Kishan Jainandunsing
(February 2006)

In this column we explain the COM Express specification in a series of tutorials. Last month we gave a brief introduction to COM Express and the COM methodology in general. This month we provide a high-level overview of the specification.

Part II - High Level Functional Overview

The Evolution Towards Low Voltage Serial Differential Interfaces

The COM Express specification specifically exploits the computer industry’s shift towards low voltage serial differential (or LVDS) interfaces. LVDS is superior over traditional non-differential voltage signaling schemes, in that it offers:

• Superior noise immunity
• Longer interconnects
• Easier signal routing on a PCB
• Minimal consumption of PCB real estate

Figure 1 below illustrates how COM technology evolved from parallel interfaces, including PCI bus and ATA, to COM Express with all LVDS interfaces.

Figure 1. Evolution from legacy and processor platform-bias to all-LVDS and processor platform-agnostic

LVDS and SDVO Interfaces
LCD interfaces were the first to migrate from a parallel to a LVDS signaling interface. Therefore, adoption had already taken place in legacy COM products before and COM Express continues incorporating LCD LVDS interfaces.

COM Express further incorporates SDVO (Serial Digital Video Out) interfaces. These evolved from a parallel version, called DVO (Digital Video Out). This type of interface provides flexibility in system video outputs, as it can be converted to any combination of DVI (Digital Visual Interface) connections for flat panel monitors and any HD/SDTV and conventional type video output, such as HDMI, S-Video, Component Video or CVBS composite video. In addition an SDVO interface can also be converted to LCD LVDS interfaces or any other proprietary video or data transmission interfaces.

Serial ATA and Serial Attached SCSI Interfaces
COM Express replaces Parallel ATA (PATA or IDE) and SCSI interfaces with Serial ATA (SATA) and Serial Attached SCSI (SAS) links, respectively, while maintaining an option for PATA to facilitate CompactFlash™ - widely used in embedded systems - and ATA optical drives. COM Express has been specified to accommodate not only today’s SATA and SAS speeds, but also future speeds as defined or in definition at present.

PCI Express Interfaces
COM Express replaces pin-prolific PCI, PCI-X and AGP interfaces with pin-anemic PCI Express links, while maintaining an option for a PCI bus interface to accommodate the slower pace at which the embedded and the semiconductor industry replaces legacy technology. With COM Express computer-on-modules for the first time are enabled with a high-speed graphics interface in the form of PCI Express Graphics, which replaces the AGP interface. The sheer amount of pins needed for an AGP interface made this impractical and not economical in legacy products.

USB Interfaces
COM Express replaces parallel ports, keyboard, mouse, floppy disk and serial ports by USB interfaces. The adoption and proliferation of USB is progressing at such a fast pace, with costs having dropped so low that the legacy interfaces no longer hold an advantage.

Processor architecture Agnosticism
COM Express is processor architecture agnostic by being only focused on system level interfaces. This means that x86 processors and companion chipsets are as good a choice as RISC and other processors and companion chipsets. The choice of processor architecture and companion chipset is therefore application driven and not dictated by the COM Express specification.

Absolute Performance and Bandwidth

As embedded applications become increasingly more demanding, legacy COM solutions are starting to lag in performance and I/O. COM Express modules surpass legacy solutions in both I/O- and performance-density. This is illustrated in Figure 2.

Figure 2. Performance and IO density

Due to the pin-out and PCB-area efficiencies of LVDS signaling, COM Express modules pack significantly more I/O bandwidth in the same or smaller footprints than possible with parallel ATA, PCI, PCI-X and AGP. And since PCI Express’ scalable bandwidth contributes to a higher performance overall of the host subsystem, performance density is much higher than achievable by legacy COM products.

COM Express Functional Blocks

A COM Express module can be thought consisting of the functional blocks shown in Figure 3.

Figure 3. COM Express functional units

At the core resides the CPU and companion chipset. Surrounding these are the functional interfaces defined by the specification. These include Ethernet ports, SATA/SAS links, PCI Express links, graphics links, such as LVDS, SDVO and PCI Express Graphics, USB and TV-Out. The legacy interfaces include PCI, LPC and IDE (PATA).

The power management functional block supports ACPI or ACPI-like interfaces, such as Save-To-RAM, Save-To-Disk, Suspend, Sleep and Wake. The system management functional block supports watchdog timer, SMBus, I2C and other system management interfaces.

Finally, system memory (expansion) is also part of a COM Express module. This further isolates a design that utilizes a COM Express module from memory technology evolutions (from DDR to DDR-2 to fully buffered DIMM, etc.) and changes in chipset memory architecture support, such as single versus dual channel memory architectures.

Table 1 summarizes the range of capabilities for the various blocks in Figure 3.

