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Interconnect Wars

Interconnect Wars? Let Peace Prevail with Interconnect Standards

COMs were developed to solve certain problems with SBCs but have some issues of their own when developing the needed carrier boards for I/O. A combination of standard interconnects may bring a solution for painlessly mating CPU to I/O.

ROBERT A. BURCKLE AND COLIN MCCRACKEN, WINSYSTEMS AND ADLINK TECHNOLOGY

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Size, cost, performance, application software development and debugging tools, wired or wireless networking, power budget, time-to-market, temperature range, expansion flexibility, RoHS compliance, ruggedness, interfacing to other peripherals, cabling, packaging, documentation, quality, technical support, certifications–whew! System designers are faced with numerous large and small decisions that impact the success or failure of their projects.

Fortunately for the embedded computing element, there are a variety of options for off-the-shelf, standard single board computers (SBCs) and Computer-On-Modules (COMs) available in all shapes and sizes. Each has its own technical advantages and limitations. The good news is that an engineering team does not have the challenge of creating a custom computer, but rather they can start with a known-good design and focus on their core competency–the application. Therefore, the question facing designers is, “What architecture and form factor should I choose as the embedded computer for my project?”

SBCs, COMs and Small Form Factors

Designers have turned toward industry standards and consequently changed from in-house proprietary designs to commercially available embedded x86-compatible products. This shift in design methodology brings well-designed, technologically advanced system components at reasonable prices, complete with device drivers and application software building blocks and tools. The amount of time available to develop an embedded system has decreased significantly with times now as short as 3 to 6 months. The popularity of embedded PCs lies in the ability of a user to buy off-the-shelf hardware and customize it with software. The result is a better product developed in a shorter period of time.

Over the last decade many different standard board form factors have emerged in the embedded industry for use in industrial, Mil/COTS, communications, transportation, instrumentation and homeland security applications. Each form factor standard defines its size, mechanical dimensions, input/output capability and expansion capabilities with other boards, mounting and certain other electrical characteristics. Many of these different form factors are managed by an industry consortium or manufacturing group of interested companies so as to monitor, improve and promulgate open standards worldwide. Some of these groups charge for the standards documentation while others are free. Independently managed groups include PICMG (www.picmg.org), VITA (www.vita.com), SFF-SIG (www.sff-sig.org), PC/104 Consortium (www.pc104.org) and others.

Many of the popular small form factors (SFFs) are shown in Figure 1. For a given form factor, a board that includes the processor, memory and peripheral controllers can be classified as either a Single Board Computer (SBC) or Computer-on-Module (COM). A COM is not a single board computer, but a system module “component” that must be plugged into a baseboard or “carrier card.” Historically, standard form factors and expansion interfaces have been unnecessarily intertwined, creating barriers to migrating from SBCs to COMs for example. Both have similar features and functions; however, they differ in how they are powered, how onboard I/O is brought out to the “real world” and how the system is expanded with additional I/O.

Applying and Managing Power

SBCs are powered directly from a power supply, whether single voltage DC, wide input range DC, multiple DC supplies with power button control (ATX-style), or other. A power cable connects the power source to the SBC. On the other hand, COMs are powered through many small fine-pitch pins on the same surface-mount connector(s) as the bus and I/O signals that run between the COM and the baseboard. Therefore, a COM cannot run by itself in a system; it must be part of a minimum two-board solution. The power connector resides somewhere on the baseboard. Enough connector pins must be allocated for power and ground nodes to present low series resistance and inductance for good DC and AC characteristics, to meet bulk current load requirements, and to keep EMI emissions and susceptibility in check.

Power management using the Advanced Configuration and Power Interface (ACPI) specification is trickier with COM architectures, as the original standard was developed for motherboards that contain a BIOS, OS, chipset, power connector, RAM, video and disk interfaces all on the same board. Whereas embedded SBCs handle most or all of the issues depending on the intended level of support, COMs that are loosely defined shift the burden from COM supplier to OEM customer to get their custom baseboard working with the “black box” module.

I/O Considerations

With an SBC, I/O is brought off the board through a set of connectors, and cables are attached directly to them. In some cases, these connectors are “PC-style” allowing a LAN cable, printer, keyboard or USB device to connect directly to the board. In other cases, these connectors are pin-headers, requiring a transition cable with another connector at the end that mounts to an enclosure bulkhead or a pin-header that connects to another PCB. In either case, the SBC is a fixed form factor and engineers must design their enclosures and cable schemes to match that of the SBC. An SBC can be used in a stand-alone configuration or expanded with off-the-shelf or custom designed I/O boards.

With a COM module, I/O is brought to a baseboard that is developed by the OEM or by a third-party design house commissioned by the OEM. This custom baseboard is a size that best fits the application and its enclosure or packaging requirements. This means that both the style and location of the I/O connectors match the mechanical constraints of the application in question.

Using a COM card is great for applications where the creation of a custom carrier card is not considered a handicap due to cost or time constraints. However, for most low- and medium-volume applications, SBCs are the better choice, alleviating NRE costs or design resources, risk and time-to-market.

Expansion Bus Considerations

In terms of how to attach additional circuitry to system buses, COMs and SBCs are similar, despite physical connector differences (slots versus pin-in-socket stacks versus SMT all-in-one connectors). Popular PC architecture buses include PCI and ISA, with PCI Express poised to replace PCI due to its higher throughput while maintaining software compatibility. ISA may be gradually fading away, with several low-speed interfaces in the modern PC chipsets now available to help migrate legacy peripherals and custom logic.

