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A Linux-oriented Intro to Embeddable Single Board Computers

Mar 9, 2002 — by Rick Lehrbaum — from the LinuxDevices Archive

What's an SBC?

Early microcomputers typically consisted of half a dozen (or more) circuit boards — plugged into a backplane — which implemented the central processor unit (CPU), memory, disk controllers, and serial/parallel port functions. These backplane-based microcomputers were used for data acquisition, process control, and R&D projects, but were generally… too bulky to be used as the intelligence embedded within devices.

By the early 80's, integrated circuit (IC) technology had advanced to where functions that previously occupied entire circuit boards could be crammed into single “large scale integration” (LSI) logic chips. LSI chips for CPU, memory, storage, and serial/parallel ports now made it practical to implement complete microcomputer systems on a single board — without backplanes. The Z80-based “Big Board” (1980) was probably the first such single board computer (SBC) that was capable of running a commercial disk operating system (CP/M).

The embedded SBC market

Like the Big Board, the “Little Board” (Ampro, 1983) used a Z80 CPU and was targeted specifically at the CP/M operating system. But it was much smaller in size, matching the footprint of a floppy disk drive (5.75 x 8.0 in.). Thanks to its unique combination of compactness, simplicity, reliability, and low cost, the Little Board made it practical for a commercial disk operating system to be easily embedded directly within devices that were not themselves computers.

Thus was born the embedded SBC market, which by now has become crowded with hundreds of SBC manufacturers producing thousands of different SBC products that target a vast array of embedded and dedicated computing applications.

Initially, every SBC product was completely unique — both architecturally and physically. This was largely due to the inherent diversity of embedded system requirements, combined with the wide assortment of processors and peripheral controllers that were available. Moreover, there were no standards to influence SBC developers' choices of functional and mechanical specs.

The dawn of the PC-compatible SBC

By the mid '80s, there was growing interest in IBM PC compatibility in embedded and other non-desktop applications, for two key reasons . . .

  • Hardware leverage — PC chipset and peripherals compatibility could produce systems that were lower cost, simpler, and easier to support

  • Software leverage — PC compatibility could make it possible to take advantage of the PC's operating systems (first MS-DOS, then Windows), languages, tools, and application software

Some of the resulting PC-compatible microcomputers were based on the form-factor of the IBM PC (“ISA bus”) plug-in card. Others were implemented as standalone (non-backplane) systems on a single board. Still others were adaptations of popular industrial backplane buses (STD, VME).

In the case of embeddable non-backplane SBCs, the trend towards PC compatibility quickly became a stampede. Consensus also emerged around several popular form-factors . . .

  • Little Board (5.75 x 8.0 in.) — complete systems on a single compact board, expandable with plug-on function modules

  • ISA “slot boards” (full-length, 13.8 x 4.8 in.; half-length, 7.1 x 4.8 in.) — SBCs in the IBM PC plug-in card format which, though backplane-oriented, could also function as standalone SBCs (without backplanes)

  • PC/104 modules (3.6 x 3.8 in.) — compact, rugged, self-stacking modules featuring a reliable pin-and-socket board-to-board expansion bus

And with the coming of PCI, these were joined a decade later by . . .

  • PC/104-Plus — PCI added to PC/104

  • EBX — PC/104-Plus added to Little Board

Not all SBCs jumped on these popular form-factors. Nor did they all go the PC-compatible (x86/DOS/Windows) route. Throughout the multi-decade history of single-board computers, there have been — and continue to be — non-standard board sizes and processor architectures that target unique application requirements or fill niches not well matched to the standardized form-factors and popular “Wintel” (PC compatible) architecture.

Strongly blowing winds of change

Today, several significant factors seriously challenge the SBC market status quo . . .

  • Exploding demand for embedded intelligence — even the tiniest and least expensive products and devices are now expected to have at least rudimentary embedded intelligence. Many also require user-friendly graphical and/or speech interfaces.

