Selecting a Processor for a PC/104-based Embedded PC

by Peter Kannegießer

Designers of embedded PC’s need to select a suitable processor for their designs with good con-sideration. This can be a quite complex task, considering that there are several competing proc-essor families on the market, each with their own distinctive features. This article shows some of the key issues to check.

The most important criterions for processors for embedded PC’s are component size, power con-sumption, cooling requirements, followed by long-time availability and computing performance.

Of these, the issue of component size is very easily grasped: Imagine a PC/104 board with its 3.550” by 3.775” measurements; it is very clear that the processor and its socket must be very much smaller than that.

Power consumption and cooling requirements are largely related, of course. If there are more than about 25 watts are dissipated by a processor, this thermal energy must be actively brought out of the die. This means that there has to be a cooling fan or a heatpipe (there are others, but those are left out for the sake of brevity). The best cooling method for embedded devices is pas-sive, with a heatsink only; mechanical fans are prone to failure, heat pipes are rather expensive and sometimes simply impossible to mount.

Performance is very difficult to assess; its absolute number depends largely on the application. However, in order to create a universally suitable module for the embedded field, one must strive to get the best performance vs. power consumption figure.

Very important in the embedded area is the long-term availability of the parts. More than five years are usually required to fit the industry’s long investment cycles and replacement needs. This is assured in most cases only for components on the so-called “embedded roadmap”, as op-posed to the consumer market devices. Today, this issue is further complicated by the upcoming legislation about lead-free components.

Traditionally, for modules with more computing performance than possible with microcontrollers, the X86 architecture was often chosen. This selection has the advantage of well-known software support with both standard desktop and real time operating systems and highly developed soft-ware tools as well.

Today, the situation is not so clear anymore. The widespread proliferation of Linux allows the use of any processor architecture that is powerful enough for running it, while at the same time provid-ing good tools for the developer and user. There are quite a number of architectures available for integration on PC/104 boards, for instance ARM, MIPS, and Power PC. It might be necessary to add hardware in order to support the ISA bus interface, but this is rather easily done.

It must be noted that almost all processors are available in different flavors; for instance, there are variants available with different clock rates or with diverse on-chip peripherals. All these variants need to be carefully considered for a concrete project. Additionally, there is the matter of the chipsets available for the selected processor; this alone could fill an article of its own and conse-quently is left out here.

Of course, the actual selection depends heavily on the application and the developer skills available at hand. It needs also be considered that a processor, which is not on its manufacturer’s embedded roadmap, can get obsolete very fast. Last but not least, there are also cost issues to be accounted for.

For its X86-based PC/104 modules, LiPPERT has chosen the Pentium M and the Geode GX 500 processors. Examples are the Cool RoadRunner 4 and the Cool FrontRunner (see pictures). These cover the performance range from clock speeds of 300 MHz all the way up to above 2 GHz, while dissipating only very little power. Thus these modules can often be operated without using a mechanical fan. As mentioned above, the software support in the X86 area is huge, which in turn allows the application developer to reach very short development times.

(Original appeared in Extension Media's PC/104 & PC/104-Plus Embedded Resource Catalog 2005)

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