Jeff Munch Column
COM Express gains a major interoperability boost
New PICMG Carrier Design Guide (CDG) enables carriers that work with multiple vendors' modules
If you have struggled to get your carrier board working with a second COM Express module, or have held off designing due to the buzz about the issues involved, good news has arrived. More than a dozen leading suppliers have set aside their differences to collaborate for a year on a design guide aimed at module interoperability. PICMG is publishing the Carrier Design Guide (CDG) this fall with critical information and example circuits to improve your module sourcing options.
The CDG contains reference schematics and explanations to help implement common peripherals on unique carrier board designs. In some cases, this means terminating a module's integrated I/O. Otherwise, PC architecture buses such as PCI Express, PCI, LPC, and SMBus can be extended to support other peripherals and standard expansion slots on the carrier board. CDG complements the COM.0 specification for COM Express modules. It does not replace or obsolete COM.0. However, it reinforces many of the details of COM.0, and acts as a "hammer" to enforce ongoing module consistency.
Merging onto the Express Lane
The general-purpose PCI Express lanes can be grouped (x1, x4, and sometimes x2) in combinations that total six lanes. These are Southbridge lanes, not the x16 PCI Express Graphics (PEG) interface from the Northbridge. The CDG reinforces the COM.0 order for using (filling) lanes. In addition, the CDG shows how to handle PCI Express (PCIe) clock buffering when more than one PCIe device or slot is used on the carrier, and what signals to connect for hot swap of slot cards. You will also learn about advanced features like SDVO and ADD2 cards in detail. High-speed differential pair signals require special routing considerations, and the CDG offers handy guidelines.
The industry-standard Advanced Configuration and Power Interface (ACPI) specification defines power-saving levels when the system is not operating. Implementation across COM Express modules has been inconsistent in the past. The CDG addresses this point by defining the on/off state of up to eight power rails (nets) that may be present in the system, including the separate standby rails that are powered when the main rails are at zero volts. In addition, suspend status signals and wake up events are defined more thoroughly. The CDG describes how to provide power to peripherals so that they continue to operate as desired during the suspect and soft off states. After all, these peripherals and part of the Southbridge must stay awake while the rest of the system is off, otherwise the system won't wake up without a complete re-boot
Free to be legacy-free?
Standard desktop connectors like SATA, LAN, and USB seem innocuous, given that the signals are "free" in the chipset and in standard controllers. But taking into consideration such things as USB over-current and Ethernet transformer tap voltage consistencies will quickly dispel a carrier designer's overconfidence with these circuits. The CDG delves into these topics with enough detail to design a working carrier the first time.
A contentious topic is how to implement serial ports and other "legacy" peripherals within an architecture that is touted as "legacy-free." The CDG does an important job of warning designers about the limited "supported" usage of the Low Pin Count (LPC) bus for such peripherals. Initialization of carrier LPC devices is heavily intertwined with the BIOS on each vendor's module. Instead, designers are advised to use the dynamic driver-loading plug-and-play model of USB, PCI, and PCI Express devices for serial ports and other legacy ports.
The CDG also contains a wealth of "fine points" pertinent to designs. Such information can make the difference between a carrier that operates and one that doesn't. Among the details drilled into are:
- Coupling capacitors
- Bypass capacitor types
- Bulk capacitance on power and standby rails
- Pull-up resistor values
- Series and parallel terminations
- Ferrite beads
- Reset circuits
- Strapping options
A word to the wise
Finally, a bit of caution is in order. While the CDG is thorough in providing designs for interchangeability of COM Express modules, it is absolutely essential to use each module vendor's documentation in order for carrier boards to be designed properly. There is no substitute for proven design methodologies. Designers without much PCI Express experience would be wise to leverage the combined experience of the module and carrier designers who contributed to CDG. Finally, what is not supported or recommended is as important to know as what is supported.
Although it will take some time for the consistencies presented by CDG to ripple through participating vendors' modules, there is no question that PICMG has served the industry well in the publication of this new document.
Jeff Munch is Chair of the AdvancedTCA subcommittee and also CTO of ADLINK Technology. Mr. Munch has more than twenty years experience in the hardware design, software development, and engineering resource management. Before joining ADLINK, Mr. Munch spent five years at Motorola Computer Group as Director of Engineering, and previously, 9 years as vice president of engineering at Pro-Log.