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New impulse for embedded computer modules

A comparison of the designs and applications of COM Express, XTX, ETX and Qseven.

Since the appearance of the first computer-on-module (COM) definition in 1999, the technology has found its way into almost all embedded computer applications. Following the introduction of the Intel® Atom™ in February 2008, PC technology became much more economical with respect to power requirements. This distinct trend continues to be mirrored in new COM developments, but not every COM concept is optimally suited. In order to accommodate mobile and power-saving technologies, a new COM definition, Qseven, was adopted in July 2008.

The “Q” in the name Qseven comes from the word “quadratic” and “seven” is the module’s 7x7 cm dimension. This base area allows the deployment of a powerful and efficient x86 PC platform with extensive interface options whilst at the same time retaining the compact dimensions needed to facilitate its integration in handheld equipment.

Unlike most of today’s module standards which are fitted with expensive “board to board” plug connectors, Qseven employs a reasonably priced MXM card plug with 230 pins arranged on a 0.5 mm grid. This plug is also used for fast laptop graphics cards and can handle the high data transmission rates required by PEG (PCI express graphics). Despite its small dimensions, it is of very robust construction and therefore suitable for all mobile applications. Three manufacturers offer this plug type in two different heights and in a rotated variant. These differences give additional flexibility when designing carrier boards.

The Qseven computer-on-module, unlike other memory modules, is not held in place by the card plug itself, but is mounted using 4 screws and a spacer (2.7 or 5 mm, depending on plug height). This type of mounting allows high shock and vibration specifications to be achieved.

Qseven is doing away with much outdated technology. Only the latest interfaces are used in order to support the performance of future mobile chipset/CPU combinations. Old “legacy interfaces” such as parallel IDE and PCI bus have been deliberately left out, in order to save on the complexity and accompanying additional costs, which production of these "relics" entailed.

The following interfaces are defined by Qseven:
- 4x PCI Express x1 lane
- 2x SATA
- 8x USB 2.0
- 1x 1000BaseT Ethernet
- 1x SDIO 8 bit
- LVDS 2x 24 bit
- SDVO / HDMI / Display Port (shared)
- HDA (high definition audio)
- I²C bus
- LPC (low pin count bus)
- Fan control
- Power management signals
- Battery management
- 5V power (max. TDP 12 Watt)
- Thermal cooling interface
Software API
In order to achieve compatibility between the Qseven modules of various manufacturers, the specification defines not only the hardware, but also an integrated software interface for embedded functions. The programming of the watchdog timer, the I²C bus and LCD brightness, together with access to the BIOS user memory area and the recording of system temperature are all integrally controlled. Qseven is the first, and so far the only COM definition, which allows users to make software adjustments in the event of their having to change supplier.

Graphics output
All four definitions support the “classic” direct control of a flat display by LVDS (low voltage differential signal). Modules require additional information from the connected display, in order to set the output format and data timing functions. Qseven achieves this integrally using DisplayID (see Put simply, one of the VESA (Video Electronics Standards Association) standardised data formats is read in via the I²C bus from the display unit and interpreted by the video BIOS. While this is also technically possible with the other definitions, it is not fixed in their specifications. Implementation is, therefore, dependent on the manufacturer and this reduces the compatibility of the modules.

COM Express and Qseven support as standard SDVO (Intel Serial Digital Video Out). With this interface, using either an appropriate encoder element, or a proprietary ADD-2 plug-in card, an additional DVI or TV-out interface can be implemented. With ETX and XTX no SDVO signals are envisaged – a few manufacturers do provide the signals, but only via proprietary cable interfaces.

Analogue VGA and TV Out
New chipsets, such as the Intel® US15W System Controller Hub used in many Intel® Atom™ developments, do not support any analogue video signals. Instead, new digital video interfaces such as DisplayPort and TDMS are offered. With Qseven, the necessary signals are provided together at the SDVO connections. The interface ultimately used is then a function of the baseboard design.

DisplayPort is one of the newest definitions of the VESA ( and is considered as the “prime candidate” to succeed the HDMI interface. Unlike HDMI, DisplayPort is a free standard. In comparison to DVI, TDMS and LVDS, DisplayPort offers an extendable, packet-based protocol, which is able to transmit additional information together with the actual picture data – such as audio data, for example. With just four differential circuits, up to 10.8 Gbit/s (compared to: DVI 4.95 Gbit/s, LVDS 2.835 Gbit/s) can be transmitted and, additionally, future high-resolution displays can be controlled.

