Chapter 3: Standard single-purpose processors: Peripherals
Embedded System Design, Vahid/Givargis Last update: 09/27/99 2:51 PM
Standard single-purpose processors: Peripherals
A single-purpose processor is a digital system intended to solve a specific
computation task. The processor may be a
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
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
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.