Pico-sized platform ushers in new era for I/O
Building on the small form factors Mini-ITX and Nano-ITX, Pico-ITX has driven board size to the point where the I/O scheme needs to be revisited. The Small Form Factor Special Interest Group (SFF-SIG) has defined SUMIT technology, which merges performance with easy connectivity for broad application appeal – not only on Pico-ITX, but also any other form factor.
The broad embedded PC market has thrived for 15 years based on the simplicity of interfacing both standard and custom I/O to the low-speed ISA bus. Now that the ISA bus is no longer integrated into new chipsets, SBC manufacturers place ISA bridges and legacy super I/O devices on some boards to round out their feature sets for embedded use. However, the added board space and current draw work against new requirements to reduce overall system size, cost, and power consumption.
This discussion examines the benefits of the latest highly integrated low-power chipset and Ultra Low Voltage (ULV) Eden processors from VIA Technologies, Inc., and describes an innovative new expansion interface technology called SUMIT that allows extensive I/O scalability within the tiny 10 cm x 7.2 cm Pico-ITX form factor. This interface technology is reminiscent of the small form factor innovation that took place 16 years ago.
Lessons from early computing
In 1983, desktop computers were deemed more "personal" than their predecessors, as they showered consumers with a wealth of resources in a small footprint ‚Äì 64 KB of RAM (Kilobytes, not Megabytes or Gigabytes), 360 KB floppy disk drives, DOS, and text-based applications. A year later, the aptly named Little Board SBC ushered in the x86 era of embedded computing by sporting a 10 MHz 186 processor with a full complement of amenities within the small form factor of those 5.25" floppy drives.
What would much later become the standard EBX form factor was merely providing an off-the-shelf alternative to custom designs based on other 8-bit architectures from the likes of Motorola, Zilog, and NEC. Gordon Moore had not yet proclaimed the semiconductor tenet that would eventually move the "Kilos" to "Megas" and "Gigas." The 32-bit Reduced Instruction Set Computer (RISC) architectures championed by John Hennessy and David Patterson had not yet won the bet that memory would eventually cost less than logic, and Al Gore had not yet "invented" the Internet.
By the late 1980s, subsequent Ampro Computers Little Board SBCs were accepted into broader applications because of their ability to be tailored to product requirements using a simple yet elegant 8-bit expansion bus interface. The PC's expansion peripheral interface, the ISA bus, allowed add-on cards to plug vertically into AT motherboard slots. The clever Little Board version featured a low-profile, pin-in-socket, board-to-board interface instead so that expansion cards could mount horizontally, parallel to the SBC.
Out of this small interface arose the PC/104 architecture and its governing trade group, the PC/104 Consortium, in 1992. Moore's legacy paved the way for highly integrated 386 and 486 CPUs to fit on the tiny 3.550" x 3.775" (90 mm x 96 mm) form factor for medical, military, avionics, transportation, and instrumentation applications, and ISA was king.
Back to the future
Fast-forward 16 years, and the king is dead, or more accurately, the ISA bus has been dealt a blow by its disappearance from the x86 primary market (desktop, laptop, and notebook computers). The long life-cycle embedded roadmaps are derived from these same chipsets. The embedded PC market has widely adopted the ISA bus due to the ease of attaching a wide variety of low-speed peripherals. For applications that can't move to another bus, the Low Pin Count (LPC) bus found on most chipsets can be used with an external LPC-to-ISA bridge.
Serial ports, for example, are still widely used as subsystem communication interfaces in medical devices, as bar code scanner interfaces for logistics and transaction applications, as GPS interfaces, and for low-bandwidth industrial control and monitoring. Although ISA bridges and legacy super I/O devices help, these add costs, power consumption, and precious board space, working against the requirements for smaller, more eco-friendly devices.
Alternatively, super I/O devices are available for replacement interfaces in the desktop PC architecture ‚Äì most notably, PCI, PCI Express, LPC bus, and USB. Several chip vendors sell LPC UARTs, and better still are new PCI Express UART chips that take advantage of plug-and-play automatic resource assignments for those operating systems that support plug-and-play.
By now, even the venerable 32-bit 33 MHz (or 66 MHz) PCI bus is being replaced by PCI Express, as a trip to the local computer electronics store readily confirms. The desktop world rapidly moved on; now it is the embedded market's turn to standardize a way to bring the newer architecture elements off the board in the form of an expansion interface.
Meanwhile, shrinking silicon is showcased in the latest of the ATX and ITX family of industrial motherboard form factors. Several years ago, Mini-ITX boards were introduced by VIA, and by now dozens of manufacturers build this 6.7" x 6.7" (170 mm x 170 mm, or 289 cm2 area) size board. Then Nano-ITX was introduced with a 50 percent area reduction (120 mm x 120 mm, or 144 cm2 area).
