Express104 Defines the Next Generation of Small, Stackable Embedded Computing Modules

Addressing a new generation of processors, demands for low power, compact size and modern, high-speed interconnect technology, Express104 is the first form factor to put the new SUMIT interconnects to work for embedded applications.


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The embedded computing marketplace is as fragmented as it is diverse, often creating painful “either/or” decisions for both suppliers and users of plug-in I/O boards. A new connector specification called the Stackable Unified Module Interconnect Technology (SUMIT) has been introduced to deliver a form-factor-independent interconnect that accommodates I/O boards small and large, fast and slow, simple and complex. Express104, which was introduced at the Embedded Systems Conference in April 2008, is the first application of SUMIT technology. Express104 is a small, stackable, cost-effective embedded board architecture designed for I/O-centric as well as high-performance applications. It is a bridge to the future with a legacy option for supporting the vast installed base of PC/104 modules and enclosures.

SUMIT is the creation of a new trade organization called the Small Form Factor Special Interest Group (SFF-SIG). It was formed in the fall of 2007 with a charter to develop and promote specifications to help manufacturers and integrators of electronic equipment reduce the overall size of their next-generation systems. The philosophy of the SFF-SIG is to embrace the latest technologies available for long-lifecycle systems while maintaining legacy compatibility to enable easy transitions to new chipset interfaces. The SIG’s specifications are intended to accommodate today’s and tomorrow’s lower-power and more highly integrated processors, chipsets and memory based on 90nm, 65nm and 45nm processes, as well as robust higher-density connectors and the high-speed serial interfaces that are replacing the parallel interfaces of the past.

Version 1.0 of SUMIT was released April 15, 2008, the same time as the Express104 specification. Express104 is defined as a stackable 90 mm x 96 mm size board with two 52-pin, high-speed connectors capable of supporting current and future PCI Express and USB data rates as well as other moderate speed interfaces for I/O expansion. It provides the basis for an I/O-centric, multi-board solution that is processor architecture and chipset independent. The specification contains support for very low power and ultra mobile processors that support “green” computing initiatives for compact embedded systems.

Express104 supports the following I/O connectivity technologies:

• Two PCI Express x1 links

• One PCI Express x4 link

• Presence detect signals to allow SBCs to disable unused clocks

• Three USB channels

• Two SPI/uWire channels

• System Management Bus (SMbus/I²C)

• Low Pin Count (LPC) bus with SERIRQ

• PC/104 (Legacy Option)

The SERIRQ signal is necessary to support ISA-style interrupts, for example with an external ISA bridge. Some legacy software and I/O still depend heavily on these interrupts, and SUMIT acknowledges the importance of a legacy-friendly architecture.

With all these features, the Express104 Specification enables small, rugged and reliable SBCs and I/O modules for new computer systems that are powerful, easy-to-use, cost-effective and scalable. Low-cost, high-performance computer systems can be built for a variety of different embedded applications. These application areas include transportation, medical, industrial automation, Mil/COTS, homeland security, energy, and communications sectors.

SUMIT Connector Technology

SUMIT is built on a very small, dense, high-speed connector containing 52 pins arranged in two rows. As shown in Figure 1, a ground blade is incorporated between the rows to help provide the impedance control that is critical for handling high data rates. Additional benefits include EMI performance and DC ground current return. The Samtec QFS/QMS Micro High Speed Series of pin-and-socket connectors, with a fine pin pitch of 0.635 mm (0.0250 inch), meets SUMIT’s requirements for density, signal integrity and ruggedness. Actual signal integrity test results demonstrate that a stack of Express104 modules will support data rates of 5 GTransfers/s, which is required for PCI Express Generation 2 and USB 3.

The connectors are small, rugged and engineered to work with standoffs in a multi-board stack. Taking up a small fraction of the board real-estate required by traditional pin-and-socket connectors, the SUMIT connectors consume only 4% of the total board area of an Express104 card yet offer tremendous bandwidth and I/O connectivity.

There are two major reasons to have two separate 52-pin connectors in SUMIT as opposed to a single two-bank connector. Some of the latest chipsets map very well to the first connector by itself, so SBCs are not penalized for the additional cost and precious board space of pins that are not even used. SUMIT is not only for 20-40-watt three-chip x86 solutions, but for the new sub-10-watt two-chip solutions as well. Secondly, best practices for PCI Express routing and power filtering are achieved with separate connectors and enough space in between for traces and bypass capacitors, versus a two-bank connector that doesn’t satisfy either requirement. Imposing a single two-bank connector would have meant unnecessary compromises.

On a single 52-pin high-speed SUMIT Type A (shorthand: SUMIT-A) connector, Express104 supports one PCI Express x1 (“by one”) link with 2.5 times the bandwidth of the old PCI-104 parallel PCI connector, three high-speed USB 2.0 interfaces, LPC (Low Pin Count) Bus, SPI/uWire and SMBus/I²C Bus signals. A second identical 52-pin SUMIT-B connector supports one additional x1 PCI Express link and one x4 (“by four”) PCI Express link plus additional power, ground and control signals. The total number of pins for both connectors is 104 (hence the name Express104) and its configuration is referred to collectively as SUMIT Type AB (shorthand: SUMIT-AB). Collectively, SUMIT-A and SUMIT-B unify the I/O requirements across virtually any standard or custom board form factor.

All Express104 expansion modules must include the SUMIT-A connector, which is the one on the upper right in Figure 2. All expansion boards must either include both SUMIT-AB connectors or must include a keep-out area where the Type B connector is located, both top and bottom. This keep-out area has a maximum height restriction of 5.08 mm (0.200 inch). The keep-out area ensures that modules with SUMIT-AB connectors can plug into modules with only a SUMIT-A connector without mechanical interference.

