Embedded Open Modular Architecture/EOMA-26

= EOMA-26 Specification =

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.

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)
 * I2C
 * USB (Low Speed, Full Speed, and optionally Hi Speed/480 Mbit/s)
 * RS232 UART (Tx and Rx only)
 * 4-pin SD/MMC (which must automatically support 2-pin, 1-pin and SPI mode)
 * 2 dedicated pins of General-purpose Digital I/O (GPIO) with a mandatory 8 further multiplexed GPIO (covering UART and SD/MMC)

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. The only exception is I2C (due to the EOMA-68 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, 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.

Notes:
 * Two Ground and two 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 2 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
= GPIO Expansion =

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. Examples include:


 * 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.
 * 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.