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Aerospace and military applications that are more sophisticated and complex are spawning a new breed of single-board computers (SBCs). That’s because the users in these areas have rather formidable demands, such as greater ruggedness, higher reliability, more power performance, less power dissipation, and a smaller form factor.

As a result, board and systems integrators are scrambling to find ways to pack as much technology as possible on the smallest board practical. Some companies are using commercial off-the-shelf (COTS) technology. Others are employing multicore designs, in which a board may have several processors, some coming from different manufacturers.

Designers use a range of operating systems as well, often utilizing multiple operating systems like Unix, Linux, Microsoft’s Windows XP Pro and Embedded, and Wind River Systems’ VxWorks on a single board. In addition, high-speed serial buses are favored over older parallel buses to handle the higher-data-rate and wider-bandwidth communications of modern SBCs.

Although COTS components see wide use in SBC designs, their application sometimes depends on performance requirements and the design budget. Some SBC designs requiring very high performance levels may opt to use custom components should the design budget allow it.

Multicore COTS processing that incorporates leadingedge, high-performance processors promises to revolutionize SBC designs. The multicore approach eliminates the need for individual computer boards for each application. It also allows multiple applications to run from a single board, often from a single CPU. High-performance multicore processors can be found in many SBCs and plug-in support mezzanine cards.

MULTICORE COTS PROCESSING
These multicore processors include Analog Devices’ Sharc and Blackfin; Freescale Semiconductor’s 8641 PowerPC and 8555E; Intel’s Core 2 Duo, Xeon Dual, and Pentium; AMD’s Turion 64, Opteron, and Radion HD3650; Sun Microsystems’ UltraSPARC IIi and IIIi; PA Semiconductor’s 1682; and the MIPS64 from Cavium Networks.

Jointly developed by Sony Computer Entertainment, Toshiba, and IBM, the Cell Broadband Engine (Cell BE) multicore processor architecture from supplier IPV Ltd. combines a modest general-purpose Power Architecture core with streamlined co-processing elements that greatly accelerate multimedia and vector-processing applications, as well as many other forms of dedicated computation (Fig. 1).

Ruggedness is a key parameter for aerospace and defense users of SBCs. With a 3U CompactPCI design, the S950 SBC developed by Aitech Defense Systems uses as little as 13.5 W in full operation and less than 8 W in sleep mode. It’s based on a PowerPC 750FX platform and features a radiation-tolerant anti-fuse FPGA that maintains memory control to ensure data integrity in harsh environments.

General Micro Systems also uses the CompactPCI approach in its Premonition CC279. This 6U, 100-W, conduction-cooled SBC is based on the user’s choice of two Intel Quad-Core or two Dual-Core Xeon processors (Fig. 2).

The use of field-programmable gate arrays (FPGAs) is another rising trend in SBC designs. FPGAs not only provide the required number-crunching and interfacing with high-speed serial buses, they also ease a designer’s migration from older-generation, FPGA-based designs without having to worry about the requirements for lead-free components.

An SBC can form an entire system on a board for a specific function. Or, several SBCs can be plugged into a backplane connector within a box-like metal cage for even higher performance. Some of these SBCs may not abide by SBC standards, though, since they may be entirely custom-made. Also, many specific-function mezzanine cards can plug into SBCs, like graphics accelerator mezzanine cards, display and other types of mezzanine controller cards, and memory mezzanine cards.

Developed by Mercury Computer Systems, the PowerBlock 50 crams a tremendous amount of power into a box-like product (Fig. 3). This 6- to 10-lb unit measures just 4 by 5 by 6 in., yet it delivers 100 GFLOPS of processing power. Optimized for embedded computing applications where space and weight constraints are important, the PowerBlock 50 uses the Cell BE multicore processor.

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HIGHER-SPEED SERIAL INTERCONNECTS
Because SBC designers employ so many different processor, operating- system, form-factor, and design architecture approaches, standardization is nearly non-existent. However, some major standardization efforts are under way in form-factor as well as in intra-board and inter-board communications.

