to be provided. Specifically:


 * Providing 10 Mbit/s Ethernet is acceptable
 * Providing 100 Mbit/s Ethernet and down-negotiation to 10 Mbit/s Ethernet is acceptable
 * Providing 100 Mbit/s Ethernet only is not acceptable
 * Providing 1,000 Mbit/s Ethernet is acceptable as long as down-level negotiation to both 100 Mbit/s and 10 Mbit/s is also provided
 * Providing 10,00 Mbit/s Ethernet only is not acceptable.

Requirements for SATA
All CPU Cards are required to support the full backwards-compatible auto-negotiation capabilities of SATA, up to the maximum speed and capabilities chosen to be provided. Specificially:


 * Provision of SATA-II is acceptable, but provision of SATA-II with only support for 3 Gbit/s SATA-II is not acceptable.
 * Provision of SATA-III is acceptable, provided that backwards-compatibility with all prior versions of SATA are also provided

= Pinouts (version 1.0) =

These pinouts make no attempt to be electrically or electronically compatible with the legacy PCMCIA standard. 16 GPIO pins, 24-pin RGB/TTL, USB2, I2C, 10/100/1000 Ethernet and SATA-III interfaces are included in the Version 1.0 specification. ''Note: USB2, SATA-III and Ethernet MUST support auto-negotiation, and MUST support the lower capabilities (USB 1, USB 1.1, SATA-I, SATA-II, 10/100 Ethernet). Higher speeds and capabilities are optional''.

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. Most systems will not have active cooling.''

All High-speed signals (USB2, Ethernet, SATA-III) 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 sixteen 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, which has electrical characteristics of 3.3 V TTL but requires 5 V TTL tolerance. Typical TTL high-level voltage is 2.0 volts; threshold is 1.4 V; low-level TTL voltage is 0.8 V.

Also, because the GPIO pins can be reconfigured individually bi-directional for any digital purposes, they must be made to be 5 V TTL tolerant and tri-state isolated, and Motherboards also must be 5.0 V TTL tolerant as well as tri-state isolated. Levels when any GPIO pin is used either as an input or as an output should again operate at nominal 3.3 V TTL levels, thus expect "high" voltage of 2.0 volts, threshold of 1.4 V and "low" voltage of 0.8 V, but must accept voltages from 0–5 V.

The option to use Gigabit Ethernet is also available, if a given system has it. If, however, a particular system does not have Gigabit Ethernet, the pins must not be used for other purposes, and must be left unconnected. This is to ensure that automatic negotiation of 100/1000 Ethernet occurs correctly.

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. 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 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 57 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