microprocessors

3241 Microprocessor and Application

ME3241-week01.docx

1

1 Microcomputer Architecture

In this week, we will do a short general introduction to: (1) computer and computer architecture,

(2) microprocessor architecture, (3) microcontroller, and (4)PIC 18 microcontroller

1.1 Introduction to Computer and Computer Architecture

1.1.1 Introduction to computer

What is a computer?

There are many different definitions, more or less equivalent, of what a computer is. The

definition from Wikipedia says A computer is a programmable machine designed to automatically carry out a sequence of arithmetic or logical operations. For any computer to function properly, it must have the following three functions: (1) it accepts input (usually

numerical); (2) it can perform computational functions, such as addition, subtraction etc; (3) it can

communicate results with the person using it.

(Ancient) History of Computer:

Computer, broadly interpretated, has a very long history. It is widely believed that Sumerian (todays Iraq area) people designed the first machine to help arithmetic computation abacus as early as 2500 BC. The Abacus they designed are so effective that in 1946, it won a speed

competition against a modern desk calculation machine in Japan.

The first mechanical calculator was invented by Blause Pascal in 1642. The Pascaline (aka

Arithmatique) can perform all four arithmetic operations without human intelligence.

A central concept of modern computer is the ability to store programs. This concept is first

developed by Joseph Marie in 1801 in his Jacquard Loom. Jacquard Loom is not a computing

machine it is used to manufacture rugs and clothes. It uses a punched card to store programs to control the machines operations to fabricate complex patterns.

Charles Babbage, considered as a father of the computer, invented the first mechanical computer. He started to work on a machine called Difference Engine from 1822, used to compute

values of polynomial functions. Later, he started designing a different, more complex machine

called Analytical Engine. The main difference between the two engines is that the Analytical

Engine could be programmed using punched cards. This is the first fully programmable

mechanical computer, and served as the basis of early computers. Unfortunately, the Difference

Engine was never completed, and the Analytic Engine was completed in 1888 long after his death

by his son.

Although Babbage's machines were mechanical and unwieldy, their basic architecture was very

similar to a modern computer. The data and program memory were separated, operation was

instruction-based, the control unit could make conditional jumps, and the machine had a separate

I/O unit.

In 1941, Konrad Zues developed the first program controlled computer Z3. Z3 is an electromechanical computer, using 2000 relays.


ME3241-week01.docx

2

The first general purpose electronic computer Electronic Numerical Integrator and Computer (ENIAC) - was developed in 1945-1946. Eniac weighs 27 tons and uses 1800 sqft space. It is

programmable, but rewiring is required to program the machine, hence is rather inflexible.

In 1946, Electronic Discrete Variable Automatic Computer (EDVAC) was developed. EDVAC

can store program using punch card, and is the first stored program electronic computer. Based on

EDVAC, John von Neumann draft a report which lies the foundation of modern von Neumann Machine.

Generations of (Electronic) Computer:

Since the appearance of ENIAC/EDVAC, computers have evolved for four generations each generation uses different components to make the computer less expensive, less power

consumption, more reliable and more powerful.

The first generation of computers (1947-1958) used vacuum tubes (ie. valves) as the main

components. Because of this choice, at most a few tens of thousands of switches (i.e., logic gates)

are used for one computer hence its computational power is limited. Also, the first generation

computer consumed much electricity, produced lots of heat and occupied large space (e.g.,

ENIAC).

The second generation of computers (1959-1965) used transistors as the main component. One

computer typically has many tens of thousans of switches, and is hence faster than the first

generation. Also, it is less expensive, use less power, smaller, and more reliable.

The third generation of computers (1966-1985) appeared following the inventionof Integrated

Ciruit (IC). IC, also refered as a chip, or a microchip, is a set of electronic circuits on one small

plate of semiconductor material. The third generations of computers used Small Scale Intergration

(SSI, several transistors on one chip), and later Medium Scale Intergration (MSI, hundreds of

transistors on one chip) and Large Scale Integration (LSI, tens of thousands of transistors on one

chip). Some other features of the third generation computers includes the use of Magnetic-core

memory (later replaced with solid-state memory) and batch processing operations systems.

