Build or buy. That’s always a tough decision since a wide span of variables can come into play, especially as the specifications list usually includes size, weight, and power requirements.
Large systems often are addressed with PCs or backplane-based systems using board form factors such as Compact PCI, Versa Module Eurocard (VME), and Virtual Path Cross-Connect (VPX). Smaller designs need to turn to more compact form factors. Solutions range from form factors that are unique to their vendors to standards from a number of different organizations.
The PC/104 Consortium fosters the venerable PC/104 form factor with its instruction set architecture (ISA) bus. The group’s newer specifications feature high-speed serial interfaces like PCI Express (see “An EPIC Tale: PC/104 Hitches On To PCI Express”).
Likewise, the Small Form Factor Special Interest Group (SFF-SIG) defined SUMIT, or Stackable Unified Module Interface Technology (see “SUMIT Brings Big Improvements In Small Packages”). The PCI Industrial Computer Manufacturers Group (PICMG) hosts a wide range of standards from Compact PCI to AdvancedTCA, but it also handles COM Express, a compact computer-on-module form factor.
Gumstix has a line of tiny modules (see “A Pack of Gumstix”) based on Texas Instruments’ OMAP processor (see “OMAP ARMed With Cortex”). The modules are only 4.2 mm high with a pair of 70-pin AVX connectors (Fig. 1).
The company even built a seven-node cluster with its Stagecoach product (see “A Cluster of Gumstix). The Stagecoach has a 100-Mbit/s Ethernet interface and draws under 20 W when fully populated (Fig. 2).
Small modules like the Overo often require no heatsink because they consume so little power. Sometimes a heatsink comes in handy, like the one found on one of Digi’s Rabbit Core modules, the MiniCore RCM5600W (see “What Microcontroller Is Best?”).
The RCM5600W has a 74-MHz Rabbit 5000 16-bit microcontroller with 128 kbytes of on-chip SRAM and an additional 1 Mbyte of SRAM on the module (Fig. 3). It also has 802.11b/g Wi-Fi support and 802.11i security support. The form factor is a 52-pin Mini PCI Express card, but the module uses its own pinout and does not have PCI Express support.
Taking on the same form factor as other devices is a common approach, allowing the reuse of connector technology as well as cost efficiencies. For example, Eurotech’s Quantum has a 144-pin small-outline dual-inline memory module (SO-DIMM) edge connector (Fig. 4).
The 67.6- by 50-mm module holds a 520-MHz Marvell XScale PXA270 processor and 64 Mbytes of SDRAM. It consumes less than 1.5 W and includes a host of interfaces, including thin-film transistor/super-twisted nematic/low-voltage differential signaling (TFT/STN/LVDS) flat-panel support and an AC’97 audio controller.
Logic PD is an industrial design company that provides a range of modules and development tools. Its AM1808 SOM-M1 (Fig. 5) holds the Texas Instruments Sitara ARM AM1808 microprocessor (see “ARM9 Micros Target Industrial and Medical Apps”).
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Like many modules, the AM1808 SOM-M1 hides its interconnect beneath the board. It also packs a host of peripherals and memory including an LCD controller, 100BaseT Ethernet, and up to 8 Mbytes of NAND flash plus Serial Advanced Memory Attachment (SATA) and multimedia card/storage device (MMC/SD) interfaces, all on a 30- by 40-mm card.
The InHand Electronics Fury board fits in your hand, including an LCD display (Fig. 6). It holds the Texas Instruments DaVinci DM3730 video processor and the 1-GHz Sitara AM3703 ARM processor. The Fury board is available without the LCD and case.
There are several ways to bring a lot of computing power to bear on a problem. Multicore and multiple processor modules are just one way. Opal Kelly’s XEM-3050 module does it with a Spartan 3 FPGA (Fig. 7). The module also holds 64 Mbytes of SDRAM, 9 Mbytes of SRAM, and 1 Mbyte of serial peripheral interface (SPI) serial flash.
This small collection of tiny modules just scratches the surface. Most of the form factors for these tiny modules are unique to one vendor, though most utilize standard connectors. Some modules may look the same, but different pinouts prevent them from being interchangeable. And next, a step up in size yields a plethora of processors on motherboards.
Micro-ATX and Mini-ITX devices like ITOX’s Core i7-based CP100-NRM, which features dual DVI displays and HDMI support, often fit the bill for compact platforms finding homes in vehicles and production lines (Fig. 8).
These smaller motherboards offer the advantage of expansion using PCI and PCI Express cards. The CP100-NRM has a PCI Express/PCI slot along with dual Gigabit Ethernet ports, six USB ports, four serial ports, and more. The board can also handle Core i5 and Celeron P4500 processors.
The Via Technologies Nano-ITX and Pico-ITX also have a popular following. The 120- by 120-mm Nano-ITX is smaller than the Mini-ITX and Micro-ATX boards and often foregoes expansion slots.
Fanless Nano-ITX boards like Emerson Network Power’s Atom-based NITX-300 tend to be used applications like set-top boxes. The Emerson board actually has both a x1 PCI Express socket and a Mini PCI Express socket in addition to Gigabit Ethernet, SATA, and audio and video support.
The smaller Pico-ITX is often used in a standalone fashion, but the Pico-ITXe incorporates a SUMIT connector. AcessIO’s Pico-ITXe data acquisition modules provide expansion on this smaller form factor where PC-style PCI and PCI Express (Fig. 9) slots do not fit.
PC/104 is the quintessential stacking system, but it was just the starting point for a range of stackable solutions (see “Playing The Board Game: Stack’em, Pack’em, And Rack’em”). PC/104 boards use the ISA bus. The bus and connectors allow the processor board to be in the top, bottom, or middle of the stack.