Not willing to wait, a new generation of serial links for SBCs is beginning to use higher-speed bus interconnects for communications. Serial bus standards like VPX and XMC reflect the growing importance of high-speed serial interfaces, particularly switchedfabric interfaces, such as PCI Express, Gigabit Ethernet, Serial RapidIO, and InfiniBand. HyperTransport is another serial bus, but it remains a chip-to-chip link even though it has been defined in board standards.

Serial buses feature wider bandwidths and greater throughputs than parallel buses. Also, they offer significantly greater performance levels and are more suited to modern SBC architectures that use multicore designs. They offer lower pin counts and hotswapping capability as well.

Yet parallel buses are scalable from 1X, 2X, 4X, and so on without the need to move up to higher-link transfer rates. Additionally, parallel buses like the VME bus, CompactPCI, Embedded Technology eXtended (ETX), PC/104, and PC PCI/SA, each with variations and alternatives, are the mainstays of bus standards for SBC communications.

VME traces back to the 1970s, when it was called the VERSAbus, and it still constitutes the lion’s share of buses for SBCs. It has continuously grown, from 32-bit, 40-Mbit/s to 64-bit, 80-Mbit/s, and to 2eSST 320-Mbit/s implementations.

According to Ray Alderman, executive director of the VME International Trade Association (VITA), VME is a billion dollar market that’s expected to grow another 10% this year. VITA has been pushing for VPX (VITA 46), which is the latest specification to gain acceptance by the SBC industry. Ratified by the American National Standards Institute (ANSI) last year, it seeks to address the needs of critical embedded-system designs.

VME addresses both the needs of older 6U form factors as well as newer 3U factors. It also complements the VME standard that dominates most available SBCs. Both VME and VPX embody baseline specifications that define mechanical and electrical parameters for SBCs.

The VPX standard can support data communications over a range of 3 to 100 Gbytes/s. It raises the amount of power handled in an SBC board slot to 115 W at 5 V, from the present 90 W at 5 V for a VME bus slot. In addition, it allows for power levels of 384 W at 12 V or 768 W at 48 V.

VPX supports existing standards for cooling SBCs, but it also provides for more stringent cooling requirements via VITA’s Ruggedized Enhanced Design Implementation (REDI), formerly known as the VITA 48 standard. And though VPX is largely compatible with VME, it has a new type of connector developed by Tyco known as the MultiGig RT2.

The 6U board includes six 16-column, seven-row connectors and one eight-column, seven-row RT2 connector. The 3U board has two 16-column, seven-row RT2 connectors and one eightcolumn, seven-row RT2 connector. The MultiGig R2 boards aren’t compatible with VME connectors, though VITA envisions the use of a “hybrid” chassis to mate with VME boards, as allowed in the VPX standard.

Designers at GE Fanuc Intelligent Platforms believe that focusing on VPX offers the best way to provide high-performance computing for present and future rugged environments. “We fully expect VME and CompactPCI to continue in production for years, but VPX is the platform of choice for many new designins,” says Richard Kirk, SBC global product manager. GE Fanuc makes a number of VPX SBCs, like the 3U VPX SBC330 and 6U SBC, both based on the Freescale 8641D Core Duo PowerPC.

The VXS (VME switched serial) bus combines the event-driven VME parallel bus with enhancements to support switch fabrics. It allows for data communications over a wide range of 3 to 30 Gbytes/s. Like the VPX standard, it can plug into VME bus backplanes.

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According to Pentek, its model 4207 is the industry’s first VME/VXS SBC to integrate PowerPC, FPGA, and multiple high-speed gigabit serial interfaces (Fig. 4). It “combines so many standard interfaces and protocols, making it an extremely flexible single-slot solution,” says Rodger Hosking, a Pentek vice president. These interfaces and protocols include VXS, PMC, PCI-X, PCI Express, Gigabit Ethernet, Serial RapidIO, Xilinx’s RocketIO Ethernet transceiver, Fibre Channel, Xilinx’s Aurora FPGA technology, and VME6x technology.

