A tabula rasa for portable green computing

A fresh start - Intel Atom-based SBCs allow the flexibility to optimize power consumption and Interoperability.

Handheld battery-powered devices, remote solar-powered sensors, and other equipment used in smart energy applications demand low power consumption and I/O flexibility in a lightweight, portable, thermally efficient package. An Intel Atom-based SBC with a SUMIT connector provides a blank canvas allowing designers to choose from a broad palette of options to minimize peripheral power consumption and maximize I/O expansion

Embedded OEM designers developing small, next-generation portable computing systems based on mid-range x86 processors need the lowest available power consumption combined with minimum size and weight. To be lightweight and portable, these devices require 5 W processor/chipset platforms on tiny circuit boards with efficient thermal solutions.

The ideal design platform for this type of application is a stand-alone, small form factor SBC that does not automatically require either a card cage or a custom carrier board. Yet it must provide enough flexibility to accommodate a wide variety of product designs with different I/O requirements, as well as upgradeability, via I/O expansion cards.

Power and portability requirements

Smart energy application examples include devices for meter and instrument readers, oil and gas pipeline monitoring/troubleshooting workers, and solar-powered sensors such as sensor buoys and weather balloons. Whether autonomous or manually operated, the system must be small, lightweight, and able to work in the field for several hours before needing its batteries recharged.

SBCs optimized for this market must be designed for +5 V supplies, so that only two 3.6 V lithium ion cells connected in series can create a minimal, lightweight solution, as long as designers allow for the reduced voltage over time caused by discharging. Neither a +12 V Computer-On-Module (COM) nor an SBC designed for ATX supplies that uses both +5 V and +12 V to operate would work in this type of portable device application because they are too bulky and heavy. A similar consideration applies to other battery topologies.

The system board for these applications must be the lowest possible weight, so it cannot be part of a two-board stack. It must consume the lowest possible amount of power; 5 W is desirable for the entire module, and up to 7 W or 8 W could be acceptable if limited by current technology. Greater power consumption reduces the continuous operating time until battery recharging is required.

Furthermore, the board must be the smallest possible size, under 4" x 4", to fit a Solid-State Drive (SSD) and other small accessories and modules behind a typical 5" to 7" LCD and keypad. It must have optional expansion interfaces for application-specific I/O, also no bigger than 4" x 4". Additional requirements include analog inputs, several channels up to 16-bit resolution each, and a 5 V or 10 V range, as well as application-specific I/O such as serial ports, CANbus, 802.11, cellular modem, and GPS receiver. The entire system should be as thin as possible, no more than 1.5" thick, to allow for a mezzanine I/O module with real-world I/O to complete the feature set.

Clearly, this is a three-dimensional design problem, where overall volume and weight supersede SBC form factor considerations alone.

SUMIT-ISM offers latest-generation connectivity

Diamond Systems – with these needs in mind – designed Aurora, an SBC based on the Intel Atom Z530 CPU and the industry-standard Stackable Unified Modular Interconnect Technology – Industry Standard Module (SUMIT-ISM) form factor. The Z530 was chosen for smart energy applications because it offers a balance of performance and power consumption without sacrificing low cost. All Z-series Atom processors consume 30 to 50 percent less power than Intel’s N270 and N450 processors (refer to Figure 1). The SBC’s small size is due to its SUMIT-ISM form factor; a deliberate choice of companion ICs and expansion bus connectors; and next-generation improvements in circuit board fabrication, component sizes, and manufacturing process technology.

Figure 1: The power comparison chart affirms the Atom Z-series platform for long battery life and efficient thermal design.
(Click graphic to zoom by 1.2x)

An integrated heat spreader with mounting holes attaches directly to the enclosure to reduce heat buildup in the box, increasing mean time between failures and preventing thermal runaway in fanless enclosures. The Atom Z530 processor and companion US15W chipset are positioned on the bottom surface of the board to take advantage of the heat spreader, resulting in fanless, wide temperature operation. This configuration protects heat-sensitive chips while leaving the SBC’s topside free for access to memory, onboard I/O, and expansion interfaces.