One remarkable invention during the third generation is the micro-processor. A micro-processor

is one IC that contains the whole CPU. The first microprocessor appeared in 1973.

The fourth generation of computers (1986-current) use Very Large Scale Intergration (VLSI,

millions of transistors on one chip). Other features include the use of solid-state memory, time-

sharing operation systems, and supporting high level programming language (C, Java, etc).

Many techniques have been developed during the fourth generation. For example, microcomputer

- a computer that uses microprocessor as its CPU was invented by IBM. Also, computer networks, email, graphical user interfact, among many others, are invented in the fourth

generation.

Because of the wide use of VLSI, the fourth generation computers are small, light, and cheap.

This makes embedded system possible. Embeded system is a system that contains a computer to

control its operation.


ME3241-week01.docx

3

Classes of Computers:

Depends of the difference in speed, instruction repertoire, number of CPU registers, word length,

main memory size, complexity of I/O modules, operating system complexity, physical size, cost,

virtual address space, secondary memory size, degree of multiprogramming, modern computers

can be catergorized in to three clases: Mainframe, Minicomputer, Microcomputer. Notice that

speed is not the only difference, and often microcomputer can be faster than minicomputer or

even mainframe. Also, division between the computer classes are increasingly unclear.

Mainframe is a powerful computer used for critical application. The difference of mainframe

with other classes is not so much of spped, but high reliability and security.

Minicomputer is a mid-range computer. It is originally designed to connect microcomputer with

mainframes. It uses own Unix operating system, and is often incompatible with other machines.

Minicomputers are giving ways to microcomputers.

Microcomputer is a computer whose Central Processing Unit (CPU) is a microprocessor.

Recall notion of microprocessor: a processor that contrains the whole CPU (including control unit,

arithmetic and logic unit, and register) one one single IC chip.

Compare with the notion of micro-controller: the whole CPU and all peripheral functions (i.e.,

the whole computer system) that is implemented on one IC chip.

1.1.2 Introduction to Computer Architecture

Computer architecture is the art that specifies the relations and part of a computer system. One

early example was von Neumanns First Draft of a Report on the EDVAC, which described an organization of logical elements. The machines following such organization is called the von

Neumann Machine. The von Neumann Machine architecture is still dominant most currently used machines are von Neumann machines. Below is a diagram of the von Neumann machine.

Figure 1: Diagram of von Neumann machine


ME3241-week01.docx

4

Components of von Neumann Machine:

A memory for holding both instructions and data required by the instructions

A control unit for fetching the instruction from memory

An arithmetic processor for performing the specific operation

Input/output and peripheral devices for transferring data to and from the system

Architechture of Microcomputer:

A von Neumann Machine can be a mainframe, a minicomputer or a microcomputer. We now

discuss the architechture of microcomputer. Almost all microcomputers are von Nermann

machines.

A microcomputer is made up of three fundamental elements: a central processor, memory and

input/output device - minimum structure. Notice that compare with the general von Neumann

machine structure, the control unit and the arithmetic processor are integrated together as the

central processor. (Why?)

Figure 2: Diagram of Microcomputer Arcitecture

Central Processor Unit (CPU) refers to the main processor in the system. Memory refers to any

component that stores data and programs used by the processor. Input/Output (I/O) refers to any

subsystem that has the responsibility of receiving data for the processor (input) and sending data

out from the processor (output). We will discuss each component in detail later.

System interconnection:

In microcomputer, system interconnection is done with the Bus Interconnection Scheme, see

below figure. Here, Bus refers to a set of 2 or more conductors grouped together to form a parallel


ME3241-week01.docx

5

information path to and/or from the processor. In a microcomputer, typically three buses coexist,

namely, data bus, address bus and control bus.

Data bus is responsible for transferring data between processor and memory or processor and I/O.

