When development teams at system and device manufacturers discuss waysin which development times and costs can be reduced, they usually endup turning to the use of Computer-On-Modules (COMs) for an answer.
In addition to increased flexibility and scalability, this approachalso leads to many other benefits. The most common standards are ETX (defined in 2000), COM Express (2004), XTX (2005) and the recentlyintroduced Qseven.
Since the start of the COM idea back in 2000, chip manufacturershave brought several new technologies to market that, for obviousreasons, could not have been foreseen when these standards weredefined. New interfaces have been defined, computing performance hasincreased dramatically and energy consumption ” thanks to ever smallerchip structures ” has seen significant decreases (Figure 1, below ).
This article describes the differences and developments of these COMstandards, compares them to each other and shows their respectivestrengths and weaknesses depending on the application.
COMs are now well established in the x86 embedded computing segment.The modular concept allows simple customisation of standardised PCmodules for the most diverse of end applications.
The custom built carrier board turns an 'off-the-shelf PC' into acustomised solution. In particular, the small dimensions and highnumber of available interfaces make the module the ideal processingcore for countless industrial applications.
|Figure1. The evolution of single board computer-on-modules, left to right,from ETX, COMExpress, XTX and now Qseven|
Eight years have passed since the first embedded PC in a modularformat ” ETX ” was defined based on a Pentium platform with PCI ISA andlegacy I/O interfaces. Today, this standard can no longer meet theneeds of current chipsets and computing power hungry applications.
In contrast to the independent development of ETX by an independentindustry group, a number of big players in the market finally joinedforces within the PICMG (PCI Industrial ComputerManufacturers Group) to develop a successor technology in a modularform that provides good investment security, namely the COM Express PCmodule. Thanks to its two available sizes and five different pin-outsfor the two 220 pin connectors, this module can be used for nearly allareas of the embedded PC segment.
This can be expensive for users, however. COM Express is in no waycompatible with its ETX 'predecessor'. Dimensions, connectors andinterfaces were modified or newly defined. Most of the COM Expressmodules currently available are Type 2 and have dimensions of 125 x95mm (basic module). Moreover, there are five different pin-outsdefined within the PICMG specification and there are more than fourmodule sizes used from module vendors.
The XTXConsortium had the strong impression that this would confuse theusers. Instead of reinventing the wheel, a module was designed thatbuilds on the already successful ETX standard while still providing newinterfaces for current chipsets that use the existing four 100- pinconnectors on the carrier board.
The solution to the problem was quite simple, in fact; the pin-outof the connector that carries the ISA bus signal for ETX was redefined.The remaining three connectors, dimensions and cooling solutions stayedunchanged. In contrast to COM Express, XTX is thus a 'true' ETXsuccessor.
The latest standard, Qseven (Figure2 below ), was recently defined by an independent consortiumwhich was founded by Congatec in Germany and Seco in Italy. Today,there are more than ten members supporting this definition.
The name Qseven is derived from 'quadratic', which is represented bythe Q and 'seven' referring to the 7 x 7cm size of the module. Such afootprint enables the development of a high-performance and extremelyenergy-efficient x86 PC platform with comprehensive interfacepossibilities.
While COM Express is directed towards highest computing performanceand a maximum power consumption of 188Watt, Qseven focuses on ultramobile applications and do not support more than 12W power dissipation.
Qseven cuts all legacy interfaces and provides a feature set thatwill be available for many years from now. Featuring only differentialserial interfaces with high bandwidth and good EMC behaviour, it is anideal platform for ultra mobile technologies, such as Intel's Atom Z5xxseries processor and the Intel System Controller Hub US15W. Thisoptimised combination consumes less than 5W.
Unlike the other COM standards, Qseven is specifically designed formobile and battery-operated applications. Additionally, its interfaceslook to the future and are compatible with state-of-the-art mobilechipsets.
Neither does it require a board-to-board connector. Instead, itutilises an MXM card slot with 230pins in a 0.5mm configuration. Thisslot is already being used for graphics cards in laptop computers,demonstrating it is capable of the high speed PEG (PCI ExpressGraphics) data transfers. Furthermore, the CPU module has edge fingersthat are inserted into the slot, so an additional connector on themodule is not required, thereby reducing costs.
PCIe vs PCI
The PCIe bus is a requirement for applications with faster I/Ointegration. Qseven, XTX and COM Express are therefore better in thisrespect. The theoretical bandwidth is just 133MByte/s with the 32bitPCI bus of the ETX.
An x1 PCIe lane allows for a full 2.5 GBit/s in both directions.Type 4 and 5 COM Express modules even allow for an x16 PCIe connection.However, the necessary chipsets for this are not yet available. XTX aswell as COM Express also support the legacy 32bit parallel PCI bus.
