ch3 - Chapter 3 Standard single-purpose processors...

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Chapter 3: Standard single-purpose processors: Peripherals Embedded System Design, Vahid/Givargis Last update: 09/27/99 2:51 PM 3-1 Chapter 3 Standard single-purpose processors: Peripherals 3.1 Introduction A single-purpose processor is a digital system intended to solve a specific computation task. The processor may be a standard one, intended for use in a wide variety of applications in which the same task must be performed. The manufacturer of such an off-the-shelf processor sells the device in large quantities. On the other hand, the processor may be a custom one, built by a designer to implement a task specific to a particular application. An embedded system designer choosing to use a standard single- purpose, rather than a general-purpose, processor to implement part of a system’s functionality may achieve several benefits. First, performance may be fast, since the processor is customized for the particular task at hand. Not only might the task execute in fewer clock cycles, but also those cycles themselves may be shorter. Fewer clock cycles may result from many datapath components operating in parallel, from datapath components passing data directly to one another without the need for intermediate registers (chaining), or from elimination of program memory fetches. Shorter cycles may result from simpler functional units, less multiplexors, or simpler control logic. For standard single-purpose processors, manufacturers may spread NRE cost over many units. Thus, the processor's clock cycle may be further reduced by the use of custom IC technology, leading-edge IC's, and expert designers, just as is the case with general-purpose processors. Second, size may be small. A single-purpose processor does not require a program memory. Also, since it does not need to support a large instruction set, it may have a simpler datapath and controller. Third, a standard single-purpose processor may have low unit cost, due to the manufacturer spreading NRE cost over many units. Likewise, NRE cost may be low, since the embedded system designer need not design a standard single-purpose processor, and may not even need to program it. There are of course tradeoffs. If we are already using a general-purpose processor, then implementing a task on an additional single-purpose processor rather than in software may add to the system size and power consumption. In this chapter, we describe the basic functionality of several standard single- purpose processors commonly found in embedded systems. The level of detail of the description is intended to be enough to enable using such processors, but not necessarily to design one. We often refer to standard single-purpose processors as peripherals, because they usually exist on the periphery of the CPU. However, microcontrollers tightly integrate these peripherals with the CPU, often placing them on-chip, and even assigning peripheral registers to the CPU's own register space. The result is the common term "on- chip peripherals," which some may consider somewhat of an oxymoron. 3.2
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This note was uploaded on 06/02/2011 for the course CS 550 taught by Professor Young during the Spring '11 term at New York Institute of Technology-Westbury.

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ch3 - Chapter 3 Standard single-purpose processors...

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