COM Express Design Guide improves interoperability

It is widely accepted that COM Express is the most important Computer-On-Module (COM) standard. Since its introduction in 2005, countless applications have taken advantage of the modular approach. The idea behind the standard was to separate the customized functions on the carrier board from the standard PC features located on the COM Express module, enabling the choice of computing performance class as well as independence from a single vendor.

However, experience has shown that this is not always reality. In some cases, malfunctions occur when using COMs from different vendors on the same carrier board. The root cause usually can be found in the implementation of the customized carrier board. To avoid those kinds of faults, PICMG decided to create a vendor-independent COM Express Design Guide (COMDG) for carrier boards.

The development of the COMDG is an excellent example of cooperation among multiple COM Express vendors. As an editor of the Design Guide, I started by merging the best parts of four different COM Express design guides from multiple vendors into one document. After that, the 15 participating companies reviewed and enhanced the technical details and added some new chapters. Engineers discussed the technical details during countless phone conferences. After 18 months of hard work, all of the participating companies were proud to complete this impressive 160-page document (see sidebar).

Sidebar 1: About the COM Express Design Guide (click graphic to zoom by 1.8x)

It is important to note that the COMDG is not a documented specification, but more like a cookbook for carrier board designers. The COM Express specification is still the main reference for module designers; however, the Design Guide is the most important information source when carrier boards are created.

The COMDG describes all defined COM Express interfaces. First, the single signal lanes are described, then one or more example schematics are shown. Each interface chapter closes with detailed routing considerations for PCB designers. This also includes relevant information about the maximum allowed trace length, definition of trace width, and impedance definitions. Some useful PCB stack configurations are also described in detail to help ease PCB design.

While the COM Express specification does not describe power sequencing, a whole chapter (including examples) in the COMDG provides clear information on how to create carrier board designs with maximum interoperability. In total, the COMDG contains about 50 example schematics. The examples range from a simple, but important, real-time clock battery implementation with UL-compliant reverse current protection to two different implementations of Low Pin Count (LPC) SuperIO chips, as well as some different PCI Express device implementations.

The COMDG is a major step toward improving COM Express module interoperability. The COM Express specification itself will also continue to improve. New graphics interfaces and higher performance of future generations of PCI Express, USB, and SATA will require changes to the specification and updates to the COMDG. PICMG has already started a technical subcommittee to address these items, keeping COM Express advancing for embedded designers. ➤

Christian Eder is VP of marketing and cofounder of congatec AG, based in Deggendorf, Germany. He has nearly 20 years of experience in technology, including management positions at CPCI Systems (Force Computers), JUMPtec AG, and Kontron AG. Christian received his degree in Electrical Engineering from the University of Applied Sciences, Regensburg, Germany.

congatec AG +49 991 2700-0 christian.eder@congatec.com www.congatec.com