Page 278 - Embedded Microprocessor Systems Real World Design

no disk drive or keyboard, you still have those interfaces on the CPU board that
you buy. CPU board vendors do not carry a large number of CPU boards to fit every
need. Since silicon is relatively cheap, they carry just a few boards that contain
nearly everything a user might want. There is little choice about this since these
boards must be designed using off-the-shelf chipsets (discrete logic would be slow,
expensive, and consume enormous real estate). Most of the chipsets that inter-
face to x86 family processors contain standard PC peripheral functions. The idea
is to shrink the standard functions to the smallest size/cost possible for PC
motherboards.

Hardware Development

For standard interfaces, off-the-shelf boards are available. However, if a proprietary
interface is required or if some unavailable function is needed, hardware must be
designed anyway. A distributed system, with low-level motor controllers and inter-
faces, probably has a PC as a central controller, and everything else is custom-made.
The more hardware that must be designed, the less leverage an off-the-shelf CPU
provides.

Keyboard and Monitor

The standard PC has a keyboard and monitor attached. They are bulky and some-
times unnecessary for the specific embedded application, but they must be there.

Parts Availability

Try to buy a PC motherboard, and then try to buy the same motherboard a year
later. This is nearly impossible to do-the designs just change too often. This can
be a real problem, especially if every iteration of the design requires new EM1 or
safety agency investigation.

Not Real Time

PC operating systems, such as DOS and Linux, do not operate in real time. Some
PC operating systems are multitasking, but that still does not mean they are real
time. PC operating systems are not real time because they are not deterministic-
you do not know how long an operating system function takes to execute. Some
applications do not care if the operating system goes away for a quarter of a second
to get something from disk; others do.

Mass Storage
This is an advantage if you need it. If you do not, you still need the disk drive from
which to load your operating system and your programs.

Industry-Standard Embedded Platj&rms 259