Freescale Semiconductor 68HC11 User manual

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
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
2
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.
4
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.
5
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
6
Connection to the Devices
Outside of Processor
Address Data Control
Addresses
from
Registers
Control
to/from
Registers
Control
to
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
7
b
7
b
0
b
15
b
0
b
15
b
0
b
15
b
0
b
7
b
0
b
15
b
0
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
8
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
IR
MDRMAR
Control
Unit
Addresses
from
Registers
Internal
Processor
Data Bus
Control
to/from
Registers
Control
to
ALU
Figure 1.6 Components of the Processor Control Block
Introduction to Computer Hardware

instructions, the HC supports many additional instructions. The HC supports
6-bit by 6-bit divide instructions, twelve instructions that support bit-manipulation,
and various instructions that support a second address register (Y), none of which was
available on the original 6800.
The HC MCU is an 8-bit processor, which supports a 6-bit address bus. It contains
an 8-bit ALU that can be configured to perform some 6-bit operations.
HC11 Processor Re isters
The HC has a complete set of processor registers. This set of registers is called the
programmer’s mo el, as shown in Figure .7. It has two 8-bit accumulators that can be
configured as a 6-bit accumulator for some operations. It has two 6-bit address
registers, a 6-bit stack pointer and a 6-bit program counter. Finally, it contains an
8-bit status register that contains three control bits and five status flags. None of these
registers is addressable as memory; they can only be accessed via instructions. These
registers are described in the following sections.
Accumulator A. An 8-bit register that is used as the primary data-processing register.
All arithmetic and logical instructions can operate on data in this register. Many
special-purpose instructions can only operate on data in this register. This register is
referred to as the “A” register or as “AccA.”
Accumulator B. An 8-bit register that is identical to Accumulator A in function. Some
instructions have versions that access Accumulator A only and do not support
Accumulator B. There are also two instructions that utilize Accumulator B and do not
support Accumulator A. This register is referred to as the “B” register or as “AccB.”
10 Figure 1.7 HC11 Programmer’s Model (
adapted with permission from Motorola )
Technician’s Guide to the 68HC11 Microcontroller

Accumulator D (A:B). A 6-bit register that is the two 8-bit accumulators joined
together (the colon between A and B indicates the joining of the two registers). It is
not a separate hardware register. Although the HC is technically an 8-bit processor,
several instructions are provided that allow 6-bit operations. This joining of A and B
is referred to as the “D” register or as “AccD.”
In ex Register X. A 6-bit address register that is used by the indexed addressing mode
instructions. It can also be used as a general-purpose, 6-bit data register. When used with
the indexed addressing mode instructions, it contains a 6-bit base address. When used
as a general-purpose data register, 6-bit data of any type can be stored and processed in
this register. This register is referred to as the “IX” or simply the “X” register.
T he role of the memory addressing registers is explained in chapter 2 during the presentation on memory
addressing modes. T he use of these registers to address memory is emphasized in chapter 5.
In ex Register Y. A 6-bit address register that is identical in function to index register
X. This register is referred to as the “IY” or simply the “Y” register.
Stack Pointer. A 6-bit register that always contains the address of the next available
stack memory location. It is referred to as the “S” or the “SP” register.
T he use of the stack pointer and the function of the memory stack are explained in chapter 6.
Program Counter. A 6-bit address register that always contains the address of the next
location in memory that will be addressed. It is referred to as the “PC.”
Con ition Co e Register. An 8-bit status and control register. Five of the eight bits
(H, N, Z, V and C) are status flags, which indicate the results of the last processor
operation. The remaining three bits (S, X and I) are control bits for advanced processor
functions. This register is referred to as the “C” register, the “CCR” register and the
“Status” register.
T he function of the status flags is presented in chapter (Instruction Set). T he function of the
control bits is presented in chapter 10 (Interrupts).
HC11 Processor Modes
The HC supports four hardware modes: Single Chip, Expanded, Special Test and
Bootstrap. Each hardware mode configures the HC to perform a special class of
hardware functions.
Single chip mo e is the normal operating mode. As the name implies, the single chip
mode requires that all software needed to control the processor is contained in the
on-chip memories. External address and data busses are not available. Expan e mo e
is an alternative normal operating mode. It allows for off-chip memory connection. In
this mode, the PORTB and Port C pins are converted to a multiplexed address and
data bus. In addition, the strobe A and strobe B control lines become the address strobe
and read/write control lines.
Special Test mo e is a special mode intended primarily for testing during the chip
production process. The bootstrap mo e is another special hardware mode, designed 11
Introduction to Computer Hardware