This platform has supported and spawned a range of related motherboard form factors like Explicitly Parallel Instruction Computing (EPIC), EPIC Express, and Embedded Board Expendable (EBX), to mention a few that utilize one or more variations of PC/104’s stacking architecture for expansion.
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Many new standards are coming together to give life to old form factors. WinSystems’ EBC-Z5xx is an EBX-style motherboard, but it sports a pair of SUMIT connectors providing PCI Express support in addition to the PC/104 ISA expansion that’s part of the SUMIT-ISM layout (Fig. 10). The industry-standard module (ISM) is essentially PC/104.
These stackable systems can handle half a dozen or more modules, but the typical stack size is just 1.5 boards because the motherboards often provide the required peripheral support. Having the expansion capability is often enough, but plenty of systems employ a number of expansion boards.
USB is another mechanism that is becoming more popular inside the box for connecting processors to peripherals. There is even a stacking USB standard called StackableUSB from Micro/sys (see “USB Thinks Inside The Box”). The small versions of the StackableUSB boards are one-quarter the size of the PC/104 boards (Fig. 11).
Of course, USB expansion does not have to stack. Cabled solutions, even inside the box, are a viable option. The USB devices can be as small as Toradex’s OAK USB sensor (Fig. 12). These sensors take power from the USB cable.
Moving up in size are boards like those available from AcessIO (see “USB Module Provides 96 or 48 Digital I/Os”). Like most USB devices, they work equally well outside the box. AcessIO provides most of its USB boards with a metal case for use outside the box. The boards can also be stacked inside the box, although the boards are connected via a USB cable.
There is a host of proprietary stackable form factors as well. The Boston Engineering FlexStack is one example of a tiny stack (see “Make Or Buy: Module Mania”). Freescale’s Tower system is another example, though it’s strictly designed as a development and prototyping platform (see “What Microcontroller is Best?”).
But back to the realm of standards.
One computer-on-module form factor standard that has garnered a large following is COM Express, which spans a range of form factors with a common connector configuration. In fact, PICMG has defined a number of combinations. The standard COM Express boards measure 125 by 95 mm, while the microETXexpress version is 95 mm square.
Kontron’s nanoETXexpress highlights the smallest COM Express form factor at 55 by 84 mm (Fig. 13). It uses Intel’s latest 1.6-GHz E6xx “Tunnel Creek” Atom processor (see “Tunnel Creek Takes A Number”).
The E6xx meshes well with COM Express, which was designed around PCI Express. It provides four 1x PCI Express lanes instead of using a proprietary front-side bus. The chip is already showing up in stackable PCI Express form factors as well.
COM Express isn’t the only multivendor computer-on-module standard. Qseven modules like Portwell’s PQ7-M100G are available from a number of sources including ITOX, congatec AG, Advantech, and DFI, among other vendors (Fig. 14).
Qseven plugs into a ruggedized Mobile PCI Express Module (MXM) connector. The edge connector, mounting holes, and a cooling plate connection occupy part of the 70- by 70-mm region.
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Standards like PC/104 utilize the same form factor for the processor and the peripheral. Computer-on-module boards typically plug into a custom carrier board that may provide connections to the outside world and occasionally additional peripherals.
Peripheral card form factors like Mini-PCI and Mini-PCI Express are often found in netbooks and laptops. They are typically used to provide wireless communications and GPS support. Of course, they can come in handy in other environments as well, and Mini-PCI and Mini-PCI Express connectors have found homes on a host of carrier boards.
Serial Peripheral Expansion (SPX) is another form factor from Versalogic. SPX boards are connected via a daisy-chained SPI (Fig. 15). The connection uses a standard 14-pin ribbon cable, or the boards can plug into a header on a motherboard.
In theory, the SPX modules can be used with any micro that has an SPI. In practice, they normally would be used in Versalogic’s single-board computers like the EBX Sidewinder or PCI-104 Wildcat.
Originally developed by Diamond Systems, FeaturePak is a new peripheral form factor (see “Module Packs I/O Features”). The Diamond Systems SUMIT/FP Adapter is designed to hold a FeaturePak module (Fig. 16).
The module is 43 by 65 mm and designed to fit within the confines of boards like PC/104 and the Themis Nano-ATR (see “Shootout At The VPX Corral”). It handles up to 100 I/O pins, and devices can be accessed by PCI Express, USB, or I2C, which a module typically will use.
Dev boards are less likely to wind up in production systems, but they make popular prototype platforms. They include the BeagleBoard with Texas Instruments’ dual-core OMAP3 platform with a 600-MHz Cortex-A8 and C64+ DSP (see “Open Source Bites Board”) as well as the HawkBoard (Fig. 17), which has an OMAP-L138 based around an ARM9 and a C674x DSP. They provide access to all the peripheral ports like Beagleboard’s HDMI display support. A few expansion boards are even appearing.
For a more compact platform, developers can turn to the mbed from NXP (Fig. 18). This tiny $59 module is built around an NXP LPC1768 Cortex-M0 microcontroller and plugs into a 40-pin socket (see “Cortex-M0 MCU Challenges 8- And 16-Bit Micros Across The Board”). Several base boards are available.
If you’re looking for a bargain, check out the $10 STM32 Discovery Kit from STMicroelectronics (Fig. 19). The module is a bit larger than the mbed but provides full access to the 24-MHz, 64-pin STM32F100RBT6B microcontroller. The chip has 128 kbytes of flash. Almost half the module is dedicated to the USB-based ST-Link debug interface.
A built-in USB debug interface and free development software are a couple of the things these dev boards have in common. Minimally, they are a good way to get a handle on the microcontrollers.
One thing is clear. There are many choices. That doesn’t necessarily make the choice easier, but it does mean there is likely to be an option that comes close to your design requirements.