COMs vs SBCs

New Impulse for Embedded Computer Modules

The Qseven definition allows the application of Computer-on-Modules to be extended to small and mobile systems. Since it has abandoned legacy interfaces, selection of a COM approach can be aided by comparing it with COM Express, XTX and ETX.


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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 and provided an added benefit in the reduction of power requirements. This distinct trend continues to be mirrored in new COM developments, however, not every COM concept is optimally suited to take advantage of the latest mobile and power-saving technologies. Therefore, a new COM definition called Qseven was created in July 2008.

New Technologies Require New COM Specifications

When development teams at system and device manufacturers discuss ways in which development times and costs can be reduced, they usually end up turning to the use of Computer-on-Modules (COMs) for an answer (Figure 1). In addition to increased flexibility and scalability, this approach also leads to other benefits. The most common standards are ETC (defined in 2000), COM Express (2004) and XTX (2005). Since then, chip manufacturers have brought several new technologies to the market that could not have been foreseen when these standards were defined. New interfaces have been defined, computing performance has increased dramatically and even energy consumption–thanks to ever smaller chips–has seen significant decreases.

While COM Express permits a maximum performance of 188 watts, contemporary processors such as Intel’s Atom Z5xx series, feature chipsets (Intel System Controller Hub US15W) that consume less than 5 watts. We can expect to see the development of even more energy efficient x86 processor platforms in the future. Unlike previous module standards, Qseven is specifically designed for mobile and battery-operated applications. Additionally, its interfaces look to the future and are compatible with state-of-the-art mobile chipsets.

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

Unlike most of today’s module standards populated with expensive “board-to-board” plug connectors, Qseven employs a reasonably priced MXM card socket with 230 pins arranged on a 0.5 mm grid. This socket is also used for fast laptop graphics cards and can, therefore, handle the high data transmission rates required by some interfaces such as PCI Express Graphics (PEG). Despite its small dimensions, it has an extremely robust construction and therefore is suitable for all mobile applications. Three manufacturers offer this type of socket in two different heights as well as a mirrored version. These optional variants provide additional flexibility when designing carrier boards.

The Qseven Computer-on-Module, unlike memory modules, is not held by the card socket itself, but is instead secured using four screws and a spacer (2.7 or 5 mm, depending on socket height). This type of mounting allows high shock and vibration specifications to be achieved.

Qseven does away with a lot of 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 (Figure 2):

• 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

In order to ensure compatibility between the Qseven modules from various manufacturers, the specification defines not only the hardware but also an integrated software interface for embedded functions. The programming of the watchdog timer, 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, that allows users to change suppliers without having to reconfigure their software. Table 1 provides the best way to compare COM Express, XTX and ETX module definitions to Qseven.

Graphics Support and Choices

All four definitions (Qseven, COM Express, XTX and ETX) support the “classic” direct control of a flat panel display via low voltage differential signal (LVDS). Modules require additional information from the connected display in order to set the output format and data timing functions. Qseven handles this task using DisplayID (see Put simply, one of the VESA (Video Electronics Standards Association) standardized data formats is read in via the I²C bus from the display unit and interpreted by the video BIOS. This is also technically possible with the other definitions but 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 standard Intel Serial Digital Video Out (SDVO). 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.

New chipsets, such as the Intel US15W System Controller Hub used in many Intel Atom developments, do not support any analog 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 carrier board design.

DisplayPort is one of the newest definitions of VESA ( and is considered the “prime candidate” to succeed the HDMI interface. In contrast to HDMI, DisplayPort is a free standard. In comparison to DVI, TDMS and LVDS, DisplayPort offers an extendible, 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 Gbits/s (compared to: DVI 4.95 Gbits/s, LVDS 2.835 Gbits/s) can be transmitted and, additionally, future high-resolution displays can be controlled.

Connectivity Choices

PCI Express is a serial, differential communications circuit able to provide a data transmission rate of 2.5 Gbits/s in each direction using only two cable pairs. In contrast to the parallel PCI bus, which manages only 0.13 Gbits/s, only one device per lane can be connected. Faster devices can accommodate more lanes. The PEG graphics port consists of 16 PCI express lanes, specially reserved for control by external graphics cards. After allowing for data overhead (10 data bits need to be transmitted to provide 8 user bits), Qseven and XTX applications are provided with data transfer rates of around 8 Gbits/s per direction. COM Express provides up to 44 Gbits/s, of which 32 Gbits/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 HAD (high definition audio) on the same plug. The remaining signals, as well as the mechanics and cooling concept, are completely identical. ETX applications not implementing the 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 (Figure 3).

On the storage side, Serial ATA (SATA) is the logical successor to Parallel ATA (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 any 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.

Finally, power consumption can make a significant difference for small, mobile and deeply embedded designs. While 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 toward the most up-to-date mobile chipsets.

“Legacy free” versus “Legacy”

COM Express has not only moved toward new, faster interfaces with a viable future, but it has also shed its antiquated ports. Along with the aging ISA bus, interfaces such as COM, LPT, floppy and PS/2keyboard/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 given that there are adequate USB peripheral replacements. 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 Form factor is preferable over COM Express. ETX, 2xCOM, 1xLPT, 1xfloppy (shared with LPT) and PS/2 keyboard/mouse are all supported, as 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 the LPC port–basically, an ISA bus with a few limitations and reconfigured for serial signals.

It is already clear that the PCI and EIDE parallel interfaces are going to disappear from the development programs of market leaders, such as Intel. 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, as well as adding 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. The Type 5 COM Express variants look to the future and replace PCI and IDE with additional Ethernet ports and PCI express lanes. This allows COM Express to offer the maximum possible I/O performance. Based on performance and power requirements, Qseven clearly remains the preferred solution for all future mobile and battery-operated applications.

All of the above mentioned module standards utilize onboard graphics, which are integrated into the chipsets. The performance of these graphic engines has clearly increased over the last few years, although true “gaming quality” has still not been achieved. COM Express’s PEG port allows the control of external high-performance graphics cards.

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