to allow loading of permanent programs and other production related programming
tasks.
NOTE: This text focuses only on the normal modes and primarily on operation
in the single chip mode. Some examples and explanation of the use of the
expanded mode are provided in chapter 8 on memory. The special modes will
not be discussed, because of the advanced nature of their function.
The hardware mode is selected by the logic levels on two mode control pins
(MODA and MODB) when the processor is in the reset state. Figure .8 shows
the logic levels required to select each of the four modes. The MODB bits selects
between the normal modes (Single Chip and Expanded) and the special modes
(Bootstrap and Test).
Self-Test Questions 1.2
1.3 Introduction to Memory
Memory is a term that refers to any component that stores data and programs
for the processor. The programs (software), as well as the program data, are stored
in the memory during execution and processing. Memory consists of storage
locations, where each location can contain one byte of information. A unique
address is assigned to each storage location in memory so that it can be individually
accessed. The HC uses memory that contains a byte of data in each storage
location.
A memory a ress is an n-bit binary number that the processor uses to select a specific
memory location. If the processor wishes to communicate with memory location 7, it
produces the 6-bit binary address for this location (%0000 0000 0000 0 ) and places
this address on the address bus. The number of address bits in a system determines
the number of unique addresses that can be created by the processor, as shown in
12
MODB MODA Mode
0 Single Chip
Expanded
0 0 Bootstrap
0 Test
Figure 1.8 Hardware Mode Select ( courtesy of Motorola )
. What are the three main components of a central processor?
2. What is the function of each of the three components of a processor?
3. What registers make up the M68HC programmer’s model?
Technician’s Guide to the 68HC11 Microcontroller

Equation . . The HC uses a 6-bit address. Therefore, it can address 2 6 or 64K
unique memory locations.
# of unique addresses = 2nEquation .
where n is the number of address bits in the system.
Although an address can be generated from each combination of binary bits in the
address, there may not be physical memory at all locations in the address range. A
memory map designates the memory addresses that are connected to physical memory
locations and indicates which locations are unused. For example, a system may have a
6-bit address bus, which allows it to address 64K memory locations. However, the
system may have only 6K of physical memory connected. In this case, the memory
map indicates the address range within the 64K range that will be used for this 6K
device. Figure .9 illustrates how this memory map would look.
The ata is the information stored at each memory location. Since all data is just a
group of binary bits, it all appears about the same to the processor. It is impossible by
simple inspection to know if a value in memory is a number, a special code or a piece
of a larger address. The real value of the data is how it is used. The meaning of the
data comes from the order in which it appears in the memory, as well as how it is used
by the processor.
The process of storing data in memory is called a write operation. When a specific
memory location is addressed, the data can be written to the data bus by the
processor, as shown in Figure . 0. The processor generates the address $0005
and sends it to the memory device on the address bus. Then the processor places
the data $CF on the data bus. The processor tells the memory what type of
operation to perform via the control signals. Writing to memory is a estructive
process, which means that any data previously in the memory location is overwritten
by the new data and the old data is lost. After a write operation, the data will
remain in the memory location until it is overwritten by another write operation.
Data can also move from the memory location to the processor via a rea operation,
as shown in Figure . . A read operation causes a copy of the data to be sent
to the processor from the memory. The processor initiates a read operation by
placing the address ($0007) on the address bus. It then signals the memory to 13
$0000
16K device
$3FFF
$4000
$FFFF
unused
Figure 1.9 Example of Memory Map
Introduction to Computer Hardware

place the data ($CE) from the addressed location on the data bus. The processor
waits a specific length of time and then reads the data from the data bus. The
signal between the processor and the memory that controls the direction of data
movement is usually called read/write. Reading from memory is a non estructive
process, which means that data in the memory location is not affected.
Self-Test Questions 1.3
14
Address
Bus
Address is
decoded from
address bus.
Data is
copied
from
data bus
to selected
memory
location.
Data
Bus
R/W
Control
$ 0005 $ CF
$ CF
6
5
4
0
Processor
Memory
Figure 1.10 Processor Write Operation
. What is an address?
2. How many unique addresses can be generated from a 2-bit address bus? A 20-bit
address bus?
3. In which direction does the data move during a read operation?
Address
Bus
Address is
decoded from
address bus.
Data is
copied from
selected
memory
location to
processor
data bus.
Data
Bus
R/W
Control
$ 0007 $ CE
$ CE
$ AA
$ 86
8
7
6
1
Processor
Memory
Figure 1.11 Processor Read Operation
Technician’s Guide to the 68HC11 Microcontroller