The new Garden of Eden
Last year, VIA went even smaller, announcing Pico-ITX at the Consumer Electronics Show with another 50 percent form factor reduction to 72 mm x 100 mm, or 72 cm2 area. Refer to Figure 1 for a size comparison. The first two Pico-ITX boards on the market are so tightly packed with components that the challenge is getting enough edge area to bring the I/O off the board.
At the heart of the new Pico-ITX boards are the ULV Eden processors, starting at only 1 W at 500 MHz. The ULV processors are prime examples of power efficiency. Such power efficiency also has the added benefit of low heat dissipation, making the processors even more desirable for embedded applications and computer systems that need to run continuously. After all, it makes no sense to rack up cents in electric bills.
With power-efficient processors gaining traction in the embedded market, VIA had to acknowledge that across many market segments, applications demand more than just power efficiency; they want performance as well. The perception of modest-performing processors had prevented Eden from having mass appeal.
Enter a new architecture: Isaiah. This architecture combines power efficiency with some oomph. Think of it as a power-efficient processor on steroids. While maintaining a similar power envelope as the previous C7 processors, the new Isaiah processor architecture attains up to 400 percent of the performance.
VIA continues to push the processing envelope while not forgetting compatibility and migration considerations. A listing of low-power processors with five-year life-cycle commitments appropriate for small form factor SBCs is shown in Table 1.
Pico scales toward the SUMIT
Simplified cabling and the ability to tailor the Pico-ITX family to the needs of a variety of applications demand a technology breakthrough, much like the Little Board's dilemma 20 years ago. To realize its full market potential, Pico-ITX hankers for the same proven approach ‚Äì an expansion interface and a trade group to drive form factor standardization, multiple SBC sourcing, and I/O ecosystem development.
A brand-new industry technology addresses this challenge with increases in performance, functional density, modularity, and connectivity, all within a backdrop of green computing. The new board-to-board interface is called SUMIT (pronounced "sum it"), which stands for Stackable Unified Module Interconnect Technology. Adding expansion modules is a robust and reliable way to provide additional I/O without a complex rat's nest of cables.
Figure 2 shows the size and location of SUMIT I/O modules relative to the Pico-ITX SBC. The 60 mm x 72 mm I/O module covers 60 percent of the SBC, placing a height restriction of approximately 10 mm in the zone under the module. The other 40 percent is the tall component zone, allowing tall heat sinks to be placed on hotter components.
The SBC can change as CPUs improve over time, while the application portion, the I/O complement, can stay the same. That partitioning comes with a roadmap to faster, lower-cost, lower-power computing in the future without changing boxes and wiring, thereby preserving up-front investments in development and certification. This partitioning is similar to Computer-On-Modules (COMs) except that I/O modules are easier to design because they avoid issues such as custom carrier boards having to tweak the power sequencing for each given CPU module.
Easy, I/O-centric interfacing benefits applications
To target a broad range of applications, the SUMIT interface is defined with a great deal of flexibility. Two separate high-density expansion connectors are available. The connector pinouts are defined with a diversity of standard buses and interconnects. All interconnects are high-speed differential pair, low-speed serial, or multiplexed for optimum space efficiency.
Connector A features Serial Peripheral Interface (SPI), I2C, LPC, and USB ports, plus a PCI Express lane to cover high-bandwidth I/O such as FireWire 800, GbE, and hot-pluggable ExpressCard storage, a second video controller with modest performance, and other devices that used to be on the space-inefficient parallel PCI bus. This allows easy interfacing to A/D and D/A devices, traditional UART serial ports, and USB peripherals directly without complex bus bridges. Video signals can vary from chipset to chipset and do not need to occupy precious expansion connector pins just to be directly cabled to an LCD; display connectors are located on the SBC. Applications for Connector A include automation, data acquisition, control, HMI/Panel PC, logistics, transportation, low-end signage/advertising, battery-powered portable devices, medical instruments, and patient monitors.
Connector B adds more PCI Express lanes for multi-LAN network appliances, RAID storage, and scientific applications. Either or both connectors are valid configurations, selected at the discretion of each SBC vendor depending on the chipset used for a particular board.
With such a limited amount of real estate for off-board expansion, signal selection must be handled with great care. Parallel buses are deliberately excluded from the SUMIT interface. More importantly, the choice of SPI, I2C, and USB clearly demonstrates an effort to reach out to the hundreds of OEMs that must continue to support low-speed peripherals.
Many different expansion PCI Express lanes could have been chosen but would have required multiple space-consuming and expensive bridges to achieve the desired simple interfaces. High-bandwidth applications such as x16 PCI Express video are out of scope for the smallest form factors and best left to the much larger boards that can spread out the 50-70 W of graphics power dissipation. Pico-ITX with SUMIT expansion is the answer to the widespread call for a simple standards-based solution for the rest of the market.