Embedded projects start with selecting hardware. Based on the requirements of the project, such as cost, battery life, communications, and user interface, a few processors and technologies become obvious choices. For example, low-power consumer devices frequently use ARM processors, and industrial control applications frequently use Intel x86 parts. The hardware selection doesn’t occur in isolation; the team selecting the hardware must at the same time consider the peripheral devices and driver availability.
The trend is toward purchasing off-the-shelf hardware and peripherals and putting them together with standard interconnects, like a PCMCIA or USB bus. Because most chips are SOC designs, that means the system on the chip is most of the hardware found on a typical computer; the peripherals that aren’t on the chip are a bit esoteric, proprietary, or costly, and drivers may not be available in all cases. For instance, wireless network interfaces aren’t part of an SOC and are sold as a separate module that attach to the PCI, USB, or PCMCIA bus on the board.1 Many SOC designs don’t contain storage or RAM, because those are higher cost and power consumption items. In addition, RAM and storage requirements vary greatly between designs.
Embedded projects fall into one of two categories:
• Those that ship with standard-sized hardware boards (COTS): A perfect example of this sort of project is a kiosk containing a consumer application. The case around the device is sufficiently large that the size of the board is of little concern. In fact, for a kiosk, you want to create a casing large and heavy enough to prevent tipping or theft. Although the marketing group at your company may try to give the device a fancy appearance, a PC-AT device can fit into the case with room left over for some eight-track tapes, a turn table, and a boom box. The size and form factor of the board aren’t important because there’s plenty of room due to the design of the case itself. Industrial devices also fit into this classification; most factory floors aren’t space constrained, and the appearance of the end product isn’t as important as it is for a device used by a consumer.
• Those that require custom hardware design (custom): Cell phones are great examples of these devices. For these projects, an industrial designer has authored a case that the hardware must fit into without altering the design too much. For a cell phone, the appearance as much as the capabilities matter to the end user. Products that fall into this class usually have strict power budgets for two reasons: battery life is a selling point, and the battery is the largest and heaviest (and sometimes costliest) component in the design. Making the most out of the battery means more freedom for the designers and a greater profit margin for the manufacturers. Frequently, these projects are built in high volume, so the work put into reducing the per-unit cost can be amortized over tens of thousands of units, making the effort worthwhile.
A half step is emerging between these categories: the stamp form factor, where a small board contains the processor and a few peripherals, like memory or flash, and has leads so it can be plugged into a daughter board. The task of the hardware designer is much less complex than when designing a complete board but much more involved than using a COTS board. The hybrid approach means lower hardware-design costs, a smaller form factor, and more control over the power consumption of the board, because the hardware designer can pick and choose the exact set of peripherals included on the board.
Understanding which category your project falls into helps you understand the development approach and how Linux can be used. In both cases, development begins on a (aptly named) development board that has nearly all the peripherals supported by the chip. Chip vendors like to target their chips to certain market segments (such as cell phone, automotive, or industrial control); they sell development kits containing a board with the processor bundled with the peripherals used by those developers so the engineers can quickly prototype a system. For example, a cell phone development kit includes a small screen, a numeric keypad, and GPRS hardware so software and hardware engineering can begin quickly. The final hardware design includes the same, or a similar, CPU chip on a customdesigned board with the necessary peripherals for the design that are similar to those included with the development kit.
However, teams of the COTS variety select a development board that best meets their requirements and use it as is for the product. Such users pick a standard form factor (like mini-itx) so the company making the case can put the mounting hardware in the right place inside the kiosk or the right case can be sourced for the project. When you’re working on a COTS project, the board used for development is identical to the board deployed in the product. Although a COTS board is more expensive on a per-unit basis, there is little hardware engineering fixed cost to amortize over the production run. Designing a custom board costs at least $50,000, and tooling costs for fabrication are at least that much again, so a product with a production run of a few hundred units has an average cost much higher than the average cost of purchasing an off-the-shelf board.
Source of Information : Pro Linux Embedded Systems
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