SUMIT I/O Expansion Guides Next- Generation Intel® Processors into Express Lanes

Robert A. Burckle, WinSystems Inc.

As Intel continues to roll out high-performance, lowpower processors, it’s important for a system designer to consider how to access and utilize their I/O expansion options. With the evolution and relentless march of personal computers (PCs) into more applications, we’ve seen the I/O architecture change and improve. New high-speed serial buses are replacing the slower and more cumbersome parallel ones. LPC and SPI replaced the ISA bus while PCI Express® replaced PCI, SATA replaced PATA, and the Universal Serial Bus (USB) replaced serial and LPT ports. For desktop and mobile environments, the roadmap is fairly clear. Yet I/O becomes more challenging for the industrial/medical/military-commercialoff- the-shelf (COTS) markets, which need to consider legacy and future interfacing as well as connectivity issues.

Although industrial embedded PCs can run the same programs as commercial PCs, they’re tailored to unique technical and other key requirements of their target application. Examples include long-term availability, reliability, size, operating temperature, ease of integration, and migration path from current designs. Processor and matching I/O hub architectures support various combinations of PCI Express®, USB, LPC, SPI, SDIO, SMBus, SATA, Gigabit Ethernet, high-definition (HD) audio, video, and general-purpose I/O. Questions for an embedded-systems designer include, “Which interfaces do I need and how are they best implemented on and off the main processor board in the most cost-effective manner?” Although there are different ways of bringing I/O off a board, one of the best ways to implement it on various board sizes is through stacking either a single board (as a mezzanine) or multiple boards without the need for a card cage.

Two years ago, a new group called the Small Form Factor Special Interest Group (SFF-SIG) was formed by technology leaders of the embedded-systems community. This independent, nonprofit industry group develops, promotes, and supports small-form-factor board, I/O, and storage specifications. Although the group embraces the latest technologies, it also has a philosophy of maintaining legacy compatibility and enabling smooth transitions to next-generation interfaces. In 2008, the SIG identified trends in processor innovation as well as the migration to serial I/O architectures. It quickly responded by defining a form-factor-independent interconnect solution called SUMIT™, which accommodates I/O boards of different sizes.

SUMIT Defined

SUMIT™, which is pronounced “sum it,” stands for Stackable Unified Modular Interconnect Technology. This electromechanical connectorization specification uses two 52-pin, high-speed connectors that can support current and future PCI Express and USB data rates as well as easy-attachment moderate-speed interfaces for I/O expansion. Actual signalintegrity test results demonstrate that a stack of modules will support data rates of 5 GT/s, which is required for PCI Express 2.0 and USB 3.0. The specification is optimized for very-lowpower and ultra-mobile processors, which support “green” computing initiatives for compact embedded systems.

In the “A” connector, SUMIT supports one x1 PCI Express lane and four high-speed USB 2.0 channels as well as the Low Pin Count (LPC) Bus, SPI/uWire, SMBus/I²C Bus, and ExpressCard™ signaling. The second “B” connector supports one additional x1 PCI Express lane and one x4 PCI Express lane plus additional power, ground, and control signals (see the Table). The SUMIT B connector is suited for applications requiring more channels and higher data bandwidth. Notably, the PCIe x4 lane can be bifurcated into four PCIe x1 lanes for a total of six PCIe x1 lanes with both SUMIT connectors. SUMIT-based I/O modules each use one or more of the buses and pass unused resources further up the I/O stack.

Aside from low power, cost is a key element in any design. SUMIT is defined in such a way that only the SUMIT A connector can be used for lower-cost and low-to-moderate I/O-speed applications. This saves printed-circuit-board (PCB) space plus the cost of the additional SUMIT B connector. For higher-speed I/O requirements, both the SUMIT A and B connectors are recommended.

Even though SUMIT defines two high-speed connectors and their respective signal assignments, it doesn’t address the specific location (placement) requirements for the connectors on any specific form-factor boards. Only the relative location of one connector to the other and their relationship to a mounting hole are specified. Doing so ensures proper routing of the signals that are passed from one connector to another as they continue up in a stacked architecture. SUMIT’s goal is to provide a compact, stackable, multi-board solution for future embedded-systems designs. Unifying the expansion interfaces of many single-board-computer (SBC) form factors has the potential to consolidate I/O ecosystems, which could improve economies of scale for off-the-shelf I/O.

