Here COMes the future!

Five years on, the PCI Industrial Computer Manufacturers Group (PICMG) has now released a new revision of the COM.0 Computer-on-Module standard that responds to new functionalities that Intel, AMD and other manufacturers are investing in their upcoming processor families.

Several legacy interfaces were taken out of the design to create space for possible future technologies. The growing significance of graphics and displays is evident here.

This optimisation now makes smaller COM Express form factors possible. Nonetheless, the pinout types produced to date will continue to be available with new generations of chips, thereby ensuring the scalability of existing applications.

In the COM Express COM.0 specification, PICMG defines the standard for a computer-on-module (COM) as a bootable host computer in the form of a single large-scale integrated component.

The aim of the standard is that the defined interfaces and form factors should give designers and solution providers a firm basis on which to develop products that are future oriented and promise long-term availability.

This is where PICMG is plotting the right path with the new COM specification. OEMs that set up their medical diagnostic apparatus, industrial robots and vending machines, POS and kiosk systems, test and measurement applications, their surveillance cameras or unmanned transport vehicles on COM Express modules have chosen a sustainable and innovative solution for the future.

When they decide to migrate to a more compact form factor, COM Express COM.0 Rev 2.0 will also give a seamless transition.

Seven pinout types are defined in Rev 2.0 - two more than previously. These can be split into two groups differing, initially, in the number of connectors they have. Pinout types 1 and 10 are supplied with a single A-B connector that has 220 pins, which can also be found on all other pinout types.

But types 2, 3, 4, 5 and 6 have a second 220-pin connector as well - the C-D connector - so they have a total of 440 pins.

First of all, there is the basic pinout: type 1. This has one 220-pin connector - the A-B connector - and supports up to eight USB 2.0 ports, up to four SATA or SAS ports, and up to six PCI Express Gen1/Gen2 lanes.

It supports dual 24-bit LVDS, an HDA digital audio interface, gigabyte ethernet and eight GPIO pins.

As of COM.0 Rev 2.0, serial ports are again supported. The pins for this were previously used for VCC 12 V. However, dedicated manufacturers, such as Kontron, ensure compatibility with existing baseboards by a protective circuit on the module.

SPI is added to all single pinout types in Rev 2.0 on previously reserved pins. The maximum primary input voltage is +12 and that for standby mode +5 V. Some solutions such as the company’s COM Express module allow a variable input voltage of 4.75 to 14.75.

Pinout type 2 has all the stated functionality of type 1, but adds a second 220-pin connector to it as well (C-D). In this case, type 2 features a 32-bit PCI interface plus IDE ports to support legacy PATA devices such as PATA HDD and CompactFlash memory cards.

There is a total of 22 PCI Express lanes (six on the A-B connector and up to 16 on the C-D connector), 16 lanes on the second connector being intended for PCI Express graphics. The maximum input power is defined as 188 W and power consumption is matched to 137 W in Rev 2.0.

Comparing pinout type 3 to type 2, only the IDE pins are used in favour of extra gigabit ethernet capability. Consequently, it has no legacy interfaces, but now supports up to three gigabit ethernet channels. In pinout type 4, again compared to type 2, pins reserved for PCI are reallocated, creating space for 10 additional PCI Express lanes. These can be used as PCIE lanes 0-15 or as second PEG port lanes 16-31.

Type 5 fuses the changes in types 3 and 4 compared with type 2.

PICMG has added a sixth type of pinout to the COM Express standard especially to use the expanded graphics possibilities of new processor families. This pinout type is essentially based on type 2, the most successful COM Express COM.0 pinout type to date.

Legacy PCI pins are now used to support the digital display interface and for additional PCI Express lanes. Furthermore, in pinout type 6 the pins assigned the IDE interface in pinout type 2 are reserved for future technologies still in development.

One of these technologies could well be SuperSpeed USB, because the 16 free pins would offer sufficient lines to implement four of the eight USB 2.0 ports as USB 3.0 ports, which each require an extra pair compared to USB 2.0.

Although, basically, the same as pinout type 2, pinout type 6 nevertheless comes with extensive support for additional display interfaces. The graphics options have always been one of the special strengths of COM Express through support of PEG, which has been chosen by Intel to provide a high-speed bus for external graphics cards.