Data bus width (in terms of number of bits) correlates with the default processing capacity of the

processor. For example, PIC 18 has a 8-bit data bus, and the processing capacity is one byte (=8

bit). Hence, PIC 18 is a 8-bit processor.

Address bus is responsible for transferring addresses from processor to memory or to I/O. The

width of address bus is hence correlated with the size of memory that the computer supports. For

example, a 16-bit address bus can support 65k memory.

Control bus is responsible for the control signals necessary to interface the various devices within

the microcomputer.

Figure 3: Bus interconnection Scheme

Memory:

Memory refers to any component that stores data and programs used by the processor. Since

1970s, the use of semiconductor memory (aka solid-state memory) has been dominant. Semi-conductor memory has been through many generations: 1K, 4K, 16K, 256K, 1M (in late 1980s)

and now 1G bits on a single chip.

To access the memory, memory address and memory map are needed. Memory address is an n-bit

number that the processor uses to select a specific memory location. Hence, the number of unique

addresses =2n. A memory map designates the memory addresses that are connected to physical

memory locations and indicates which locations are unused. That is, it maps the address with the

physical memory. See below diagram.


ME3241-week01.docx

6

Types of memory:

There are two types of memory: RAM (Random access memroy) and ROM (Read only memory).

RAM is volatile, i.e., it needs power to sustain data. It is called RAM because same amount of

time is required to access any location on the same chip. There are two types of RAM: Dynamic

random-access memory (DRAM) which needs period refersh, and Static random-access memory

(SRAM) which doesnt. DRAM is typically used for primary memory and SRAM for cache.

ROM can only be read but not written to by the processor. Mask-programmed read-only memory

(MROM) can only be programmed when being manufactured, while Programmable read-only

memory (PROM) can be programmed by the end user via a special equipment. PROM can only

be programmed once. EPROM can be programmed many time, but it must be done in a bulk

manner ie the whole chip in one erasure operation. EEPROM can be programmed many times, either one location, one row or the whole chip. Flash memory can be programmed many times, in

bulk manner (either a block or the whole chip).

Input/output:

Input/output refers to any subsystem that has the responsibility of receiving data for the processor

(input) and sending data out from the processor (output). A port is an I/O connection that allows

the movement of data between computer and I/O device. A serial port allows two-way transfer of

data as a serial data stream (ie one bit a time). A parallel port transfers data in parallel (multiple bit simultaneously). Analog interface is needed to cnvert analog to digital data or vice versa.

Central Processing Unit (CPU):

CPU refers to the main processor of the computer. It has three main components: Arithmetic logic

unit (ALU), register and control unit. ALU performs computation (both mathematical and logical

operations) on data, and return result of operation to register or memory. Register (aka register

file) is used to temporarily store data values and memory addresses, and to contain status and


ME3241-week01.docx

7

control information. The control unit is the specific component of CPU that controls the CPU

operations, by issueing control signals or instructions.

The internal CPU interconnection is illustrated in the below figure:


ME3241-week01.docx

8

Early computers have many chips for each different components of a CPU. Later on,

microprocessor contains all three units of the CPU in one chip. For modern microprocessor

(especially those general purpose ones), the CPU also contains memory on chip (aka cache

memory) for faster access.

A special type of microprocessor is microcontroller. Microcontroller contains all components of a

computer system (i.e., CPU, memory, I/O) in one single chip. See below table for comparison

between microprocessor and microcontroller.

1.2 Processor Design

1.2.1 Processor Microarchitecture

We now briefly processor microarchitecture the art to specify how the processor is built. Recall the CPU has three components control unit, register and ALU.

The control unit performs two basic tasks: sequencing and execution. Sequencing refers to the

control unit causes the CPU to step through a series of micro-operations in the proper sequence,

based on program being executed. (instruction sequencing) Execution refers to the control unit

causes each micro-operation to be performed. (instruction interpretation).