The board-to-board stack height is 15.24 mm (0.600 inch) and is measured from the top of one board to the bottom of the next. This is the mated height of the Samtec QMS/QFS pair, as well as that of the appropriate standoff length for mounting boards together. A total of four 0.125-inch (inside diameter) holes are defined for threaded spacers that are used to provide accurate board separation and rigidity.

The stacking order for the Express104 modules is significant. An Express104 expansion module with a PCIe x4 link must be closest to the root SBC. Next would be an Express104 expansion module with a PCIe x1 link. Stacked above that would be any USB, SPI/uWire, SMBus/I²C and/or LPC modules. Finally any legacy PC/104-compatible (ISA Bus) modules would be on the top of the stack. Alternatively, SBCs can support PC/104 Bus I/O cards below the SBC by using a stack-through connector on the SBC.

PCI Express on Express104 Boards

Routing signals up a board stack can be a challenging task, especially when it comes to high-frequency signals. PCI Express in particular requires careful consideration when laying out a processor or I/O expansion board. In fact, for any extremely high-speed differential signaling environment, the symmetry of the circuit is of utmost importance.

Matching each segment pair length, matching left hand and right hand turns for the pair, placing vias or components symmetrically in the signal path, routing the trace pair symmetrically to these features, and using only the outer layers of the board are critical to minimize impedance, reflection and flight time mismatches that degrade signal quality at these frequencies. Great care has been taken with the design and specification of Express104 to make sure that the high-frequency signals can be easily routed according to the PCI Express guidelines with the fewest number of vias both on the CPU and the I/O cards (Figure3). The PCI-SIG is the primary source for details about rules and best practices.

Also, each PCI Express x1 link utilizes an auto alignment topology routing up the stack in which links that are used by the expansion card are automatically selected and the remaining unused signals are simply shifted to the lower link’s pins on the top connector for use by the next board. This passing of signals “up the bus” is analogous to the way PCI interrupts have been routed in a rotating method on PC motherboards for more than a decade. Cards can use one or more links from their connector while the remaining links are routed to the next board or connector, justified back to the first pin. All expansion modules are exactly the same from the connector pin definition perspective.

One of the design features of an Express104 stacking I/O expansion module is to not require any jumpers for address or slot alignment. This would be required due to the nature of implementing a point-to-point architecture in a self-stacking design compared to previous generations of parallel bus technologies using through-hole stacking connectors. By contrast, Express104 uses a pair of surface mount connectors that allows one or more PCI Express x1 controllers to be mounted on an I/O expansion module. This feature allows automatic link alignment, which eliminates the need for jumpers or special stacking order. Boards not supporting PCI Express simply pass all signals straight up the stack from one connector to another.

An Express104 expansion module with a single x1 PCI Express controller is always wired to Link A on the “bottom” side connector. On the “top” connector, Link A will be wired from Link B on the bottom connector. Link B has no connections on the top connector of a card that consumes and re-aligns PCIe signals (Figure 4).

An Express104 expansion module that consumes the x4 PCI Express link has no connections on the top connector for these pins. The SUMIT-B connector is on the bottom of the module only. PCI Express links are point-to-point only, not multi-drop like legacy parallel PCI. When not using the x4 link, pass the signals straight up the bus from top to bottom. When passing through any PCIe x1 link, associated clock and presence detect pins for that link must also be passed up directly or auto aligned with the parent signals. There is a clock for each link so that all Generation 1 and Generation 2 devices can be supported. SBCs may choose to disable unused clocks to further save power and reduce EMI, a nice feature of SUMIT for “green” computing that is lacking in other board-to-board interconnects.

Legacy Support

Even though Express104 only needs SUMIT connectors, a special configuration has been defined to support PC/104 modules. PC/104 I/O modules are still heavily in use by system OEMs today. There are many I/O cards available that offer a wide variety of I/O interface solutions especially for embedded applications. Since PC/104 is well known and easy to use, a designer can either develop his or her own specialty I/O board in-house or select from among hundreds of off-the-shelf solutions. The reliable, easy-to-interface PC/104 Bus and a great number of various inexpensive I/O modules are worth preserving.

Express104 can maintain legacy support for the vast number of PC/104 standard and custom expansion I/O modules and enclosures available worldwide. This is done by defining the location and use of the legacy PC/104 connector, which maintains its same placement, physical dimensions and mounting holes. However, Express104 has the SUMIT-AB connectors located in the area where the PCI-104 connector was placed for PC/104-Plus and PCI-104-compatible boards and modules. This yields twice the bandwidth in a quarter of the space.

Express104 provides a sensible transition from current-generation parallel bus-based systems to next-generation systems based on high-speed, point-to-point interconnects such as PCI Express. Further, with Express104, there’s no need to make a sharp break from the past when it’s not necessary. This lets equipment makers continue to use legacy hardware and software while features and performance capabilities are still fine for their application.

Express104 provides an inclusive next-generation vehicle for compact “green” embedded computers, providing a common set of well-supported low-, medium- and high-speed serial interfaces. The connector specified by SUMIT is both robust and low cost, and it satisfies a broad gamut of performance requirements ranging from an A-only implementation of a simple, low-speed serial bus to a high-bandwidth A/B implementation with two x1 and one x4 PCI Express lanes. That’s a set of characteristics to satisfy a broad slice of embedded applications and to offer unification as an answer to the fragmentation of the embedded computing world today.

Small Form Factor SIG

Arlington, TX.
(817) 274-7553.