1.4 Memory Types
The physical memory locations can be represented by various types of memory devices.
Two different types of memory made from semiconductor materials will be described:
volatile and nonvolatile. Volatile memory is a memory that retains the data only when
the power is applied. If the power is removed, the data stored in the memory locations
will be lost. As the name implies, nonvolatile memory is not volatile. A nonvolatile
memory retains the data with or without power being applied to the device.
RAM
RAM stands for Random Access Memory; however, it is better described as read/write
memory. Most memory on modern computers is ran om access, which means that it
can be randomly addressed (the data is accessed in any order the processor chooses).
Sequentially accessed memory is used only for special processing and timing applica-
tions and will not be described in this text.
RAM is volatile, retaining data only when the power is applied. If the power going to
RAM is turned off, all data contained in it is lost. When power is reapplied, the content
of RAM is undefined, which means the data can be any value. RAM is used for
temporary storage of programs while they are being executed, as well as for temporary
storage of data while it is being processed. RAM is relatively fast memory when
compared to the whole family of various memory devices. RAM is also relatively less
expensive, uses less power and is smaller than most memory devices.
ROM
ROM stands for Read Only Memory. ROM is used for permanent storage of programs
and data. It is a nonvolatile memory; therefore, it always retains its data, whether power
is applied or not. Standard ROM must be loaded with the permanent programs and
data during the manufacturing process. Other types of ROM have been developed that
allow programs and data to be written to them after the manufacturing process.
Programmable Read Only Memory or PROM is designed to be programmed in the
field, after the manufacturing process. This provides the user the ability to purchase
one chip and use it for various purposes dependent upon the field application. PROM
can be programmed one time. If a program or data contained in a PROM needs to be
modified, the PROM must be discarded and a new one must be programmed.
Erasable Programmable Read Only Memory, EPROM, is a type of PROM that can be
erased by applying an intense ultraviolet light to the memory circuitry. Once it has been
erased, it can be reprogrammed using a special programming fixture. Chips that
contain EPROM have a small window on the chip package that allows the light to reach
the memory circuitry. Once an EPROM is programmed, this window is covered to stop
light from entering the device and inadvertently erasing the memory locations.
Electrically Erasable Programmable Read Only Memory, EEPROM, can be erased with
electrical signals and then reprogrammed. EEPROM is used in various devices that 15
Introduction to Computer Hardware

require permanent memory yet need the convenience of electrical erasure. Single
locations within an EEPROM can be erased and reprogrammed.
HC11 Memory
The HC E9 contains three types of on-chip memory, RAM, EEPROM and ROM, as
shown in Figure . 2. There are 5 2 bytes of RAM located in the memory map, from
$0000 through $0 FF. In addition to the RAM, the HC contains 5 2 bytes of EEPROM
and 2K of ROM. The EEPROM is located from $B600 through $B7FF in the memory
map, and the ROM is located from $D000 through $FFFF. In addition to this memory, a
64-byte register block is located at $000 through $03F within the memory map.
Refer to chapter 8 for a further presentation regarding HC11 memory.
Self-Test Questions 1.4
1.5 Input/Output
The third element of every computer system is the I/O. All computers must input data
from at least one source. The data is processed internally and then output to an external
device. Not all I/O capabilities are related to specific devices. For example, most
desktop computer systems have general I/O ports. A port is an I/O connection that
allows for the movement of data between the computer and an I/O device. There are
two major types of ports on computers: serial ports and parallel ports. A serial port is
16
Figure 1.12 Memory Map on the HC11E9
$ 0200
$ 0000
$ 1000
$ B600
SINGLE
CHIP
$ FFFF
$ D000
512 BYTES RAM
64-BYTE REGISTER
BLOCK
AVAILABLE
FOR
EXPANDED
MEMORY 512 BYTES EEPROM
FFFF
B600
B7FF
D000
1000
103F
0000
01FF
12 KBYTES ROM
$ 0FFF
$ 1040
$ B5FF
$ B800
$ CFFF
. What two types of memory devices make up the memory system of a computer?
2. What type of ROM can be erased by electrical signals?
3. How much RAM is contained on the HC E9?
Technician’s Guide to the 68HC11 Microcontroller