Expansion buses and integrated I/O are limited by Menlow’s two-chip solution. Therefore, several ICs are incorporated to round out the I/O required in embedded applications. Latest-generation connectivity such as Serial ATA (SATA), Gigabit Ethernet, and PCI Express (PCIe) ensures long lifetime and top performance. A full MAC+PHY Ethernet controller is required since the MAC is not integrated. An IC converts Parallel ATA (PATA/IDE) to SATA. Additional USB ports and a PCIe x1 lane are available on the SUMIT-A expansion interface for new off-the-shelf or custom mezzanine modules. Four serial ports come from a quad UART chip. Finally, an embedded controller IC provides precise power rail sequencing.

All of this connectivity was integrated into a 3.55" x 3.775" x 0.6" (90 mm x 96 mm x 15.2 mm) footprint using high-density circuit board fabrication and assembly process techniques to design an appropriate use of expansion bus connectors without consuming too much board space. Additionally, the board’s PC/104 ISA bus bridge accommodates installation of the largest stacking I/O ecosystem. PC/104 expansion maintains compatibility with legacy applications, while SUMIT expandability provides a migration path to current and future high-speed I/O.

The SBC’s inclusion of a SATA interface instead of PATA/IDE benefits portable applications by connecting directly to modern, low-cost, long-life SATA SSDs. Alternately, the onboard USB flash disk pin header for optional flash disk support provides the modest amount of flash memory that many of these applications require. Conventional rotating hard drives are not appropriate for these applications due to the excessive size and weight burden of a larger SATA drive. The prices of flash components have dropped enough to allow SSDs to fit inside the size, weight, power, and cost envelope of many portable devices.

Trade-offs and other design decisions

Considering the importance of every feature and design decision made on Aurora is analogous to determining “opportunity cost” in the business world. The “cost” of each feature is compared to the value of another equally important feature that could occupy the same board space. For example, choosing a SUMIT-B connector instead of a SUMIT-A connector would have required a PCIe switch. This would have forced the SODIMM memory connector onto the board’s bottom surface, rendering the RAM less accessible between the heat spreader and the board. Consequently, the relocation of the SODIMM connector would increase the SBC’s overall height and slightly decrease the heat spreader’s thermal conductivity due to the raised height of the SBC’s parts, which would then contact the processor and chipset. The extra board space consumed also would have eliminated one of the following valuable features: SATA, four serial ports, or the USB flash disk header.

SUMIT-A comprises a multitude of buses for enormous I/O flexibility. Even with a growing off-the-shelf ecosystem of I/O cards, OEMs often must design a custom board to optimize the size, weight, and cost of high-volume applications. A SUMIT expansion card thus resembles a blank easel, with PCIe, USB, LPC bus, and SMBus as the broad palette. The artist can choose one, multiple, or all interfaces to attach circuitry. SUMIT-A provides +5 V and +3.3 V for the I/O card designer to minimize peripheral power consumption.

With its 0.025" (0.635 mm) pitch surface-mount connector, SUMIT-A occupies only a tiny fraction of the circuit board, leaving room for the SBC’s rich feature set. SUMIT-A is intended for designs that need a minimum of PCIe lanes. This works well with a Menlow-based SBC, as the US15W chipset has only two PCIe lanes, one of which is used for the onboard GbE LAN controller. Applications that need to fan out the number of lanes can do so on the I/O card. That way, only system OEMs who need to use a PCIe switch pay for it in chip cost and board real estate.

The entire kit consists of relevant combinations of the boards, LCD with touch screen or keypad/buttons, battery cells, and an Operating System (OS) such as Windows Embedded Standard, Windows Embedded CE, or Linux. The challenge is to run a rich, desktop-like OS that has enough performance without exceeding the power budget. Componentized OSs like these allow many unneeded functions to be eliminated at OS build time.

Overall, the design decisions show how to use the latest silicon and expansion interfaces to design ultra-low-power SBCs for portable or solar-powered smart energy applications. Each system design is tailored to its unique requirements via the expansion palette, as it leaps from the easel into the real world.

Colin McCracken is VP of marketing at Diamond Systems Corporation. He has more than 20 years of experience in the embedded systems market in various marketing and engineering capacities, most recently at ADLINK Technology and previously at Ampro Computers. Colin is also a board member of the SFF-SIG (www.sff-sig.org), which he cofounded in 2007.

Diamond Systems www.diamondsystems.com