Within the control unit, there are some special components: (1) Memory Address Register (MAR)

which contains address of current data word that is being addressed; (2) Memory Data Register

(MDR) that buffer and control the movement of data into/out of the processor; (3) Instruction

Register (IR) which contains the opcode for the current instruction; and (4) Instruction Decoder

that decodes the instruction in IR and controls the execution of the instruction. Notice that

although these component is called register, then do not belong to the registers of the CPU since they can not be directly accessed by the program. Below is a diagram.

Example: to execute the instruction Fetch(address), three steps are needed 1. Load address into MAR. 2. Decode address in MAR. 3. Copy memory location contents into MDR. To execute

Store(address, value), four steps are needed 1. Load address into MAR. 2.Load value into MDR. 3.Decode address in MAR. 4.Store contents of MDR into that memory location


ME3241-week01.docx

9

There are two ways to implement the control unit design. The hardwired control unit views the

control unit as a sequential logic circuit to generate fixed sequences of control signals. The

advantage is its speed of operation. The disadvantage is very inflexible to changes after built, and

costly to design and debug. The microprogrammed control unit uses microprograms to interest

and execute instruction, and hence easy to design and modify.

ALU is the unit to perform mathematical and logical operations on the data. Modern processors

often contain separate units for integer and floating point computation (floating point unit). Also,

modern processors use multiple execution units to execute instructions in parallel (e.g., dual-core,

quad-core etc).

Register is a small storage area to temporarily store data that the microprocessor is using. There

are some special regiserts which we elaborate. Accumulator is a special register that is directly

linked to ALU to assist with arithmetic operation (i.e, store one operand and the result of the

operation). Program counter (or instruction counter) keeps track of address of the next location in

memory that will be accessed. Status register contains bits to indicate certain results/status of the

last operation. Other special registers include stack pointer, address register etc.

1.2.2 Instruction Set Architecture

Instruction set architecture considers designing machine instruction and instruction set for the

processor.

Typically, an instruction needs to provide the following information

where are the data located (register, memory, etc) what to do with the data where to store the result.

The general format for instruction Z = X op Y is as follows: Operator 1

st operand 2

nd operand Result

op X Y Z

The various methods to identify locations of the operands are called addressing modes. Each processor has its own addressing modes. We will discuss in detail the ones for PIC18 later this

course. In general, there are the following types of address modes:


ME3241-week01.docx

10

Immediate addressing: when the operand is part of the instruction. e.g., acc+3-> acc, here number 3 is immediate addressed

Absolute addressing: when the address of an operand is held in the instruction. Eg acc + [0x0010] -> acc, here [0x1000] is absolute addressed, and the processor

will fetch the data store in memory address 0x1000

Register direct addressing: when the operand is held in an addressed register. Eg acc + [0x0010] -> acc, here acc is register direct addressed. The processor will

use the data in accumulator

Register indirect addressing: when the address of the operand location is held in a register. Eg, fetch [acc]. Here the address of the data is stored in accumulator, and

the machine will find the data stored in this address

Relative addressing: when the address of the operand is computed by adding an offset held in the instruction to the contents of specific register. Eg, fetch [acc+1].

Here the address to fetch is the address stored in accumulator plus 1.

Instruction set design:

There are three types of instruction set Minimal Instruction Set, Reduced Instruction Set (RISC), and Complex Instruction Set (CISC). The difference are increasingly unclear.

Instruction Execution and Pipeline

To execute an instruction, the processor must perform the following:

Fetch the instruction read the instruction opcode from the memory (opcode is the code indicates which instruction to execute).

Advance program counter (PC) PC is updated to pointing to the address of the next instruction.

Decode the instruction Opcode is decoded by the instruction decoder Perform the instruction executed the instruction based on built-in sequence for the

specific instruction

The status register is updated to reflect the status of the operation

Thus, to execute one instruction, the process needs to do several tasks sequentially, and may take

some time. To reduce the execution time, pipelining is widely used. Pipelining overlap the

execution of several instructions to reduce the total time. In particular, the execution of one

instruction is divided into sections, and place latches between them.