  • Ubiquitous connectivity — there is growing need for everything electronic to be interconnected, whether by wires or wireless. These devices must often be capable of inbound or outbound Internet connectivity and must support numerous standardized protocols (TCP/IP, PPP, HTTP, FTP).

  • Evolving peripheral and bus interfaces — although popular interconnection standards can sometimes seem immortal (consider Centronics and RS232), new interfaces do gradually supplant the old. Nearly two decades after the PC's birth, the ISA bus has finally been replaced by PCI. USB is now replacing the venerable serial, parallel, and PS/2 ports. Ethernet is everywhere and FireWire (IEEE-1394) is beginning to make a strong showing. SCSI never made it to mainstream in PCs (other than the Apple's). We may well stand on the verge of backplane-free systems whose only expansion mechanism is via medium- and high-speed serial interfaces (USB, IrDA, FireWire, Ethernet, . . .).

  • Application-oriented system-on-chip processors — numerous highly integrated ARM, MIPS, PowerPC, and x86 based one-chip systems are being developed to match the specs of a wide array of high volume and cost sensitive appliance-like products. Today, these “application-on-chip” processors represent tantalizing fodder for a new breed of high-integration, high-performance — and highly cost-effective — SBCs. Many of these SOCs have abandoned x86 compatibility for the sake of cost/power/integration benefits.

  • Embedded Linux — in just a few short years, Linux has exploded onto all aspects of the computing scene, offering a low cost, open source solution with strong support for open standards, networking, communications, Internet, graphics, . . . and more all the time. Despite its origins as a Unix clone for PCs, Linux now supports as broad a range of processors as any “traditional” embedded OS. Consequently, full-featured OS support for diverse architectures (beyond x86) has increased dramatically in the last several years, due to the rapidly evolving capabilities and growing architectural neutrality of Linux, resulting in a more level playing field among competing processor architectures.

Considering all these factors, it becomes evident that conditions are ripe for change in the embedded SBC market!

A little chaos theory

Prior to the embedded SBC market coalescing around the PC architecture and a handful of form-factor standards, it was nearly impossible to locate two SBCs that bore much similarity with to other. The PC architecture brought a degree of order (in several shapes and sizes) to that chaos, by serving as a unifying force — a situation which persisted for nearly two decades.

Today, with the established norms disrupted by new interfaces (USB, FireWire, Bluetooth), architectures (MIPS, PowerPC, ARM), and operating systems (Linux), the embedded SBC market had better prepare itself for a new phase of its lifecycle — one that will initially be characterized by a heightened diversity of operating systems, processor architectures, peripheral interfaces, and physical form-factors.

A few words about Linux on Embedded-PC SBCs

These days, most manufacturers of PC-compatible single board computers claim to support embedded Linux on their products — either directly, or via third-party relationships with embedded Linux software providers. However, be careful not to get too complacent when the sales rep says “Sure, we support Linux.”

In general, Linux support for PC-compatible embedded SBCs tends to be straightforward — provided: (1) the chipsets used are mainstream and fairly current; and (2) the chipsets have been used in the normal manner. Always ask the SBC vendor what specific versions of Linux they have tested, how they conducted the tests, which interfaces on the SBC were exercised, and what functions are either untested or unsupported.

Specifically, areas to watch out for include . . .

  • Display controller modes beyond VGA
  • LCD panel control signals
  • SCSI
  • PCMCIA
  • Onboard solid-state disks
  • Nonstandard functions like watchdog timers, digital I/O, and analog I/O
  • Ethernet (in some cases)

Knowing that Linux drivers or in-kernel support exists for the chips used is encouraging, but that's not sufficient. SBC manufacturers often take shortcuts to save money or board space, and in the process may unwittingly sacrifice compatibility. In short: there's no substitute for testing!

Fortunately, if you are using Linux in your embedded SBC based system, you can at least take heart in the knowledge that driver source code is readily available and that there are plenty of Linux-aware programmers around who can help you untangle any problems that may arise.

Continued



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