COM Express Revision 2.0: What’s new in the latest specification?

Additionally, the I²C bus receives two small updates. It now has the ability to support multiple I²C masters and its main power rail was moved from nonstandby to standby power. This transition has become necessary because of the optional support of a carrier EEPROM that stores information about carrier board capabilities (e.g. amount of PCIe slots and lane configurations). The respective module may be able to read this information before powering up and pre-configure itself to fit best to the carrier board's abilities.

Other than the COM Express module specification, the COM Express Embedded EEPROM specification (EeeP) is available free of charge from the PICMG web page (http://www.picmg.org/pdf/PICMG_EeeP_R1_0.pdf).

This EeeP addresses the following issues:

• Device revision identification;
• Standardized cross vendor cross platform I²C EEPROM data storage and retrieval (including product, marketing, and branding information);
• Generic approach to data storage;
• Vendor specific data storage in a multivendor environment;
• Consolidation of data currently stored in multiple discrete devices, into 1 device (e.g. PICMG EEPROM, OEM specific EEPROM, Display Device EEPROM);
• Maximum code reuse;
• Improve EEPROM access characteristics and increase EEPROM device selection flexibility.

In addition to the existing pinouts of Type 1 through 5, Rev 2.0 of the COM Express specification adds two new pinout definitions called Type 6 and Type 10.

New Single Connector Pinout Type 10

The Type 10 pinout is a refresh of Type 1 and uses a single 220 pin connector. Type 10 COMs take advantage of one modern display connection, which can either be used as TMDS (HDMI/ DVI), DisplayPort or SDVO, and have had the number of LVDS channels halved so only one LVDS channel is available now. An additional Type 10 pin (formerly a 12V pin) on the A-B connector allows the carrier board to distinguish between Rev 1.0 Type 1, Rev. 2.0 Type 1 and Type 10 modules.

Up to this point, the Type 2 pinout is the most commonly available COM Express pinout. The root of its success may be the variety of interfaces, such as PCI, PCIe, IDE, SATA and USB 2.0, which also correspond best with most of the current x86 platforms. The rarely used Types 3 to 5, Type 4 and Type 5 are more less specialized for single industries that require more Ethernet connections or more PCIe lanes.

New Connector Pinout Type 6

The newly defined Type 6 builds on the same success factors as Type 2 but also takes into consideration future interfaces. The A-B connector has nearly the same pinout as Type 2 Rev. 2.0. Only some reserved pins are used for UART, FAN (PWM), Lid and Sleep signals. The UART pins (2x SER_Tx/Rx) were added for debugging purposes (e.g. console redirection) due to their simplicity. Additionally, a new pin has been defined to indicate physical presence to an optional TPM chip located on the module.

The fact that these pins are used on Rev 1.0 systems for supplying the module with power requires that module and carrier board designers spend some additional time to ensure that these interfaces can withstand a connection to a 12 V rail. This provides the ability to connect a Rev. 1.0 module to a Rev 2.0 carrier board, or vice versa.

In contrast to the previously mentioned small changes on the A-B connector, the C-D connector has been completely redefined. It gets rid of parallel legacy interfaces such as PCI and IDE and provides support for current and future interfaces. These changes can be separated into 3 main categories:

• Up to four USB 3.0 Interfaces;
• Up to three Digital Display Interfaces;
• Up to 2 additional PCIe 2.0 lanes.

Digital Display Interfaces (DDI)

Digital Display Interface is a sum of differential pairs that carry display data. The new Type 6 pinout allows the usage of up to 3 independent DDIs. The first DDI channel is capable of supporting SDVO, Display Port and TMDS. TMDS can be used as HDMI or DVI, depending on the I/O connector. The 2nd and 3rd DDI channels only support TMDS and DisplayPort.

As a result of this general definition, the decision about the proper interface for an application is up to the carrier board designer. When using a DDI channel as DisplayPort, it can easily be connected from the COM Express connector straight through to the I/O connector on the carrier board, whereas a TMDS output requires an additional level shifter on the carrier board because of the different signal levels.

Some vendors, including congatec, recognized the potential of these Digital Display Interfaces and offer them together with the latest Type 2 Rev. 1.0 compliant modules, but to do so there must be a trade-off, which means giving up the PCIe Graphics (PEG) port. Type 6 modules like the conga-TM77 support PEG Port and DDI allowing for maximum connectivity.