The COM Express specification was first released in 2005. Its main target was, and still is, to define the mandatory requirements of COM Express modules and carrier boards as far as it is necessary to ensure interoperability between the products of different vendors. Nevertheless, with continuous technical progress, there is also the need for adjustments of the common interface — that being the COM Express connector.
Now, more than 5 years after the initial release of the COM Express Specification, the new Revision 2.0 prepares the COM Express standard for the future. So, what changes were made to COM Express Revision 2.0?
Compact Form Factor
Rev 1.0 defined two form factors for COM Express modules: Basic (125mm x 95mm) and Extended Size (155mm x 110mm). Whereas the rarely used extended form factor is especially suited for high-power-consuming modules, e.g. server technology, the more commonly used basic form factor is based on less power consuming mobile and embedded technology. Shortly after the official release of Rev. 1.0, multiple vendors created space-saving 95mm x 95mm modules that were widely known as “Compact” modules. The small size of the Compact modules allows for an easier integration when it comes to low power application with space limitations.
New Interfaces and Major Changes
TV Out is no longer part of the latest revision of the COM Express standard. Since all modern displays and TVs accept digital high bandwidth signals, e.g. HDMI, there is no longer a need for analog video transmission via the outdated TV Out.
Although the support for PCIe Gen. 2 implies no change to the pinout of the COM Express module, it nevertheless affects the layout of the carrier board. PCIe Gen. 2 provides twice the speed of PCIe Gen. 1, which is achieved by doubling the frequency. To ensure sufficient signal quality at the receiver, designers have to take care of higher losses in the PCB, increased crosstalk etc. Furthermore, the differential PCIe clock signal and the PCIe clock buffer must be able to provide the increased jitter requirements of PCIe Gen. 2.
Some changes made by the transition from Rev. 1.0 to Rev. 2.0 are more or less due to the evolution of interfaces. With Rev. 2.0, it's possible to optionally use the existing GPIO pins as a SDIO interface. Beside the typical mass storage SD and SDHC cards, the SDIO interface is also capable of handling I/O cards such as WLAN, Bluetooth and GPS, based on the same size as a SD Card. USB port 7 (last port) can now be used as USB client port, so that, if supported by the module, the whole COM Express system can act as a USB device.
Another update is the official support of HD Audio. Due to the lack of AC'97 support on modern embedded platforms, HD Audio was already used with Rev. 1.0 compliant modules, although it is not part of the Rev. 1.0 COM Express Specification. With the introduction of Rev. 2.0, the former AC'97 pins of the COM Express connector can now carry either HD Audio or AC'97 signals, depending on the module vendor. Although both interfaces, AC'97 and HD Audio, use the same pins, they are not compatible due to different protocols.
In order to allocate pins for additional signals and future extensions, ten 12V pins were reclaimed. This doesn't limit COM designers because the new maximum possible power consumption of 68W for single connector modules and 137W for dual connector modules is still more than enough.