But these days it is not only a matter of satisfying the growing need for performance, but of supporting different output devices. And these are precisely the kind of requirements for which pinout type 6 has been scaled.

Like virtually all the other pinout types (except type 10), it continues to support the familiar analog VGA, the standard interface for RGB/CRT devices used in many industrial applications. Because of the analog transmission of the picture signal, VGA is not entirely suitable for graphic resolution of more than 1280×1024. Low-voltage differential signalling (LVDS) devices can also be driven directly by all pinout types. This is important for applications with LCDs, for example, which mainly use this transmission standard.

Here it should be noted that the dual 24-bit LVDS channels are designed for one display; the second channel serves solely to process the increasing data rates caused by higher resolution and frequency. The connected LVDS display defines how many channels are needed for what resolution.

Pinout type 6 goes far beyond these graphics options, though, because ports 1, 2 and 3 are dedicated to the new DisplayPort interface here (digital display interface: DDI).

This is a response to the fact that Intel is now offering three configurable DDI ports in its new chipsets. The developer can configure these ports individually for high-definition multimedia interface (HDMI) or DisplayPort (DP) and DDI port 1 additionally for serial digital video output (SDVO).

This is because SDVO is not multiplexed on the PEG port in type 6, which has been possible with type 2 up to now, but on digital display interface port 1.

With the SDVO interface supported only by Intel chipsets, COM Express is flexible in supporting a whole variety of graphics signals. So the developer can now additionally implement digital visual interface (DVI), for example, and do so at relatively low cost, say for digital monitors and dual-display solutions.

In the past, COM Express did not officially support SDVO but it has also become part of the COM Express standard in the new COM.0 Rev 2.0.

The developer can also operate modern DisplayPort and HDMI graphical interfaces through the DDI. DisplayPort is a universal and - unlike HDMI - royalty-free interconnect standardised by VESA, which should ensure its widespread popularity. DisplayPort not only has a much higher data transfer rate of 10.8 Gbps (compared with 2.835 Gbps with LVDS and 4.95 Gbps with DVI), but also a micro-packet protocol, allowing simple expansion of the standard.

DisplayPort, furthermore, supports an auxiliary channel that allows a bidirectional connect to control devices by VESA standards E-DDC, E-EDID, DDC/CI and MCCS for example. This enables genuine plug-and-play operation. The auxiliary channel can be used, for example, for touch panel displays, USB connects, cameras, microphones and suchlike.

DisplayPort could eventually replace HDMI, popular on the consumer market which, as already mentioned, is also supported by the COM Express standard on the DDI. Although HDMI might seem an ideal solution for consumer electronics due to its high data rate, its connector concept (audio and video on one cable) or its backward compatibility, this interface was not developed for the embedded market.

Its attachment does not make a particularly stable impression, plus long-term availability could also be a problem, with drivers or mechanical requirements frequently changing.

With this extensive support for the new graphics and display functionalities of upcoming chipsets, pinout type 6 is a promising follow on to pinout types 2 and 3 and comes at the right time.

Kontron already anticipated this development in a number of areas, so developers who want to make full use of the new graphics possibilities of COM Express are well served by the embedded specialist. Given its experience, the module producer can provide developers with optimal support when migrating from pinout types 2 or 3 to pinout type 6 and help ensure a seamless transition.

The major innovation that COM.0 Rev 2.0 represents is definition of the new pinout type 10, a kind of twin brother of pinout type 1. Type 10 addresses the requirements of newer and highly compact processors more explicitly. A close look at the pin assignments reveals the differences to watch out for when migrating from type 1 to this new type, although both pinout types are compatible with each other.

In type 1, for instance, SATA ports 2 and 3 are assigned pins in rows A and B, but these are no longer reserved in type 10. The pins could still be used as SATA ports, but are now reserved for alternative purposes such as USB 3.0. So, in designs for pinout type 1 as for type 10, Georg Vogl, product manager at Kontron, advises against wiring SATA 2 and 3 over the module connector.

The modules then remain compatible, and they are ready for USB 3.0 at the same time.

In type 10, the pins for PCIe lanes 4 and 5 are no longer reserved and can also be used for upcoming technologies.

The background in both the above cases is as follows: processors of a small form factor, at which type 10 aims, support up to two SATA interfaces and four PCIe lanes. The vacated pins on the module connectors of the ultra standard can, therefore, be used for new purposes.

