Embedded Open Modular Architecture/EOMA-68/Tablet
The Tablet Motherboard
Popular by decree, but only successfully-sold when the price is stunningly low yet the feature-set rich, tablets are the "must-have" for all ODMs and OEMs who aspire to a chunk of the large apple pie. Key goals for this motherboard are therefore to be small, slim, low component count and based on a low-risk development strategy. Thanks also to the modular design, the board is sufficiently simple that it may even be possible to do as a single 2-layer PCB, thus reducing costs even further.
Connectors and Components
The connectors required are:
- 1x USB2
- PCMCIA Connector "inline" (signals conforming to EOMA/PCMCIA Standard)
- 5V Power
- 1x PCI Express "inline" (supporting USB Wifi, not PCI-e Wifi, such as RT2070 and RT8191)
- 1x Stereo Speakers and Microphone
- 1x RGB/TTL LCD Output (with LED Backlight)
- 2x Battery Connectors
Major components are:
- An STM32F103RBT6 Embedded Controller (same as in the Micro Engineering Board)
- A 4-port USB-2 High-speed Hub (e.g. GL850G)
- 12.5Mhz XTAL (for the GL850G)
- Power Management ICs (Buck Converters for 3.8v Lithium to 5.0v; 3.3v LDOs)
- An I2C EEPROM
- An RT2070 or RT8191 USB-compliant MiniPCIe WIFI Module
- An Antenna for the WIFI Module
- A 7in LED-backlit LCD
- A slim-line PCMCIA Ejector Assembly
The estimated BOM is therefore around the $30 to 35 mark, excluding the EOMA/PCMCIA-compliant CPU Card, and including the batteries, case and WIFI module. The most expensive component is the LCD Panel, whilst the 2nd most expensive one is the batteries.
The STM32F106 is chosen because of its low cost, flexibility and capabilities. The STM32F106 has 2 ADCs that can be used to monitor up to 16 simultaneous channels. Some of these channels can be used for a Resistive Touchscreen, whilst others can monitor the Battery Charge state. Also, the GPIO pins can be used for buttons, powering up of devices (e.g. for saving power, the WIFI module can be disabled) as well as doing PWM-driven Brightness Control of the LCD's Backlight. The STM32F106, costing around $1.50 to $1.70, replaces approximately $6 to $8 of discrete components and ICs, and does the exact same job, using software.
For developing the formware for the STM32F, there is a GPLv3 STM32 Library. Uwe Hermann describes a number of Software Libre Tools which are available that assist in the programming of STM32F devices. Also the OLPC OpenEC Project has pre-existing firmware source code which can be ported and adapted, saving several man-months of development effort.
The primary reason for choosing a modular USB-compliant Mini-PCIe WIFI Module is because laying out PCBs involving R.F. components is particularly tricky. Not only is it possible to simply plug in a WIFI module, but also it is possible to leverage better pricing without redesigning the product.
Overall, the reduction in the complexity of the PCB means that a single-sided 2-layer board is a distinct possibility. Combined with the low component count, the pricing of manufacture of such a product based around this design concept will be extremely good.
Diagram of Tablet Motherboard Layout
From this diagram, it can be seen that there is very little involved. Like the Odroid, it's possible to have a product where the connectors and buttons define the size of the PCB more than the ICs and discrete components. In this case, many of the major connectors (such as USB-OTG, HDMI, Micro-SD and Headphones) will already be on the EOMA/PCMCIA-compliant CPU Card, leaving nothing left for the motherboard than to provide USB2 and Power connectors! An alternative revision is also shown which takes a USB 3G Modem, in PCI-e form-factor.
Diagram of Tablet Construction
This diagram shows the physical construction of the tablet's components. Critical to achieving a thin tablet, even when allowing for a 5.5mm height PCMCIA CPU Card, is to use an inline PCMCIA header as well as an inline PCI-e connector. If one is used at all (rather than having the PCMCIA CPU card factory-installed) the PCMCIA assembly is shown being attached directly to the tablet's outer casing, rather than being attached to a PCB. Additionally, two batteries are used (in parallel) rather than one, in order to prove a balanced weight distribution to the device. The larger battery sizes, which could even be up to 6mm thickness, potentially allows for cheaper Lithium Polymer batteries to be used - again, reducing the overall cost of the device.