PATA vs. SATA and SDIO
SATA is the logical successor to the EIDE Interface. Until recently,the parallel IDE interface was frequently deployed in embeddedapplications to control the tough and affordable CompactFlash cardsused as bulk memory. However, over time SDCards have become the cheaperalternative and are now also available for industrial applications.
The SDIO interface in the Qseven standard enables SDCards to be usedas bulk memory, but the interface also allows further flexibility. TheSecure Digital Standard enables memory devices and applications such asWLAN, Bluetooth, RFID etc. with the same card format. These add-oncards are very compact, tough and ideal for extending mobile systems.
PEG vs. onboard graphics
Although the graphics controllers that are integrated into the chipsetcontinue to accommodate more functions and performance needs, thesecontrollers still cannot fulfil the demands of certain applications.Similar to the normal desktop PC sector, gaming applications pushgraphics performance in the embedded sector as well. And this isprecisely the great advantage of COM Express.
It is the only standard that supports the PCI Express Graphics Port(PEG), which allows for the connection of a superfast video controllervia dedicated x16 PCE lane. The other standards use the integratedgraphics controller with shared memory architecture. While these alsooffer decent performance, they cannot compete with the external PEGsolutions.
LVDS, SDVO, DisplayPort and HDMI
For the 'classical' control of a flat panel display all four moduletypes support LVDS (Low Voltage Differential Signal). The COM modulesrequire additional information about the connected display so that theycan adjust to the output resolution and data timing. This isaccomplished using DisplayID (see www.vesa.org).
In a nutshell, a standard data format, as defined by VESA (VideoElectronics Standards Association), is read from the display unit withthe help of the I2C Bus, and then interpreted by the video BIOS. Thisenables 'plug and play' functionality and makes complicated displayadaptations a thing of the past. In addition to the local LVDS display,a second graphics port enables an additional display for COM Expressand Qseven.
The physical signals of this graphics port are used by SDVO. A hotplug mechanism detects what type of interface the customer is using andthen configures the graphics controller accordingly. Alternatively toSDVO Qseven does support DisplayPort or HDMI.
DisplayPort is one of the latest VESA definitions (www.vesa.org) andis being treated as a 'hot candidate' to be the successor of today'sstandard HDMI interface. Unlike HDMI, DisplayPort is a free openstandard, thus guaranteeing widespread adoption.
Compared with DVI, TDMS and LVDS, DisplayPort offers an extendable,packet-based protocol that can carry additional information, such asaudio, in addition to pure display data. With a mere four differentialchannels, up to 10.8GBit/s (DVI 4.95GBit/s, LVDS 2.835GBit/s) can betransferred. This is the equivalent of simultaneous transfers of 6 HDTVvideo channels.
All COMs are equipped with additional functions for industrialapplications. Examples of this include Watchdog Timer, I_C Bus, LCDbrightness control, BIOS user storage area and the reading of systemtemperatures.
Due to the fact that no standardised software interface for thesefunctions has been defined to date, the theoretical exchangeability ofCOMs has in practice proven to be more difficult than expected.
In order to generally avoid the software modifications that suchsituations would require, the Qseven specification includes aconsistent software API (Application Program Interface). Qseven modulesfrom different manufacturers can thus be easily exchanged withoutmodifications to hardware or software.
Legacy vs legacy free
The COM Express standard is not the only way to new and fast interfacesthat offer investment security. Rather, the standard also moves awayfrom old and slow legacy ports. COM Express is a true example of a'legacy-free' standard; the ISA bus and COM, LPT, floppy and PS/2keyboard & mouse interfaces are no longer supported.
The lack of parallel, floppy, and PS/2 ports may not bother mostsystem designers, as there has long been an adequate range of USBperipherals, the missing COM ports will, however, be cause for morescepticism because they are still the preferred communication anddebugging interface, particularly in the industrial sector.
It is for precisely this reason that the XTX form factor scorespoints over COM Express. Qseven is the most legacy free standard. It nolonger supports the PCI bus, but features SDIO and the new panelinterfaces; DisplayPort and HDMI.
Market leader Intel is, however, already starting to eliminate theparallel PCI bus and parallel ATA and new developments for ETX areclearly affected. At present, the ETX-compatible 'successor'- XTX – isalready the appropriate alternative for all applications that canfunction without the ISA bus and will remain on the market considerablylonger due to its clearly 'modern' allocation of the ISA bus connector.
Those who have reservations about a completely new design and whotherefore see no advantage in ETX compatibility will fall back on COMExpress for the high end approach, or Qseven for small size and lowpower consumption.
Christian Eder is MarketingManager at Congatec AG.