A further difference is that type 10 uses the second LVDS channel, TV out and VGA to support the SDVO port (or alternatively DisplayPort or HDMI) via DDI. That is no real loss, as VGA will only play a minor role in future. But dual display support is implemented with the ultra-compact modules, which will continue to support an LVDS channel.

The differences are not likely to be of much consequence for buyers of Kontron’s COM Express modules as the manufacturer has already reserved the appropriate pins for SDVO support, for instance, on the former VGA and second-channel LVDS pins in its nanoETXexpress-SP modules.

Further changes affecting all the types of modules available are as follows: the COM Express connector in the present form is now also approved for PCI Express Gen2 signals. In technical terms, that means no alteration to the connector and its pinning, but the developer must still adhere to new rules for PCIe Gen2 when routing the module and baseboard.

Additionally, the AC97 pins are now used to support AC97 and HD audio.

The following changes have additionally been made in the new version: COM modules type10 and type 6 now also support SDIO, multiplexed on the existing GPIO signals. Optionally, two 3.3 V TTL serial ports are added - many a legacy application requires it - and here the standard again shows its flexibility in responding to the needs of the market. Both ports can be used for different purposes, eg, RS232, RS485, the CAN bus or other two-wire interfaces.

One change shown in the new specifications affects all pinouts: in addition to the previous firmware hub, there is a new BIOS interface for an internal and external boot implemented in the new generation of processors.

This is a serial peripheral interface, the future interface for BIOS flash on the module and carrier board. Ready reserved pins are used for this purpose.

Generally, PICMG allows a choice between two SPI chips, the new COM.0 Rev 2.0 specifying external BIOS support for all module types. The LPC interface was used for this purpose in the earlier version.

The new modules must support SPI, but may still additionally flash BIOS externally through LPC, ie, if the chipset continues to support it. The reason for this change in BIOS flash is that the new small form factor processors only support SPI boot devices.

Inclusion of the smaller compact form factor in the standard is a major innovation. It means that the most popular type 2 can also be used in applications with space constraints now. COM.0 Rev 2.0 defines its dimensions as 95 x 95 mm. Aside from this, the physical requirements are exactly the same as those of the successful basic form factor.

The company has already been producing modules with these specifications for more than two years under the brand name of microETXexpress, and is claimed to be the first to market. Only recently, it marketed a new product from this module family: the microETXexpress-XL with an Atom Z520PT processor and US15WPT system controller hub. This is a COM Express COM.0 pinout type-2 computer-on-module with a compact form factor, specially developed for use in the E2 industrial temperature range from -40 to +85°C.

The microETXexpress-XL with an Intel Atom Z520PT processor and 1.33 GHz supports up to 2 GB of soldered DDR2 RAM and also has space for an onboard solid-state drive. It makes full use of the bandwidth of the COM Express pinout type 2 connector with 1 x gigabit ethernet, 1 x serial ATA, 1 x PATA, 8 x USB 2.0 and 2 x PCI Express, plus PCI for custom additions.

With the SDVO port, it is simple to implement a DVI output and, together with the 24-bit LVDS single channel, the device presents possibilities for connecting a whole variety of displays and monitors.

The updated specification for COM Express modules is an appropriate response to trends in the sector, because the standard can only be sustainable if it adapts flexibly to new demands such as miniaturisation and graphics performance - as a number of manufacturers have already shown.

The PICMG COM Express COM.0 Rev 2.0 embedded developer is a good indicator of what the future holds. And, if the current PICMG design guide is speedily matched to the new challenges, that will create further confidence.

Following on from the standard for DIMM-PC passed in 1998, the one for ETX that appeared in 2000, and then COM Express type 1-5 published in 2004, COM Express COM.0 Rev 2.0 is now the fourth standardisation for computer-on-modules in 12 years. That may seem a lot at first glance, but if you consider the speed at which processor families have advanced in the same period, this development is rather a sign of deliberation and of the kind of stability and continuity that are seldom seen in this sector.

Rev 2.0 is a standard that has been expanded for the same form factors and matched to the future. The COM Express concept has remained intact - only the pinning has been varied slightly. That’s a firm foundation for the next five or six years.