Freescale Semiconductor 68HC11 Manuel utilisateur

Objectives

Intr odu ct ion to Com put e r H ardw are

After completing this chapter, you should be able to:

◗Describe the fundamental elements of every computer system: proces-

sor, memory, and input/output

◗Compare elements of the HC block diagram to the fundamentals of

every computer system

◗Describe the use of busses to connect computer elements

◗Explain the three major functional units of a processor

◗Illustrate the typical registers inside the processor

◗List the HC processor registers

◗Discuss the HC processor modes

◗Compare and contrast various memory types

◗Describe the on-chip memory of the HC

◗Specify input/output functions present on most computers

◗Use some basic BUFFALO commands to control the EVBU

Outline

◗. Elements of Every Computer

◗.2 Elements of Processors

◗.3 Introduction to Memory

◗.4 Memory Types

◗.5 Input/Output

◗.6 EVBU/BUFFALO

Introduction to Computer Hardware

chapter 1

Introduction

Computer systems have been developed for a variety of functions and purposes.

General-application desktop machines are the most common. They run a variety of

software applications, such as word processing, financial management and data

processing. They have all but replaced the typewriter as a necessary business tool.

Computers are also present in automobiles, appliances, airplanes and all types of

controllers and electromechanical devices.

Despite the differences among these computer systems, they all share fundamental

components and design. The purpose of this chapter is to provide an understanding

of the fundamental components of a computer system. A conceptual presentation

regarding the elements of every computer system is made with sufficient detail to

establish a foundation for these concepts. The concepts will then be extended to the

HC hardware.

1.1 Elements of Every Computer

All computers are made up of a group of three fundamental elements: a central

processor, memory, and input/output devices. Figure . shows a block diagram of a

computer that includes these three elements. The examination of any kind of desktop

computer, workstation or computer control system will reveal at least this minimum

structure. Many computing devices will have multiple processors, multiple memory

types, and numerous input/output devices. In many cases, the input/output devices

contain all three elements as a unit. Video cards for personal computers, for example,

always contain a video processor and memory in addition to their inherent input/out-

put capability.

Central

Processor

(CPU)

Connection to the

Outside World

Memory

Input/Output

Data Bus

Address Bus

Control Bus

Figure 1.1 Fundamental Block Diagram of a Computer

Technician’s Guide to the 68HC11 Microcontroller

HC11 Hardware Block Dia ram

The HC is a computer system on a single chip that contains the three functional

blocks of a computer system. Its internal central processor is a member of the 6800

family of processors; it has on-board memory and sophisticated on-chip input/output

capabilities.

The HC block diagram is shown in Figure .2. This block diagram is specific to the

M68HC E9 version of the HC . This version of the HC is used on the development

board (EVBU), as well as in the examples used throughout this text. Three types of

memory are included on-chip: RAM, ROM and EEPROM. The HC also supports

expanded off-chip memory. It contains five on-chip input/output functions, an

analog-to-digital converter, and a sophisticated timing system that supports numerous

event-driven functions. The address, data and control busses are not shown in this

diagram; the processor is connected to the memory and input/output functions inside

the HC chip in the manner illustrated in Figure . .

Figure 1.2 M68HC11E9 Block Diagram 3

Introduction to Computer Hardware

NOTE: Throughout this text, references to the HC presume the M68HC E9

version of the chip; thus, the shorter, more general name “HC ” will be used,

unless otherwise specified.

Central Processor

The processor is the device at the center of the machine. It has the responsibility to

execute instructions, manipulate data and perform arithmetic functions. It controls

and manages the activities of the entire machine. The human brain is the ultimate

processor. It can receive and process instructions, process data (like visual images and

sounds) and perform arithmetic calculations. However, the human brain is much,

much more than a processor because it has the ability to think and to reason. Computer

processors cannot think in the same sense.

The term Central Processing Unit (CPU) is used to refer to the main processor in a

system. The CPU often works in conjunction with a set of processors to complete a

whole system. Modern computers contain additional processors, other than the CPU.

They contain video processors, input/output processors, memory controllers, interrupt

controllers and math co-processors, to name a few. Since these other processors are

subordinate to the central processor, they are often called sub-processors. Many

peripheral devices, such as harddrives, printers and video projection systems, have

dedicated processors embedded into their control circuitry.