With a blend of high-bandwidth PCI Express lanes, USB, and lower-speed multiplexed and serial buses, SUMIT can be added to a variety of board form factors. Plus, it’s flexible and compact enough to meet a very broad range of application requirements. Currently, SUMIT connectors have been designed into standard and custom boards to provide I/O expansion. Well-known SBC form factors are now available as host SBCs. Examples include the 5.75-x-8.0-in. EBX, 115- x-165-mm EPIC, 90-x-96-mm Industry Standard Module (ISM), and 72-x-100-mm Pico-ITXe.

SUMIT-Based I/O Modules

Currently, two I/O-expansion-board standards are defined and available in the market: Pico-I/O and SUMIT-ISM. An industry-standard module (ISM) is defined as a small, 90-x- 96-mm board outline plus mounting holes without regard to bus expansion. ISM modules are small, rugged, easy to use, and scalable. Pico-I/O is defined as a tiny, 60-x-72-mm stackable I/O expansion module. The Pico-I/O specification defines a 4320-mm2 board footprint with four dedicated corner mounting holes for stand-off mounting. A Pico-I/O module is half the size of an 8640-mm2 SUMIT-ISM™ module. A Pico- I/O module can plug into virtually any standard form factor or custom SBC with SUMIT expansion and mounting holes for a Pico-I/O-size module stack. Pico-I/O is unique in that it combines very small size, modularity, flexibility, and interface versatility in a single stack of I/O modules.

Through the SUMIT interface, both SUMIT-ISM and Pico-I/O offer a compact, stackable, I/O-centric, multiboard solution for small-form-factor embedded systems. They provide the I/O portion of a stackable multi-board solution that’s neither processor-architecture- nor chipsetdependent. SUMIT-ISM and Pico-I/O represent a significant milestone in embedded I/O by defining universal I/O in smaller form factors. They can therefore take advantage of the denser electronics available for the ever-shrinking size of processor modules in the embedded-computer marketplace. This reduces cost and bulk while increasing mounting and packaging options for small-form-factor embedded systems. With all of these features, SUMIT-based systems enable small, rugged, and reliable computer systems that are powerful, easy to use, cost effective, and scalable.

Product Availability

At the 2009 Embedded Systems Conference in Boston, an Atom-based single-board computer with the SUMIT I/Oexpansion connectors was introduced by WinSystems. The EBC-Z530-G SBC includes a 1.6-GHz processor, 512 KBytes of on-board SDRAM, a microSD slot, and System Controller Hub (see the Figure). On-board I/O includes the following: two Gigabit Ethernet controllers, four USB 2.0 ports, a PATA interface, HD audio, LVDS and CRT video, 4 COM channels, 48 lines of digital I/O, LPT, PS/2 keyboard, and mouse interfaces. The SUMIT connectors provide two PCIe x1, two USB 2.0, LPC, SPI, and SMBus signals to a SUMIT-ISM module stack. This SBC will operate from -40° to +70°C for applications in industrial and fanless environments. Other embedded-SBC technology leaders have introduced SUMITexpandable products in 2009. In doing so, they contributed to the forward momentum of a standard that enables a lowcost, low-power I/O ecosystem.

Figure: Pictured is WinSystems’ EBX form-factor SBC, which is powered by an Intel® Atom™ processor with SUMIT I/O expansion connectors.

About the SFF-SIG

The SFF-SIG is an international organization devoted to identifying, creating, and promoting standards that help electronics-system and device manufacturers and integrators move to small-form-factor technologies in their products in order to protect their investments over the long term. A free, downloadable copy of the SUMIT specification is available at

Robert Burckle is vice-president of WinSystems Inc., based in Arlington, TX. He has 30+ years of experience in embedded computing. Burckle holds bachelors and masters degrees in electrical engineering from the University of Louisville and an MBA from the University of North Texas.