As an example, suppose there is a processor which needs five steps to execute an instruction,

namely: fetch instruction (IF), decode instruction (ID), read registers (RR), execute instruction

(EE), write result into register (WR). Each step takes the same time, and the total time is 100ns.

Now consider the pipelined version, it divides this 100ms into 5 stages, as follows:

Fetch instruction + latch (one clock cycle) Decode instruction + latch (one clock cycle) Read register + latch (one clock cycle) Execute instruction + latch (one clock cycle) Write result into register (one clock cycle)

Suppose the latch latency is 3ns, then each execution cycle time = 100ns/5+3ns =23ns. In one

clock cycle, the processor is IF, ID, RR, EX and WR for 5 different instruction, and thus in the

long run, each instruction is executed with in one clock cycle. The below is a diagram illustrating

this:


ME3241-week01.docx

11

1.3 Microcontroller

Microcontroller (C or MCU) is an integration of all of the computer components (i.e., one or

more microporcessors (Ps), memory and I/O devices) on one chip. it is typically intended as a single chip solutions for systems requiring low to moderate processing power

Examples of early microprocessors include the following:

Intel 4044 (1971): first chip to contain all components of a CPU Born of microprocessor

Intel 8008 (1972), Intel 8080 (1974) 8-bit microprocessors built by Intel

Other 8-bit microprocessors built in Mid 70s Motorola: M6800, Signetics: 6502, Zilog: Z80, Texas Instruments: T9900,

National Semiconductor: IMP-8 etc.

Example of microprocessors developed since late 70s and beyond:

in late 70s Intel 8086

Microcomputer XT Intel 8088 Motorola 68000 Zilog Z8000 etc

Later - CISC Intel 80X86

Microcomputer AT Intel 80286 Motorola 680X0

Motorola 68020


ME3241-week01.docx

12

Later - RISC Power PC SUN Microsystems

Examples of micro-controllers:

Intel: 8031, 8051, 80188, 8096 etc Motorola: MC6805, MC68HC11, MC68HC12 (16-bit) etc Microchip: PIC16, PIC18

Classification of Microcontrollers:

Microcontrollers are typically classified according to the processing capacity of the processors equivalently the width of the data bus. It can be catergorized as 4-bit microcontroller, 8-bit

microcontroller, 16-, 32-, 64-bit microcontroller. The larger the data bus width, the more powerful

the microcontroller (and more expensive). Hence, low end microcontrollers are used for cheap

applications (eg toys, intelligent consumer product etc), while high end ones are used for complex

machines, industrial controllers etc.

1.4 PIC18 Microcontroller

We now give an introduction of PIC18 microcontroller as well as its properties. Below is a list of

features of PIC18

8-bit CPU 16-bit instruction width, 70+ instructions 2 MB program memory space 256 bytes to 1KB of data EEPROM Up to 3968 bytes of on-chip SRAM 4 KB to 128KB flash program memory Operates at up to 40 MHz crystal oscillator Instruction pipelining

Sophisticated timer functions that include: input capture, output compare, PWM, real-time interrupt, and watchdog timer Serial communication interfaces: SCI, SPI, I2C, and CAN 10-bit A/D converter

A simpliefied block diagram see below. You should be able to tell from the diagram the data bus

width, instruction width, address bus width, and the maximum amount of memory supported.


ME3241-week01.docx

13

1.4.1 Memory organization of PIC 18:

A very important topic to understand is the memory organization of PIC18. In general, memory

consistes a sequence of directly addressable information units called memory locations. A

memory location has two components an address and its contents, it can be used to store data, instruction, and the status of peripheral devices.

In PIC 18, the data memory (aka data registers for PIC 18) and program memory are separated

(this is called Harvard Architecture), see below figure. The adavantage of separating the data

memory and program memory is that it is possible to access data and instruction simultaneously.

Data memory includes general purpose registers and special function registers.