Memory

Memory is a term that refers to any component that stores data and programs used by

the processor. Memory can have many forms. There are semiconductor memories,

magnetic memories, and optical memories. Semiconductor memories include read

only memory (ROM) and read/write memory (RAM). Magnetic memories include

floppy disk drives, hard disk drives and tape systems. Optical memories include

CD-ROM, DVD and optical disks. A thorough presentation of memory relevant to the

HC will be provided in chapter 8. Section .3 will address concepts of memory that

are applicable to all computer systems and a necessary foundation for this study of the

HC .

Input/Output

Input/Output is a term that refers to any subsystem that has the responsibility of

receiving data for the processor (input) or sending data out from the processor

(output). Input/Output is typically abbreviated as I/O and does not necessarily imply

that a particular device has both input and output capability. Typical input devices are

keyboards, mice or scanners. Typical output devices are printers and monitors. Typical

devices that perform both input and output functions are modems or tape drives.

Collectively, all these devices are referred to as I/O devices. The HC I/O capability

is discussed in detail in chapters 0 through 3.

Technician’s Guide to the 68HC11 Microcontroller

Busses

The processor communicates with the memory and I/O via busses. On a computer, a

bus is a set of two or more conductors that are grouped together to form a parallel

information path to and/or from the processor. The bus size, given in bits, is a measure

of the number of conductors that can be active simultaneously on the bus. There are

three major busses on computer systems: the data bus, the address bus and the control

bus.

The ata bus is responsible for the transfer of data between the processor and memory or

the processor and I/O. It is a bi irectional bus, because data can travel to or from the

processor and other devices. Typically, a data bus transfers data in byte widths. Thus, a

data bus is an 8-bit, 6-bit or 32-bit bus ( byte, 2 bytes or 4 bytes wide). The number of

bits, or data bus width, also directly correlates with the default processing capacity of the

processor. Typically, processors and computers are referred to by the size of their data

busses. An HC is considered an 8-bit processor because it has an 8-bit data bus and has

the default processing capability of one byte. Pentium-based personal computers are

32-bit or 64-bit machines. Thus, they can process four or eight bytes simultaneously.

The a ress bus is responsible for the transfer of addresses from the processor to mem-

ory or to I/O. The address is used to identify specific memory locations or I/O devices. It

is a uni irectional or one-way bus, because processors are the only devices that can create

an address for memory or I/O. Typically, an address bus transfers addresses over the bus

in double byte widths. Thus, an address bus is usually a 6-bit, 32-bit, or 64-bit bus (2 bytes,

4 bytes or 8 bytes wide). The HC uses a 6-bit address bus.

The control bus is responsible for the control signals necessary to interface the various

devices within a computer system. This bus is not typically structured as a fixed number

of conductors, as are the data and address busses. It is more likely to be a collection of

all other signals necessary for proper operation of the system.

Figure .3 illustrates how busses might be implemented between an HC and some

expanded off-chip memory device. This example uses a 6-bit address bus, an 8-bit data

bus and two control signals that would be part of a larger system control bus.

HC11

Chip

External

Memory

Device

Address Bus

Data Bus

Read/Write Control

Output Enable

Two signals connected to the

memory device that are part

of the system control bus

Figure 1.3 Computer Bus Example

Introduction to Computer Hardware

Self-Test Questions 1.1

1.2 Elements of Processors

Processors have the job of processing the data within a computer and controlling the

overall operation of the system. The basic block diagram of a processor includes an

arithmetic logic unit, processor registers and a control unit, as shown in Figure .4.

Arithmetic Lo ic Unit

The arithmetic logic unit (ALU) is responsible for mathematical and logical operations.

Most processors support addition and subtraction, logical AND, OR and NOT

operations and data shifting. All of these operations are performed in an ALU. The

ALU receives data from the processor registers and from external memory via the

external data bus. It does not store data, but it returns the result of the operations to

the registers or to memory. The ALU is interfaced to a block of processor registers and

Connection to the Devices

Outside of Processor

Address Data Control

Addresses

from

Registers

Control

to/from

Registers

Control

ALU

Internal Processor

Data Bus

Processor

Registers

Arithmetic

Logic

Unit

(ALU)

Control Unit

Components of the Processor

Figure 1.4 Processor Block Diagram

. What are the three main hardware components of a computer?

2. What is the function of each of the three hardware components of a computer?

3. What connects the three hardware components and allows the data, addresses

and control signals to move between these components?

Technician’s Guide to the 68HC11 Microcontroller

to the control block, as shown in Figure .4. The HC has an ALU that can perform

operations on 8-bit and 6-bit data.

