COM Express: More features, more to consider

The COM Express standard has been broadly adopted since it came into existence, and there is a variety of compliant products available today. COM Express 2.0, currently in development within the COM.0 R2.0 PICMG subcommittee, is being updated with backward compatibility in mind. The revised standard will address new and evolving digital display interfaces, legacy obsolescence issues, Serial Peripheral Interface (SPI), and support for HD Audio.

By choosing the ideal, off-the-shelf Computer-On-Module (COM) to fit their needs, organizations can reduce costs, shorten design cycles, and remain competitive. While the basic premise of using COM modules as compute subsystems won’t change when Rev 2.0 is rolled out later this year, it will affect carrier boards and current Type 2 COM designs, which are the most widely used COMs today. COM Express 2.0 will build on the success of these modules by broadening the list of supported interfaces needed to accommodate newer technologies. The refreshed standard will ensure that carriers and modules being designed today have a solid migration path from the current generation of COM Express modules to COM Express 2.0 compliance.

However, many applications have specific I/O requirements satisfied by carrier boards. With more I/O functionality moving to COMs, design teams need to carefully evaluate their carrier board design I/O choices.

Development teams will continue to focus their energies on designing carrier boards that feature application-specific, custom I/O unique to their end products while using a COM Express module for the standardized processing complex. Rev 2.0 continues the path set out in Rev 1.0 that planned for the phased obsolescence of PCI and IDE interfaces and multiplexes newer interface technologies in their place. These technologies include digital display interfaces and updated audio and peripheral interfaces, as well as updated interfaces to address obsolescence issues associated with older audio and BIOS flash interfaces.

A primary change in Rev 2.0 is support for digital display interfaces such as High-Definition Multimedia Interface (HDMI) and DisplayPort. HDMI provides an audio/video interface that can carry TV and PC video signals, including HD video, along with an accompanying digital audio stream. Standardized by the Video Electronics Standards Association, DisplayPort is a royalty-free interface between a computer and monitor. Unlike earlier digital display interfaces, DisplayPort is capable of driving external displays and internal display panels, both of which are important to COM Express applications. Support for these new interfaces ensures that COM Express adopters can easily transition from older interfaces to standardized digital display interfaces of the future.

The USB standard’s recent migration from Revision 2.0 to 3.0 offers 10 times faster transfer rates. USB 3.0 adds a new SuperSpeed mode, which uses an additional set of bidirectional differential pairs while maintaining backward compatibility with USB-enabled PCs and peripherals currently in use. Rev 2.0 of the COM Express standard adds support for USB 3.0 and the continued performance evolution of serial device connectivity enabled by USB 3.0.

The interface for the BIOS flash device in the first generation of COM Express modules was typically a firmware hub attached via the Low Pin Count (LPC) bus. Firmware hubs are quickly being replaced with flash devices attached via a SPI bus. Adding a SPI interface in Rev 2.0 of the specification while retaining support for the LPC interface ensures a smooth transition for COM Express users. The LPC interface is preserved in Rev 2.0, serving as a simple low-speed interconnect for carrier devices beyond the firmware hub. Aside from addressing firmware hub obsolescence, adding a SPI interface also allows for larger BIOS flash devices and simpler carrier implementations.

Another minor change is related to audio specifications that have progressed from Audio Codec ’97 (AC’97) to HD Audio, which allows more channels to be played back at higher quality. In the revised standard, both specifications are supported and map onto the same set of pins, even though the functionality in HD Audio is more robust. Supporting HD Audio is a requirement for rich multimedia applications.

COM Express 1.0 led to mass market adoption of Type 2-compliant modules. Version 2.0 builds on this success and promises to extend it into the future. Support for new technologies anticipated with next-generation COM Express modules means that designers will have more options to consider when creating customized carrier boards that will house these feature-packed modules. ➤

Bob Pebly is an engineering Fellow with RadiSys Corporation, based in Beaverton, Oregon. He has more than 25 years of experience in the embedded computing and telecommunications industries, including 15 years with IBM. Bob serves as RadiSys’ primary representative in PICMG, and was secretary of the COM Express R1.0 PICMG subcommittee and a contributor to R2.0. He holds a BSCS from Pennsylvania State University.

RadiSys bob.pebly@radisys.com www.radisys.com