Like VPX, the VITA 42 or XMC mezzanine standard is an open standard that supports high-speed, switched-fabric interconnect protocols on the PCI Mezzanine Card (PMC) form factor. The XMC standard specifies a fifth connector that handles PCI Express and other highspeed serial interfaces like RapidIO and parallel RapidIO.

Targeting rugged environments, the ESMexpress (ANSI/VITA 59) mezzanine standard covers SBCs that can plug into PMC and XMC SBCs. MEN Micro’s PowerPC-based ESMexpress XM50 SBC accommodates up to 2 Gbytes of soldered DDR2 SDRAM with error-correcting code (ECC), in addition to SRAM and ferroelectric RAM (FRAM) (Fig. 5). The XM50 also supports USB flash memory.

A number of SBC and plug-in mezzanine- card manufacturers are introducing products based on the VPX and XMC standards, in both 3U and 6U form factors. These include SBCs that use both Intel and PowerPC processors for graphics, mass-storage, and switching applications.

The VPX6-185 SBC from Curtiss-Wright Controls Embedded Computing features a dual-core PowerPC (a singlecore option is available), 64 kbytes of level 1 cache and 1 Mbyte of level 2 cache per CPU core, up to 2 Gbytes of DDR2 SDRAM (SDRAM), up to 512 Mbytes of flash memory, and two high-performance memory controllers (Fig. 6).

Its high-performance I/O complement includes four Gigabit Ethernet ports, two PMC/XMC ports with PCI-X and PCI Express interfaces with an XMC I/O, an optional VME64 interface, a four-channel serial I/O, and a two-channel USB 2.0 I/O. Each of its four-lane fabric ports is individually selectable to be either a serial I/O or a PCI Express port. The SBC supports VxWorks and Linux operating systems, Continuum firmware, an SSL Altivec-optimized DSP library, and IPC software for serial I/O and inter-core communications.

Embracing VPX and XMC alike, Quantum 3D’s Sentiris 5140 XMC mezzanine graphics accelerator mezzanine card offers 256 Mbytes of DDR3 memory (Fig. 7). This real-time COTS-based product uses AMD’s ATI Radeon HD3650 graphics processing unit to deliver top-notch image quality and performance, according to the company.

“We see sensor technology in aerospace applications becoming more complex and requiring that SBCs and support mezzanine cards be located in the aircraft itself, instead of data being transmitted down to a basestation, which requires very wide bandwidths,” says Alan Commike, principal high-performance computer architect for Quantum 3D.

The Sentiris 5140 is supported on x86 architectures running Windows XP Pro and XP Embedded, as well as Linux operating systems. It uses eight or 16 lanes of PCI for communications. At 16 lanes, it features 4 Gbytes/s of data transfer to a host interface. There’s also an optional PMC connector.

ADVANCEDTCA
Two of the newer serial buses used for telecommunications are the AdvancedTCA (Advanced Telecom Computing Architecture) and its sibling MicroTCA from the PCI Industrial Computer Manufacturers Group (PICMG).

These open-architecture specifications allow SBC and mezzanine-card designers to fully take advantage of low-cost COTS components using networked topologies with improved thermal performance. They reduce the cost base of a board by eliminating the need for unnecessary support circuitry for communications functions.

Emerson Network Power Connectivity Solutions takes advantage of both buses in its PrAMC-7210 processor module and 10-Gbit ATCA processor blade mezzanine card. Powered by an Intel Core 2 Duo processor running at 1.5 GHz with 4 Mbytes of L2 cache, the PrAMC-7210 suits applications that combine the advantages of multicore processing performance with high-speed serial transmissions like Gigabit Ethernet and PCI Express. The card features a 16-core Cavium Octeon processor for high-performance computing applications.