PCI Express
PCI Express is a serial, differential communications circuit able to provide a data transmission rate of 2.5 Gbit/s in each direction using only two cable pairs. In contrast to the parallel PCI bus, which manages only 0.13 Gbit/s, only one device per lane can be connected. Faster devices can accommodate more lanes. The PEG (PCI Express Graphics) port consists of 16 PCI express lanes, specially reserved for control by external graphics cards. After allowing for data overheads (10 databits need to be transmitted to provide 8 userbits), Qseven and XTX applications are provided with data transfer rates of around 8 Gbit/s per direction. COM Express provides up to 44 Gbit/s, of which 32 Gbit/s are reserved for optional external graphics.

A large degree of compatibility between ETX and XTX is guaranteed. The difference between them lies in the new plug definition. ETX uses a plug with 100 pins exclusively for the ISA bus, whilst XTX has 4 PCI express lanes, 4x SATA, two additional USBs and HDA (high definition audio) on the same plug. The remaining signals, as well as the mechanics and cooling concept, are completely identical. ETX applications which don’t have an ISA bus are able to use XTX modules without experiencing any kind of problem. Qseven and COM Express are completely new definitions, which have no compatibility at all with ETX or XTX.

Power consumption
Whilst COM express allows a maximum power consumption of 188 Watts and ETX/XTX modules use up to 40 Watts, processors such as the Intel Atom Z5xx series with Intel System Controller Hub US15W chipsets require less than 5 Watts . It is very likely that future product development will result in X86 processor platforms requiring even less power. In contrast to the module standards currently in use, Qseven is designed to have a maximum power consumption of just 12 Watts for mobile and battery-operated applications and interface layout has been orientated towards the most up-to-date mobile chipsets.

SATA versus PATA and SDIO
SATA is the logical successor to PATA and EIDE. Not only is performance increased, but there are also definite improvements in signal length, fault correction and cabling. Many embedded systems use EIDE, in order to use cheap, robust CompactFlash cards as mass storage devices. In this area, too, Qseven is proving to be state-of-the-art. Qseven supports the SDIO interface as a single COM standard, allowing extremely cheap SD memory cards to be used for mass storage, in addition to other I/O cards with such functions as RFID, WLAN or Bluetooth.

SATA is not supported by ETX in either of the 2.x versions. The ETX 3.0 update will allow two SATA plugs to be connected directly to the module. This is inconsistent with the basic idea of a cable-less module design, but on the other hand, allows for simple upgrading.

“Legacy free” versus “Legacy”
COM Express has not only moved towards new, fast interfaces with a viable future, but has also shed its old, slow and antiquated ports. Together with the ageing ISA bus, interfaces such as COM, LPT, floppy and PS/2 keyboard/mouse are no longer supported. By turning its back on the PCI bus and VGA, Qseven has moved another step closer to its “legacy free” goal. The absence of parallel, floppy and PS/2 ports should not bother the majority of system designers, since there are adequate USB peripherals. However, the absence of serial interfaces could be a cause for concern, since these are still popular communications and debug interfaces in an industrial context. It is precisely in this area that the XTX formfactor can score over COM Express. As with ETX, 2xCOM, 1xLPT, 1xfloppy (shared with LPT) and PS/2 keyboard/mouse are all supported, as indeed are PCI Express, SATA and HDA. This forms a bridge between the old and the new computer worlds. But even with a legacy free design based on Qseven, classic serial interfaces can still be put cheaply onto carrier boards using LPC port (low pin count – basically, an ISA bus with a few limitations, re-configured for serial signals).

It is already clear that the PCI and EIDE parallel interfaces are going to disappear from market leader Intel’s current chip development programmes. These interfaces are to be manufactured with increasing sophistication using bridging components. The situation with respect to the ISA bus is a little more complex. This will also be manufactured, but is going to have to do without certain functions, depending on the new chipset arrangements. This may limit the installation options of the new ETX modules.

Shedding the ISA bus and the addition of modern serially-differential interfaces makes XTX the logical successor to ETX. COM Express, with its flexible pin configuration, is already orientated to a future without PCI bus and IDE. So, the Type 5’s still unusual plug arrangement is being left out in favour of the additional Ethernet and PCI express ports. This allows COM Express to offer the maximum possible I/O performance. Performance-wise, and also in terms of its power requirements, Qseven clearly remains the ideal solution for all future mobile and battery-operated applications.

All the module standards described have been installed on on-board graphic chips, which are mostly already integrated into the chipsets. The performance of these graphic engines has clearly increased over the last few years, although true “game quality” has still not been achieved. COM Express’s PEG port allows the control of external high-performance graphics cards.

Dipl.-Ing. (FH) Gerhard Gilch, congatec AG

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