Embedded Open Modular Architecture/EOMA-26
This page describes the specification of EOMA-26. The number of pins on the interface is 26; the physical form-factor is the 34x75x5mm ExpressCard format. Typical PCB sizes are 28.7mm x 63.6mm.
Re-purposing of the ExpressCard interface and form-factor has been chosen to create portable mass-volume (100 million units and above) ultra-low-cost Embedded Computing Modules (Computer on Module).
The interfaces are:
- 3-channel LVDS (covering up to 1280x800 @ 18-bit colour)
- RS232 UART (Tx and Rx only)
- 4-pin SD/MMC (which must automatically support 2-pin, 1-pin and SPI mode)
- 1 PWM output
- 10 dedicated pins of General-purpose Digital I/O (GPIO) with mandatory multiplexed functions (covering UART, PWM and SD/MMC)
These interfaces are NOT OPTIONAL for CPU Cards. All CPU Cards MUST provide all interfaces and all multiplexed functionality. I/O Boards on the other hand are free to implement whichever interfaces are required for the device. The only exception is I2C (due to the EOMA-26 identification EEPROM), which MUST be provided by all I/O Boards.
Exactly like ExpressCard Cards, EOMA-26 CPU Cards may have absolutely any functions, any additional connectors, peripherals and so on without limitation, except for compliance with the EOMA-26 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 ExpressCard.
Target Market for EOMA-26
The target market for EOMA-26 is smaller or lower-cost devices than EOMA-68. Tablets and Laptops up to 11in in size in particular would ideally make use of EOMA-26. In essence, the EOMA-26 form-factor was designed to take advantage of the decreasing cost and increasingly-high specification of lower-end SoCs such as the A10S, A13, AM3359  and so on.
Pinouts (version 1.0)
These pinouts make no attempt to be electronically or electrically compatible with ExpressCard. Power is deliberately placed on or received from different pins such that EOMA-26 CPU Cards do not power up when accidentally plugged into ExpressCard sockets, nor ExpressCards power up when accidentally plugged into EOMA-26 I/O Boards.
- Two Ground and one 5V pins are provided.
- Power is therefore limited to around 2.5 to 3.0 watts (note: heat is dissipated passively).
- USB and LVDS are balanced / differential pairs.
- The UART Tx and Rx lines can also be GPIO
- The SD/MMC's data lines 0 to 3 can also be GPIO
- As 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 I/O boards 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.
Table of EOMA-26 pinouts
|* 1 GND|
|* 2 USB2-OTG (Data-)|
|* 3 USB2-OTG (Data+)|
|* 4 UART_TX / GPIO1|
|* 5 UART_RX / GPIO2|
|* 6 PWR (5.0V)|
|* 7 I2C Clock (SCL)|
|* 8 I2C Data (SDA)|
|* 9 PWM0 / GPIO0|
|* 10 GND|
|* 11 SDC-CMD / GPIO3|
|* 12 RIN 3- Negative LVDS differential data output|
|* 13 RIN 3+ Positive LVDS differential data output|
|* 14 SDC-CLK / GPIO4|
|* 15 RIN 2- Negative LVDS differential data output|
|* 16 RIN 2+ Positive LVDS differential data output|
|* 17 SDC-0 / GPIO8|
|* 18 CLKIN- Negative LVDS differential clock output|
|* 19 CLKIN+ Positive LVDS differential clock output|
|* 20 SDC-1 / GPIO7|
|* 21 RIN 1- Negative LVDS differential data output|
|* 22 RIN 1+ Positive LVDS differential data output|
|* 23 SDC-2 / GPIO6|
|* 24 RIN 0- Negative LVDS differential data output|
|* 25 RIN 0+ Positive LVDS differential data output|
|* 26 SDC-3 / GPIO5|
I2C Identification EEPROM
The purpose of the Identification EEPROM is to unambiguously allow an EOMA-26 CPU Card to query an I/O Board and find out what the pins are being used for, and what ICs are available. In the case of EOMA-26 this includes setting some of the multiplexing, as well as potentially identifying GPIO Expansion or MCUs that are in use.
The main requirements in summary form 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 0xA2).
- The I2C bus that is connected to the EOMA-26 interface will expect to have access to an EEPROM that is addressable (readable) at I2C address 0xA2.
- There also MUST NOT be a device on address 0xA3 (this is the EEPROM "write" address, used at factory-install time).
- 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 0xA2 and 0xA3) 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-26 Interface. i.e. the I2C bus that is connected to the EOMA-26 interface MUST remain completely dedicated to EOMA-26, and MUST NOT be shared with any peripherals on the CPU Card itself.
The severe limitations of 26 pins potentially necessitates an I/O board design approach which is already well-known: the use of GPIO Expander ICs on the main PCB, or depending on whether additional functionality is needed (ADC/DAC etc.) the use of a full Micro-controller (e.g. Cortex-M3) may be better. Examples include:
- The Freescale K20  has a low-cost 40-pin QFN option.
- The TI LM8330  which can do 8x12 keyboard matrices (up to 104-key keyboards) and has 3 PWMs (normally used to control brightness on LEDs on a keyboard or a backlight).
- The MAX7315  can be used in PWM mode if there is only one low-power LED required for a backlight.
- The MAX7304 i2c 16 bit port expander seems cheaper than 8 bit MAX7315 
- Toshiba has a range of GPIO Expander ICs  which include, in addition to I2C, an external IRQ line. This IC would be ideally suited to providing external IRQ lines for SD/MMC Card-Detect for example.
- Freescale's MC9RS08KA Embedded Controller range is ideally suited and ultra-low-cost
- NXP's PCF8574 is an ultra-low-cost 16-pin I2C-based GPIO Expander
- Holtek also have a superb 8-bit MCU product  which has 24-pins of programmable I/O, ADCs and PWMs.
- Often there is a surprising amount of GPIO available on other ICs, including:
- GSM/EDGE/3G M2M modems often have up to 16 additional GPIO pins: other models such as MiniPCIe ones typically have one or two.
- WIFI modules especially SD/MMC ones have spare GPIO capability; even USB-based WIFI modules typically have 1 or 2 GPIOs (e.g. for LED display).
- Some PMICs have optional GPIO capability: for example, the X-Powers AXP209 has 4 spare GPIO pins.
- Many USB Audio ICs, which are designed for use in USB headphones, often have GPIO for powering LEDs and for providing volume / mute button capability.
Particular consideration needs to be given to the SD/MMC "Card-detect" and to USB-OTG "ID" GPIO functionality. No proscription is given as to how I/O Boards provide these functions. An I/O Board design may consider choosing any of these options:
- To not provide SD/MMC "Card Detect" (i.e. expect the main CPU to do SD Card "polling") and to use one of the 2 dedicated GPIOs for USB-OTG "ID"
- To use both of the 2 dedicated GPIOs for both functions
- To flip the UART pins over to GPIO
- To use one of the above GPIO Expander ICs, connect the Expander's IRQ to one of the 2 dedicated GPIOs and to connect both "Card Detect" and "ID" to the Expander IC
- As above but to use a MCU (Embedded Controller) in the exact same role as a GPIO Expander IC. Examples include Atmel ATMEGAs, ST Micro's STM32F, 8051s and so on.
These latter two options are best suited to when the I/O Board requires additional functionality such as PWM, ADC, DAC etc. All of these options are required to be described accurately in the I2C EEPROM.