Processor Re isters

The processor registers are a set of registers needed to perform the instruction

execution. The registers are used to temporarily store data and memory addresses, as

well as to contain status and control information. These registers are accessible to the

user via instructions. A basic register block configuration is shown in Figure .5. Each

processor will have a unique design that may not include all the registers described

here. Often a processor will contain multiple copies of these registers for versatility and

enhanced functionality.

Accumulators are special registers directly linked to the ALU to assist with the

arithmetic operations. The results of arithmetic operations are stored in an accumula-

tor. The size of the accumulators is related to the size of the data bus. If the processor

is an 8-bit processor, then it will have 8-bit accumulators. If it is a 6-bit processor, then

it will have 6-bit accumulators. Moreover, processors often have the ability to process

data of various sizes. For example, an 8-bit processor may also be able to process 6-bit

data. If the processor has a 32-bit configuration, it can also process data in 6-bit or

even 8-bit words.

The program counter is a processor register that keeps track of the address of the next

location in memory that will be accessed. Every processor must have a register that

performs the function of a program counter. The program counter is the same size as

8-bit Accumulator

16-bit Accumulator

Program Counter

Stack Pointer

Address Register

Status Register

Figure 1.5 Basic Processor Registers

Introduction to Computer Hardware

the address bus. If the processor has a 6-bit address bus, the program counter is a

6-bit register. Instructions require one or more bytes of memory for their completion;

therefore, the program counter must be capable of incrementing through memory as

the instructions are executed. Without the program counter, the processor would not

be able to determine which instruction to execute. This register is sometimes called

the instruction pointer.

The processor has a need to temporarily store and retrieve data from memory during

the processing of instructions. This temporary memory area is called the stack. The

processor must have a register that contains an address pointer that indicates the next

available memory location on the stack. This register is called the stack pointer. The

stack pointer is the same size as the address bus. If the processor has a 6-bit address

bus, the stack pointer is a 6-bit register.

Most general-purpose processors contain registers that are used specifically by the

instructions to address memory. These registers have various names, but generally they

are called a ress registers. The use of address registers allows for simpler instructions,

because they do not need to be concerned about the address. The address required to

complete the instruction is already provided in the address register.

Every processor must have a register dedicated to reporting status. The status register

contains bits that indicate certain results of the last operation. The most common status

bits are sign flags, carry/borrow flags, zero flags and overflow flags. The sign flags

indicate the sign of the last data processed. The carry/borrow flags indicate that an

arithmetic operation produced a result larger than could fit into the register used or

that it had to borrow to complete the operation. The zero flag indicates that result of

the last operation was zero, and the overflow flag indicates that a sign overflow occurred

(the sign of the last operation is wrong).

Control Unit

The control unit within the processor is responsible for reading the instruction from

memory; it then ensures that the instructions are executed. The control function is

driven by the instructions that are decoded by the control unit. The decoded

instruction causes a series of steps to be followed during the execution. Figure .6 shows

the major components of the control unit within a processor.

The memory a ress register (MAR) is a special address register that is linked to the

program counter. The job of the MAR is to contain the address of the current data word

that is being addressed. The memory ata register (MDR) is another special register that

resides between the data bus and the various processor registers. It provides buffering

and control of the movement of data into and out of the processor. The MAR and MDR

are not accessible to the user, but are used internally by the processor for address and data

bus interface and to assist in the execution of instructions.

The instruction register (IR) is a special register that always contains the opcode for

the current instruction. This instruction opcode is read from memory during a fetch

Technician’s Guide to the 68HC11 Microcontroller

cycle and deposited into this register. The opcode remains in this register until it is

overwritten by the opcode for the next instruction.

Opcodes, operands and instructions are explained in Chapter 2.

NOTE: The instruction register should be called the opcode register, because it

contains only the opcode of the instruction. Operands and operand addresses

are not processed by the instruction register.

The instruction eco er is the main section of the control block. It has the job of

decoding the instruction in the instruction register and controlling the execution of

the instruction. It causes each step of the instruction execution to take place. If an

address needs to be read from memory, the instruction decoder will cause that to take

place. If data needs to be added, it will tell the ALU to perform an addition operation,

and so on.

T he process of fetching and executing an instruction is covered in chapter 2.

HC11 Processor

The HC is a member of the Motorola 6800 family of processors. It is compatible with

the M6800 and M680 processors. In addition to executing all M6800 and M680 9

Connection to the Devices

Outside of Processor

Address Data Control

Instruction Decoder

MDRMAR

Control

Unit

Addresses

from

Registers

Internal

Processor

Data Bus

Control

to/from

Registers

Control

ALU

Figure 1.6 Components of the Processor Control Block

Introduction to Computer Hardware

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