Functional Block	Capabilities
PCI Express	Up to 32 lanes, shared with 16 PCI Express Graphics lanes and 2 ExpressCard lanes
SATA/SAS	Up to 4 Serial ATA or Serial Attached SCSI links
LAN	Up to 3 10/100/1000Base-T links (PHY included)
USB	Up to 8 USB 2.0 ports
Memory	On-board or expansion sockets, single or dual channel
Graphics	Up to 2 LVDS channels for LCD panels Up to 2 SDVO channels for DVI/TMDS flat-panel displays x16 PCI Express Graphics link for external GPUs (multiplexed with SDVO) TV-Out scan conversion
CPU & chipset	CISC, RISC, VLIW, etc.
System Management	I2C, watchdog timer, SMBus
Power Management	Save-To-RAM, Save-To-Disk, Suspend, Sleep, Wake

Table 1. Summary of capabilities defined for COM Express functional blocks

Footprint Types

COM Express defines two footprint types or form factors - Basic and Extended. The Basic form factor fits applications where small size is of higher importance than performance. The Extended form factor fits applications where the opposite is the case. The Extended form factor allows for higher performance implementations as it has room to accommodate dual CPU and dual channel memory architecture designs, as well as larger memory expansion footprints. The two module form factors are illustrated in Figure 4.

Figure 4. Basic and Extended Form Factor footprints

As shown in Figure 4, the two footprints have overlapping assembly holes and mezzanine connector positions. This was intentionally done for interchangeability between modules of different footprints. For this reason the pin-outs on the mezzanine connectors are the same for both footprints.

Heath Spreader Definitions

COM Express specifies an optional heat spreader for the Basic and the Extended footprints. This heat spreader promotes interchangeability between similar modules from different manufacturers with a third party’s thermal solution. The heat spreader is defined for a certain area only. Outside this area variations are allowed between modules from different manufacturers. Figure 5 illustrates the heat spreader definitions for both footprints.

(a) Basic Form Factor

(b) Extended Form Factor

Figure 5. Basic and Extended Form Factor heat spreaders

The COM Express standard specifies that the main assembly holes should allow assembly screws to pass from top to bottom to create an assembly that can withstand very high levels of abuse. The specification recommends that the heat spreaders have independent assembly points to the module’s PCB to facilitate that modules can be assembled and disassembled without causing the heat spreaders to detach.

The extra assembly holes on the heat spreaders are specified as attachment points with specified threading, intended for attachment of third party thermal solutions. These assembly holes are not specified to let assembly screws pass through the module’s PCB.

Height Definitions

The presence of a heat spreader requires that a specific height requirement is met. This height is measured from the top of the carrier board's PCB to the top of the module's heat spreader and is dependent on the height of the mating connector on the carrier board. Two variations of this connector are specified, resulting in heights of 18mm and 21mm. See Figure 6.

Figure 6. Height definitions with heat spreader assembled

Mezzanine Connectors

The mezzanine connectors on a COM Express module and the mating connectors on the carrier board are configured with either 220 or 440 pins. A 220-pin configuration is referred as connector row AB and a 440-pin configuration is referred as connector rows AB plus CD.

Figure 7 . Mezzanine connector configurations

Connector row AB is required and provides the following functionality:

• Up to 6 PCI Express lanes
• Up to 4 SATA or SAS links
• Up to 2 LVDS channels
• VGA output
• TV scan conversion output
• One Ethernet interface
• LPC bus
• System management
• Power management
• Power and ground

Connector row CD is optional and provides the following functionality:

• 32-bit, 33/66MHz PCI bus or 10 additional PCI Express lanes
• One parallel ATA channel or up to 2 additional Ethernet interfaces
• One x16 PCI Express link or 2 SDVO channels or 16 PCI Express lanes
• Additional power and ground

The connectors support the data transfer rates of PCI Express Generation 2, which doubles current PCI Express Generation 1 transmission speeds to 5GHz. The Ethernet interfaces are currently defined up to 1000Base-T. However, the specification has a provision for future definition of a single 10Gbase-TX interface, which replaces two 1000Base-T interfaces.

Pin-out Types

The COM Express standard specifies 5 different pin-out types as summarized in Table 2.

Table 2. Five different pin-out types

Pin-out types 1 and 5 are completely legacy free. Pin-out type 2 provides PCI and IDE legacy interfaces side by side with PCI Express and SATA (or SAS). Pin-out type 3 exchanges the IDE signals for 2 additional Ethernet ports. Finally, pin-out type 4 exchanges the PCI bus for 10 additional PCI Express lanes.

Only pin-out type 1 modules are interchangeable with all other pin-out type modules. Whereas, all other pin-out type modules are not interchangeable with each other. The specification provides a mechanism to prevent the accidental interchange of incompatible modules. Three pins are allocated on the CD mezzanine connector rows, which are strapped high or low on the carrier board to encode the module pin-out type that the carrier board is compatible with. All module pin-out types, except modules of pin-out type 1, should implement logic that prevents powering up the module in case of a mismatch between module and carrier board pin-out type encoding.

PICMG COM Express Logos

Modules that are in compliance with the COM Express specification may be labeled by themanufacturer with a PICMG COM Express compliance logo. See Figure 8.

Figure 8 . Official COM Express logos

End of Part II