Map your way to the SUMIT

PC/104 and Small Form Factors — October 23, 2009

3Speaking of Stackable Unified Module Interconnect Technology (SUMIT), one area the team developing the specification has focused on is interoperability. Designers can quickly tell how boards go together using a new labeling system defined in the latest version of the specification.

The SFF-SIG’s SUMIT specification is intended to accommodate present and future low-power, highly integrated processors, chipsets, and memory, as well as robust high-density connectors and the high-speed serial interfaces that are replacing the parallel interfaces of the past.

The hallmark of SUMIT is how easily this stackable expansion architecture can be used with the latest crop of ultra-low-power chipsets on different small form factor boards. Vendors currently offer Pico-ITXe, Pico-I/O, EBX, and SUMIT-ISM boards with SUMIT expansion connectors. Designers use SUMIT to achieve flexibility and scalability; however, that flexibility can lead to possible confusion, misunderstandings, and incompatibilities since the signals aren’t all required to be present on a SUMIT connector.

Resources on the SUMIT connectors

Several of the interfaces tapped for inclusion in SUMIT come from the mobile and laptop computing world to leverage advances in low cost, small size, and low power. SUMIT represents a convergence of well-supported connectivity standards providing the performance needed to support embedded applications well into the future. It is designed to be independent of form factor and equally applicable to I/O boards small and large, fast and slow, simple and complex, while residing on various sorts of system boards, both standard and proprietary. I/O boards typically make use of only one of the interfaces resident on the SUMIT connector – the interface that’s most appropriate for the onboard function – but more complex designs are also possible.

High-, moderate- and low-speed interfaces can easily coexist on a single 52-pin SUMIT connector, as shown in Figure 1. On the first connector, SUMIT supports one PCI Express (PCIe) x1 lane, four high-speed USB 2.0 channels, Low Pin Count (LPC) bus, Serial Peripheral Interface (SPI)/Microwire, SMBus/I2C bus, and ExpressCard signaling. A second identical connector supports an additional PCIe x1 lane, one PCIe x4 lane, and more power, ground, and control signals. The second connector is used for applications requiring more channels and higher bandwidth. Notice that the PCIe x4 lane can alternatively support four PCIe x1 lanes for a total of six PCIe x1 lanes on both SUMIT connectors combined.

Figure 1: SUMIT connectors support several interfaces with a variety of speeds.

This flexibility required the SFF-SIG to create a simple, visual way to tell engineers about SUMIT resources/uses and explain how vendors implemented the various interface resources on their products.

Look for the SUMIT label

Considering that interface signals are not always supplied by an SBC or used by an I/O module, the SFF-SIG defined a label in version 1.5 of the SUMIT specification conveying interoperability of a vendor’s SUMIT-compatible product. The philosophy of this idea is to create a simple table that communicates the interoperability needs of different products. The purpose of the label is to quickly show sources and uses of resources on the SUMIT connector and demonstrate how vendors implemented SUMIT’s various interfaces on their products. It standardizes the sizing/aspect ratio, making the information easy to find on any labeling for SUMIT-enabled products. Though not all-inclusive, this label can convey meaningful data in a straightforward, universal format.

Resources for both SUMIT A and B connectors are listed in the label. If the SUMIT signals are not defined for the connector, the box area is gray. If a signal or interface is not supported, the box contains a dash. If one of the A or B connectors is not supported or populated, then all the boxes are gray, indicating that no resources are available.

The form factor row denotes the size of the board using the SUMIT interface such as an Industry Standard Module (ISM), Pico-I/O, Pico-ITXe, or other industry-standard or custom-size board. Room is provided at the bottom of the label for special notes calling attention to important features or design issues.

The label at work

To illustrate this labeling convention, consider the WinSystems’ EBC-Z510-G, an 8.0" x 5.75" EBX-sized SBC (Figure 2) with both SUMIT A and B connectors. This board supports I/O expansion of two PCIe x1 links, no PCIe x4 link, two USB 2.0 channels, ExpressCard, LPC, SPI/Microwire, SMBus/I2C, and various power sources.

Figure 2: An EBX-sized Atom-based SBC supports I/O expansion via the SUMIT A and B connectors.

The corresponding label for this board is shown in Table 1. Notice that this label does not specify any of the other onboard I/O expansion and peripheral interfaces such as the PC/104 connectors, video, Ethernet, serial and parallel digital I/O, and so on. The label is SUMIT-specific.

Table 1: The SUMIT label gives designers a quick overview of SUMIT resources on a particular board.

For power, a check is included in each box indicating whether the card needs or provides that resource. A dash indicates it is not supported. Also notice that the power sources are either passed through from the power supply or generated from onboard power supplies. Manufacturers must list the current supplied in their published technical literature and provide sufficient technical data to allow designers to properly understand sources and uses of the power and individual interface signals on their SUMIT products.

For more on the details

The charter of the SFF-SIG is to develop and promote specifications to help electronic equipment manufacturers and integrators reduce the overall size of their next-generation systems. Using the SUMIT label offers a basic overview of SUMIT resources. Instead of replacing a manufacturer’s detailed data sheet, this label is intended to provide a quick look for design engineers as they evaluate products for their projects. To review the complete SUMIT version 1.5 specification free of charge, visit

Bob Burckle is VP of WinSystems, Inc., based in Arlington, Texas. Bob has more than 30 years of experience in embedded computing and holds BSEE and MSEE degrees from the University of Louisville as well as an MBA from North Texas State University.

Will Ouyang works in the Product Marketing Development and Standards Group for Samtec, Inc., based in New Albany, Indiana. Will has 14 years of experience working in the connector industry and holds a BSME degree from Carnegie Mellon University.

WinSystems 817-274-7553

Samtec 812-944-6733