ME3241-week01.docx

14

A PIC 18 MCU can have up to 4096 bytes of data memory (recall 12-bit register address bus).

Data memory is implemented in SRAM and consists both general purpose registers (used to hold

dynamic data) and spcial-function registers (used to control the operation of peripheral functions).

A special property of PIC 18 data memory is that it is divided into banks. This property has

very profound impact on programming with PIC 18. Data memory is divided into 16 banks, each

bank has 256 bytes. At any time, only one bank is active. Which bank is active is specified by the

BSR register. Thus, to access data from another bank, one need to do bank switching this is an overhead and can be error-prone. To reduce bank switching, PIC implemented the access bank.

Access bank is a special region of the data memory it consists of the lowest 96 bytes and highest 160 bytes of the data memory space. One can access memory locations within the access bank

regardless of which bank is active.

The group of registers from The group of registers from 0xFD8 to 0xFFF are dedicated to the

general control of MCU operation in CPU. Two examples are WREG and STATUS register. The

WREG register is involved in the execution of many instructions. The STATUS register holds the

status flags for the instruction execution

In terms of the program memory the program memory address bus and the program counter (PC) are 21 bit long, hence the maximum amount of program memory supported is 2MB. PIC 18

also has a 31-entry stack to hold the return address of subrounting call. When power on, PIC18

starts to executre instruction from address 0. The location 0x08 is reserved for high-priority

interrupt service routine, 0x18 is reserved for low-priority interrupt service routine. Part of the

program memory is inside the chip (depends on the models, currently up to 128kb) and part of the

program memory is outside the side.


ME3241-week01.docx

15

1.4.2 Instruction Format

There are 5 different types of instructions for PIC 18. They have different formats. Most

instruction are 16 bit long, some of them are 32bit (i.e., use two locations of the program

memory).

1. Byte oriented instruction : eg. movf 0x20, W, A

2. Byte-to-byte operations: eg. movff 0x100, 0x200. Here, no bank selection is needed since

12 bits are used to address a memory location

3. Bit-oriented file register operations: eg. bcf 0x20, 2, A

4. Literal operations (literal is a number to be operated upon) e.g., movb 3


ME3241-week01.docx

16

5. Control operations instructions that change the program exection sequence and make subroutine calls they all have different formats

Addressing mode:

There are five addressing modes (ways to identify locations of the operands) in PIC 18:

1. Register direct: Use an 8-bit value to specify a data register. Eg. movf 0x20, W, A. Here 0x20 is a register that is directly addressed

2. Immediate Mode: A value in the instruction to be used as an operand. Eg., movlb 3. Here number 3 is a number that is immediately addressed

3. Inherent Mode: an implied operand. Eg, movf 0x20, W, A, here the register WREG is inherently addressed

4. Indirect Mode: A special function register (FSRx) is used as a pointer to the actual data register. Eg. movwf INDF0. This command copies the contents of the WREG register to

the data memory whose location is stored in FSR0.

5. Bit direct: deal with individual bit. E.g., BCF PORTB, 3, A. This command Clears bit 3 of the data register PORTB

Because in most instruction, only 8 bit is used to identify a file register, while we have 4096 bytes

of file register (212

bytes), we have to divide into banks- each bank have 256 bytes (28 bytes) .

Refer to page 14 for the information of bank and access bank.


ME3241-week01.docx

17

1.4.3 Pipelining

The PIC18 divide most of the instruction execution into two stages: instruction fetch and

instruction execution. Hence, up to two instructions are overlapped in their execution - One

instruction is in fetch stage while the second instruction is in execution stage, see below diagram.

Notice that this is made possible since the fetch stage accessess program memory and the

execution stage accesses data memory (i.e., file registers). Since the two memory are separated

with individual buses, pipelineing is possible. Because of pipelining, each instruction appears to

take one instruction cycle to complete. For example, suppose for unpiplined version, one

instruction takes 60ns, and assume latch requires 3ns, then for the pipelined version, each time

cycle = 60/2+3=33 ns.

microprocessors

Documents