designed to allow two-way transfer of data as a serial data stream. Serial means one bit
is transferred at a time over a single wire or line. Modems use serial connections to
interface to the computer. High-speed serial ports are becoming the standard for
communications. A parallel port is designed to transfer data in parallel. Parallel means
multiple bits are transferred simultaneously over multiple wires (typically, 8). Histori-
cally, most scanners and printers use parallel ports to interface with the computer.
A special type of I/O requires an analog interface. Many data sources in the real world
are analog. For the computer to access this data, it must be converted into a digital
form, using an analog to digital converter. When the computer needs to send data to
an analog device, a digital to analog converter is necessary. Examples of analog data
are temperature, wind speed, and humidity.
Figure . 3 illustrates various I/O devices that may be connected to a computer system.
I/O on the HC11
The HC contains several on-chip ports that provide a variety of input/output
functions. The I/O capabilities are organized as shown in Figure . 4.
Port A can be used for digital I/O as well as for multiple timer functions. The digital
I/O capabilities include three dedicated output pins, three dedicated input pins and
two programmable I/O pins. A 6-bit free running counter is included to support the
input-capture and output-compare timer functions. In addition, an 8-bit pulse accumu-
lator subsystem and a periodic interrupt generator are provided as part of the Port A
timing system. 17
MONITOR
SERIAL PORT
VIDEO PORT
SERIAL PORT
PARALLEL PORT
SCANNER
PRINTER
PARALLEL PORT
COMPUTER
SYSTEM
Figure 1.13 I/O Devices Connected to a Computer System
Introduction to Computer Hardware

See chapter 12 for explanation of the Port A timing system.
Port B is an 8-bit general-purpose output port. Port C is an 8-bit general-purpose port
that can be configured for either input or output operation.
The primary I/O functions of Port B and Port C are presented in chapter 9.
Port D is a general-purpose digital port and also supports serial communication
functions. The digital I/O function uses the six I/O pins of Port D as programmable
I/O pins. When used for serial communications, two independent serial interfaces can
be configured. They are called the serial communications interface (SCI) and the serial
peripheral interface (SPI).
The serial communications capabilities built into Port D are discussed in chapter 1 .
Port E is an 8-bit general-purpose digital input port that can be configured as an
8-channel analog-to-digital converter with an internal resolution of 8-bits.
The analog input capabilities of the HC11 are presented in chapter 11.
18
Figure 1.14 HC11 I/O Ports ( adapted with permission from Motorola )
Technician’s Guide to the 68HC11 Microcontroller

Self-Test Questions 1.5
1.6 EVBU / BUFFALO
A full explanation of the function of the EVBU is presented in the Universal Evaluation Board
User’s Manual. Refer to section 4.6 of the EVBU User’s Manual for a description of the BUFFALO
monitor commands.
The EVBU is the Motorola M68HC Universal Evaluation Board. It is a development
tool for HC microcontroller-based designs. It provides a variety of features with which
to experiment with the functions of the HC and to test designs based on the HC .
It also works well as an educational tool. In addition to the HC chip, the EVBU
contains the following hardware features:
◗M68HC68 Real-time clock chip. The real-time clock chip provides accu-
rate timing information in seconds, minutes, hours, and so on.
◗Standard serial communications port. This port allows connection to
an external computer.
◗Breadboard area. A small solder pad development area is provided on
the EVBU layout.
The EVBU has a monitor program that serves as an operating environment. The
monitor program is a piece of software that provides a controlled environment in which
the HC can operate. It also provides a user interface (UI). The UI allows a user of
the EVBU to enter some simple commands to access memory, run programs and
monitor the function of the HC . The monitor program is called BUFFALO (Bit User
Fast Friendly Aid to Logical Operations). BUFFALO works in conjunction with the
EVBU hardware to provide the following features:
◗One-line assembler. The user can enter programs directly into memory.
◗Program download. Programs that have been created externally can
be loaded directly into memory.
◗Troubleshooting capabilities. BUFFALO commands allow users to exe-
cute programs one step at a time, to stop programs at a particular in-
struction and to examine the contents of the registers and memory.
BUFFALO is burned into the HC on-chip ROM during the manufacturing process.
It occupies 8K of the 2K of available ROM. In addition, some of the RAM is used by
BUFFALO as system temporary memory. Therefore, only part of the RAM is available
for user programs and data. However, the EEPROM is unused by the EVBU and is 19
. What is a port?
2. What is the difference between a serial and a parallel port?
3. Why are analog interfaces needed?
4. How many I/O ports are contained on the HC ? What are they?
Introduction to Computer Hardware