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U. Glaeser

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FIGURE 23.1 An embedded system includes a microcomputer with electrical, mechanical, chemical, and optical<br />

devices. A microcomputer includes a processor with memory, input, and output devices.<br />

provide necessary functionality. The software together with the I/O ports and associated interface circuits<br />

give an embedded computer system its distinctive characteristics.<br />

The term “microcomputer” means a small computer. Small in this context describes its size not its<br />

computing power, so a microcomputer can refer to a very wide range of products from the very simple<br />

(e.g., the PIC12C08 is a 8-pin DIP microcomputer with 512 by 12 bit ROM, 25 bytes RAM, and five I/O<br />

pins) to the most powerful Pentium. One typically restricts the term “embedded” to refer to systems that<br />

do not look and behave like a typical computer. Most embedded systems do not have a keyboard, a graphics<br />

display, or secondary storage (disk). Embedded systems can be developed in two ways. The first technique<br />

uses the microcomputers that are available as single chips. These devices are suitable for low-cost, lowperformance<br />

systems. On the other hand, one can develop a high-performance embedded system around<br />

the PC architecture. These systems are first developed on a standard PC, and then the software and<br />

hardware are migrated to a stand-alone embedded-PC platform.<br />

One can appreciate the wide range of embedded computer applications by observing existing implementations.<br />

Table 23.1 illustrates the breadth of applications that use an embedded microcomputer.<br />

23.2 Embedded Processors<br />

Processor<br />

In the last 30 years, the microprocessor has made significant technological advances. The term microprocessor<br />

refers to products ranging from the oldest Intel 8080 to the newest Pentium. The processor or CPU<br />

controls the system by executing instructions. It contains a BIU, which provides the address, direction (read<br />

data from memory into the processor or write data from processor to memory), and timing signals for the<br />

computer bus. The registers are very high-speed storage devices for the computer. The program counter<br />

(PC) is a register that contains the address of the current instruction that the computer is executing. The<br />

stack is a very important data structure used by computers to store temporary information. It is very easy<br />

to allocate temporary storage on the stack and deallocate it when done. The stack pointer (SP) is a register<br />

that points into RAM specifying the top entry of the stack. The condition code (CC) is a register that<br />

contains status flags describing the result of the previous operation and operating mode of the computer.<br />

Most computers have data registers that contain information and address registers that contain pointers.<br />

The ALU performs arithmetic (add, subtract, multiply, divide) and logical (and, or, not, exclusive or, shift)<br />

operations. The inputs to the ALU come from registers and/or memory, and the outputs go to registers or<br />

memory. The CC register contains status information from the previous ALU operation.<br />

Software is a sequence of commands stored in memory. The control unit (CU) manipulates the<br />

hardware modules according to the software that it is executing. The CU contains an instruction register<br />

(IR) which holds the current instruction. The BIU contains an effective address register (EAR) which<br />

holds the effective address of the current instruction. The computer must fetch both instructions (op<br />

codes) and information (data). The BIU controls both types of access.<br />

When an instruction is executed, the microprocessor often must refer to memory to read and/or write<br />

information. Often the I/O ports are implemented as memory locations. For example, on the Motorola<br />

© 2002 by CRC Press LLC<br />

embedded system<br />

microcomputer bus<br />

electrical,<br />

processor bus RAM mechanical,<br />

interface<br />

registers<br />

ROM<br />

chemical,<br />

unit<br />

or<br />

optical<br />

control unit ALU I/O Ports<br />

devices

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