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Bob Burckle WinSystems VPIt’s amazing that we see so much written about the latest generation, high-performance CPUs. Intel with the Atom family and VIA with the Nano family of products are leading the charge within their x86-compatible processors and companion chipsets. The reason for their popularity is that these tiny devices offer great computing capacity with relatively little power required while still being compatible with the ubiquitous PC software base. This permits an engineer to continue to design and build systems based on their existing knowledge while tapping the vast amount of sophisticated operating systems, drivers, and development/debugging tools. Yet with all these advances, to make them useful for an embedded system (not a desktop PC), then it is still necessary for a designer to interface them to the real world.

I/O has progressed as well from the older, slower parallel buses such as ISA, PCI and ATA to high-speed serial buses such as PCI Express, USB and SATA. The new buses move more data quicker while reducing the number of pins on their connectors. This is both a blessing and a curse since it presents a revolutionary, not an evolutionary approach to the I/O. PCI Express and USB2 connections are important, but it is also necessary to be able to attach relatively low-speed serial and parallel devices as well.

Simple, low-cost I/O is important. You might say that trying to hang a relay, UART, or other low-speed device on a PCI Express lane is like trying to get a drink of water out of a fire hose! So how do you handle I/O on the chips?

Fortunately the semiconductor vendors did not totally abandon the slower buses. There is still LPC, SPI, and SMbus/I2C supported so that it is not necessary to hang multiple bus bridges, translators, or an FPGA off of a PCI Express lane to get to low-speed I/O. This prevents unnecessary cost, power, and board space. But how do you bring both low- and high-speed signals together in a coherent and cost-effective way?

A number of forward thinking embedded computer companies came together under the auspices of the Small Form Factor Special Interest Group (SFF-SIG) to define a new interface called SUMIT. SUMIT (pronounced “sum it”) stands for Stackable Unified Modular Interconnect Technology. SUMIT is an electromechanical connectorization specification that enables all common serial and legacy chipset expansion buses for next generation products. It is a stackable, I/O-centric, serial expansion approach that is independent of any particular board form factor.

SUMIT supports one x1 PCI Express lane, four high-speed USB 2.0 channels, LPC (Low Pin Count) Bus, SPI/uWire, SMBus/I²C Bus, and ExpressCard™ signaling on the first high-speed connector. A second identical connector supports one additional x1 PCI Express lane, plus either one x4 PCI Express lane or four x1 PCI Express lanes as well as additional power, ground, and control signals. The second connector is for applications requiring more channels and higher bandwidth.

To reduce cost, SUMIT is defined in such a way so that only a single, one-bank connector can be used. This saves printed circuit board space plus the cost of an additional connector if more functionality is not required. By using two smaller, separate connectors instead of one large connector, an expansion or add-in board built with only a single connector can plug directly into other processor or expansion cards populated with both connectors, further reducing overall system cost.

SUMIT connectors
Figure 1: SUMIT connectors on a WinSystems’ SUMIT-ISM Gigabit Ethernet I/O board

With the space efficiency of high-speed serial buses compared to their predecessors, it might have been tempting for the SUMIT originators to consume all of the space occupied by legacy stacking PCI and ISA bus connectors with many more PCI Express lanes. Instead, SFF-SIG members used restraint in achieving 15 times the legacy parallel bus bandwidth with 6 PCIe lanes and passing along the rest of the space savings to board designers, customers and end users. Going beyond 6 lanes would have increased product costs, reduced board features, increased power consumption, increased stack heights, and exacerbated heat removal challenges. Real-world applications took priority over extreme corner cases.

Even though SUMIT defines the high speed connectors and their respective signal assignments, it does not 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 is specified to ensure proper routing of signals that are passed from one connector to another as they continue up in a stacked architecture. Therefore, SUMIT can be used to support a single mezzanine card or to allow multiple boards on a self-supporting stack. This flexibility has allowed the connector to be designed into a number of different custom and standard size base boards including Pico-ITXe, EBX, EPIC, and the 90x96mm Industry Standard Module (ISM).

For SUMIT-based I/O expansion, there are two standard module form factors: Pico-I/O and SUMIT-ISM. Pico-I/O boards measure 60x72mm while the SUMIT-ISM board is 90x96mm. The board area for Pico-I/O is exactly ½ the area of SUMIT–ISM. Both module types are small, rugged, easy-to-use, and scalable. Either solution provides powerful I/O building blocks and that can be stacked on top of one another or used as a mezzanine. This reduces cost and bulk while increasing mounting and packaging options for small form factor embedded systems.

You will continue to see a number of product introductions with SUMIT connectors on a variety of different board form factors in ECN Magazine, online and in newsletters. As processors continues to evolve into faster, smaller, and lower cost solutions, SUMIT provides the enabling technology for small, full featured, next-generation I/O. SUMIT-based embedded systems will be able to serve any number of markets such as industrial automation, medical, MIL/COTS, transportation, security, and communications.

The SFF-SIG has charted a course to develop, adopt, and promote circuit board specifications and related technologies that will help electronics equipment manufacturers and integrators reduce the overall size of their next-generation systems. For more information about the SIG or for a free downloadable copy of the SUMIT, ISM, Pico-I/O or SUMIT-ISM Specifications, go to www.sff-sig.org.

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