Stackable Form Factors Shrink and Evolve with Outstanding Legacy Support

By Paul Rosenfeld, President, SFF-SIG

One of the driving forces for the use of Intel® architecture (a.k.a. x86) in embedded systems has been its similarity to the ubiquitous PC. The addition of a stacking architecture— whereby I/O cards are plugged piggy-back-style on top of each other instead of into a card cage—creates a small, rugged system with high resistance to shock and vibration. Developed in the early 1990s, the widely deployed PC/104 architecture and its derivatives have been the foremost examples of this stacking technology.

PC/104 enabled hundreds if not thousands of new embedded applications to use an off-the-shelf x86 solution where a custom solution might have been required. Yet stacking-architecture form factors have not evolved—even as advances in low-power, high-integration processor families like the Intel® Atom™ processor have reduced the board space required for an x86 implementation. The 90-x-96-mm PC/104 boards, which were phenomenally small in 1992 compared to 150-square-inch motherboards, are ho-hum today. They’re even considered large by 2010 standards. Many additional applications could be implemented with an off-the-shelf x86 solution if only a smaller form factor were available.

In 2009, the Small Form Factor Special Interest Group (SFF-SIG) introduced a set of specifications defining a smaller stackable architecture. The bottom of the stack, as always, is the single-board computer (SBC). The SFF-SIG SBC definition builds on the popular Pico-ITX form factor. It was originally introduced by VIA Technologies and is now also supported by Kontron, Radisys, Axiomtek, and others. The new Pico-ITXe (“e” for expandable) specification defines a 72-x-100-mm SBC using SFF-SIG’s SUMIT interface (see accompanying article) to support stacking I/O modules. SUMIT provides a wealth of bus interfaces including x1 and x4 PCI Express lanes, LPC, USB, I2C, and SPI. All that remained was the I/O module definition.

The Pico-I/O specification defines a 60-x-72-mm, SUMITbased I/O expansion module. At half the size of PC/104 boards, Pico-I/O modules include mating SUMIT connectors on both the bottom and top of the board. Due to the multi-interface SUMIT technology, a Pico-I/O board may grab the interface(s) that it requires (perhaps a single USB port or x1 PCI Express lane or even both) and pass the remaining signals up the stack. By implementing lane/channel shifting for both PCI Express and USB, the next card up the stack is guaranteed to find a USB port or PCI Express lane available on port1/lane1 if the resource remains available from the SBC.

As with the inception of PC/104, major suppliers in the x86 SBC and I/O markets are now releasing Pico-based designs. VIA has a Pico-ITXe board available today (EPIAP710) and VIA, WinSystems, and ACCES I/O all have Pico-I/O modules available as well.

Certainly, not all applications have size requirements that dictate a tiny, Pico-based solution. For many, 90-x-96 mm remains perfectly adequate. To support the needs of these applications, SFF-SIG has released specifications that define the use of the SUMIT interface on PC/104-sized boards. This SUMIT-ISM specification embraces a unique mounting concept that enables outstanding legacy support of existing PC/104 or PCI-104 I/O cards on top of the SUMIT-ISM stack. Through appropriate use of bridge technology either on the SBC or on an I/O card, a SUMIT-ISM stack can support a legacy PC/104 card with its ISA bus (bridged from LPC) or a legacy PCI-104 card with its PCI bus (bridged from a x1 PCI Express lane). This capability smoothes the evolution from older technologies to new bus architectures by maintaining support for the I/O cards that make applications unique.

SUMIT-ISM CPUs and I/O cards—together with SUMITEPIC and SUMIT-EBX CPUs—are available today from VersaLogic, WinSystems, and ADLINK. The SUMIT, Pico- ITXe, Pico-I/O, ISM, and SUMIT-ISM specifications may all be downloaded without restriction or registration from the SFF-SIG web site (www.sff-sig.org).

Paul Rosenfeld is president of the Small Form Factor Special Interest Group (SFF-SIG). His career in the embedded space spans almost 40 years of software and hardware engineering, marketing, and senior-management positions in companies like HP, Intel, Vadem, Microtec/Mentor Graphics, Ampro, and Diamond Systems. Rosenfeld was part of the team that introduced PC/104 to the market in 1990.