Shrinking the stack ñ footprint, that is

In the previous issue, I wrote about SFF-SIG’s recently released SUMIT-ISM Specification, detailing how to use the Stackable Unified Modular Interconnect Technology (SUMIT) interface on PC/104-sized 90 mm x 96 mm CPUs and I/O boards and describing the outstanding legacy support provided by this specification.

In this issue, I’d like to discuss two other recently released SFF-SIG specifications: Pico-ITXe and Pico-I/O. Together, these specifications present the first new stacking architecture standard for embedded applications in almost 20 years. PC/104 has had a terrific run as the de facto stacking architecture for embedded systems since its introduction in the early 1990s. At that time of fairly gigantic 150 square inch motherboards, PC/104 represented a phenomenal size reduction that enabled hundreds of previously unimplementable embedded applications. And PC/104 will continue to serve a variety of applications with legacy support requirements for many years.

Yet in the face of today’s credit card-sized COM modules, a 90 mm x 96 mm CPU I/O stack is on the large side. The time has come for a new standard that takes advantage of high-integration chips and shrinks the stack footprint to enable a new set of applications for which PC/104, EPIC, and EBX are simply too large.

In 2007, VIA Technologies invented the Pico-ITX SBC to showcase the company’s extreme chip-level integration, including built-in graphics sufficient for most embedded applications. Pico-ITX SBCs are also currently available from , , Axiomtek, and a few others. Notably, these 72 mm x 100 mm full-featured SBCs have no ready means of I/O expansion. If an application requires additional I/O (digital, , serial, GPS, USB, whatever), the solution starts to fail. While it is possible to expand I/O through USB ports, this would require custom mechanical mounting of USB devices with a potential rat’s nest of interconnect cables. What’s needed instead is a small-footprint stacking I/O module for Pico-ITX utilizing a high-performance, comprehensive board-to-board interface.

That’s where the SUMIT interface comes in. Introduced by SFF-SIG in early 2008, this standard uses one or two tiny 52-pin connectors to provide a wealth of interfaces to stacked I/O modules. SUMIT incorporates flexible PCI Express, USB, LPC, I2C, and SPI interfaces on one connector, with additional PCI Express support on the second connector if needed, totaling up to six PCI Express x1 lanes. Occupying a bare 7 mm x 21 mm of board space along with two or four mounting holes, a single connector interface can be shoehorned into a Pico-ITX SBC, creating a Pico-ITXe SBC (the e stands for expandable).

What remains is the definition of the I/O module, which is where the Pico-I/O Specification enters the picture. It defines a 60 mm x 72 mm footprint (about half the size of a PC/104 module, as shown in Figure 1) that leaves space for PC-style I/O connectors on the Pico-ITXe SBC. The definition also allows the Pico-I/O module to share one or two of the Pico-ITXe SBC’s four corner mounting holes, further reducing the board space consumed by the stacking interface.

Figure 1: The Pico-I/O footprint is half the size of PC/104. (Photo courtesy of ACCES I/O Products.)

As with other stacking architectures, the Pico-ITXe SBC has SUMIT connector(s) on top of the board. The Pico-I/O modules have mating connector(s) on the bottom as well as the top. Because of the multi-interface technology on the SUMIT interface, a Pico-I/O board can grab the interface it requires (perhaps a single USB port or PCI Express lane) and pass the remaining signals up the stack. By implementing lane/channel shifting for PCI Express and USB, the next card up the stack is guaranteed to find a USB port or PCI Express lane available on port 1/lane 1 if the resource remains available on the SBC.

Of course, the best way to understand the flexibility and benefits of this new, tiny stacking architecture standard is to read the specifications. Both are freely available to the public at Even better, you can start using Pico products today, with the EPIA-P710 Pico-ITXe board from VIA and Pico-I/O modules from VIA, WinSystems, and ACCES I/O Products.

Small Form Factor SIG 408-480-7900

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