In the realm of buses, the greatest similarity between SBCs and COMs exists between the PC/104-Plus bus (PCI and ISA) and the ETX module de facto standard (which also offers PCI and ISA). Outside of those two, SBCs and COMs tend to diverge quickly. With the all-in-one COM connector approach, supporting newer interfaces like PCI Express, LPC and I2C becomes a simple matter of assigning pins within the chosen connector. Industrial motherboards take an analogous approach by using standard desktop PCI Express, PCI and ISA slots, where expansion cards plug in vertically (at a right angle to the motherboard). This puts pressure on the overall size, but does tend to spread the heat dissipation evenly around a large volume.

Stackable SBCs have lagged due to divergent philosophies about what to support going forward, since tiny small form factor boards do not have the space to support everything. Tough choices are made by architects with different application priorities in mind. Furthermore, in the small form factor realm, thermal issues get complicated very quickly.

Meeting in the Middle

Given the expense, time and risk of developing a custom baseboard, many embedded system OEMs prefer to start prototyping with existing standard SBCs. In some cases, enclosures were designed around legacy or EOL single board computers based on the EBX form factor, for example. COMs offer the ability to easily replace obsolete processors and chipsets in order to keep systems in production with minimal impact. A hybrid approach using COMs on standard-form-factor production baseboards avoids the tasks of turning a huge ATX-style reference design into a custom small carrier. The off-the-shelf EBX-size baseboard shown in Figure 2 accepts COM Express modules, forming a hybrid SBC+COM solution intended to drop into existing EBX-based applications. Alternatively it serves as a small form factor baseboard reference design when volumes justify a custom baseboard.

One drawback of existing COMs defined by true standards bodies 3-5 years ago is that the modules are too large to fit on small form factor SBCs without blocking the expansion bus interfaces. The baseboard in Figure 2 supports many features, but has a PCI Express x1 slot for expansion instead of a stackable interface. A second example is the design approach shown in Figure 3, which places the COM on the bottom of the EPIC form factor baseboard so that the top side can have a PC/104-Plus expansion interface without being obstructed by the COM module. This works well for new designs, but could require minor modifications to existing enclosures to drop in.

However, with the advent of tiny, ultra-low-power processors like Intel’s Atom and VIA’s Nano CPUs, it is now possible to define a new, smaller COM standard so that the module can fit easily on the top side of the baseboard without bumping into expansion I/O cards whose locations were previously defined by EBX and EPIC form factor specifications. The challenge is met by a new standard called COMIT from the Small Form Factor Special Interest Group (SFF-SIG). “COMIT” stands for Computer-On-Module Interconnect Technology, and can be implemented on modules as small as 62 x 75 mm as shown in Figure 4.

Examining the Atomic Structure

The COMIT module at the left edge of Figure 4 steers clear of the vertical space needed for expansion I/O cards. COMIT takes on the role of a pluggable CPU core, much like a Socket 370 or Socket 478 processor. COMIT is a ground-up new computer-on-module architecture, defined by rugged SBC vendors for rugged SBC applications, that targets the space-efficient, low-power Atom and Nano processors. A rugged connector pair from Samtec has been selected to live up to the quality and reliability demands of military, avionics, transportation and industrial applications.

Another new standard from SFF-SIG is the SBC expansion interface called “SUMIT,” for Stackable Unified Module Interconnect Technology. COMIT allows newer x86 interfaces like PCI Express, LPC and SPI to go to the baseboard, and a mid-size SBC form factor can add circuitry like Gigabit LAN and legacy serial ports and then pass the rest of the unused resources and shared buses to the SUMIT interface where off-the-shelf and custom I/O cards can be added. The SUMIT-A and SUMIT-B connectors are at the center of the board in Figure 4.

Worst case, rounding out the I/O means designing a simple SUMIT card to some standard small dimension, which is much simpler than taking on the power supply, power management and termination of standard I/O that are unintended burdens passed along to the OEM by COM manufacturers. The value of being up and running on day one is “priceless.”

COMs can enable SBCs, showing more collaboration than competition between the two approaches. COMIT is designed to work with EBX, EPIC or custom form factors to “right-size” a board and its processing power to an application. It also helps to future-proof the system design due to the “socket” approach and philosophy of the CPU core element.

SBCs and COM products coexist peacefully in a growing off-the-shelf board market where OEM design expertise, design time frames and design and product costs drive solutions either to the COM side or the SBC side. The real question is, “How much processing power, bandwidth and expansion capability is needed for a small form factor solution for your embedded application?” Computer-on-Module products are typically used for higher volume applications where the designer can benefit from implementing a custom I/O board to minimize or eliminate cabling and reduce assembly costs. A COM module must be mounted on the baseboard for all additional I/O features and connections, just like a large socketed component.

However, a custom baseboard design requires an in-house design team or contract design for the development, which means added cost, more time and limited flexibility for new or additional requirements (sometimes known as marketing “improvements”) for the applications. This expense must be amortized over a large volume of boards to be cost-effective. Modern board-to-board interconnects and off-the-shelf (standard product) baseboards are making it easier for OEMs to get the benefits of both SBC and COM architectures. Together, COMIT and SUMIT once again make it possible to use off-the-shelf components when volumes are modest, with a path toward custom baseboards later when volumes grow. Information about the new standards can be found at www.sff-sig.org.

WinSystems
Arlington, TX.
(817) 274-7553.
[www.winsystems.com].

ADLINK Technology
San Jose, CA.
(408) 360-0200.
[www.adlinktech.com].