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EMBEDDED SYSTEM

Being an electronic engineer, you might have seen PC desktop's motherboard; it's an

embedded system. It has microprocessor (Pentium or Athlon), memory (DRAM DIMM module

and onboard SRAM), I/O interface (keyboard, mouse etc.), Peripheral communication interface

(PCI, USB port, etc). This PC system's architecture is designed for application such as net

surfing, excel, word, powerpoint. Suppose you want to use the same computer to monitor the

engine of your bike or car. Can you think of using big PC for that purpose? It's so impractical.

The input and output are totally different, here comes customizing your own

microprocessor/microcontroller, memory, display, i/o and peripheral interface and also theoperating system. This field of designing application specific computer systems is called

embedded systems development. If the response of this computer system need to be real time and

highly reliable then it's called Real Time Embedded System. The real time means, for example in

a control system where the speed of motor need to be varied at the moment at which some

parameter deflects from it's original value, then it's real time; with no waiting or hanging.

To define in a sentence, Embedded System is a special purpose computer system/board,

which encapsulates all the devices such as processor, memory, interface and control in single

package or board to perform only a specific application task.

Every home has several examples of embedded computers. Any appliance that has a

digital clock, for instance, has a small embedded microcontroller that performs no other task than

to display the clock. Modern cars have embedded computers onboard that control such things as

ignition timing and anti-lock brakes using input from a number of different sensors.

Embedded computers rarely have a generic interface, however. Even if embeddedsystems have a keypad and an LCD display, they are rarely capable of using many different

types of input or output. An example of an embedded system with I/O capability is a security

alarm with an LCD status display, and a keypad for entering a password.

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In general, an Embedded System:

Is a system built to perform its duty, completely or partially independent of human

intervention.

Is specially designed to perform a few tasks in the most efficient way.

Interacts with physical elements in our environment, viz. controlling and driving a motor,

sensing temperature, etc.

Figure 1:Sample block diagram of a typical embedded system.

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Figure 2: PowerPC based embedded board.

Most embedded systems are time critical applications, meaning that the embedded system

is working in an environment where timing is very important: the results of an operation are only

relevant if they take place in a specific time frame. An autopilot in an aircraft is a time critical

embedded system. If the autopilot detects that the plane for some reason is going into a stall then

it should take steps to correct this within milliseconds or there would be catastrophic results.

What are Embedded Systems used for?

The uses of embedded systems are virtually limitless, because every day new products are

introduced to the market that utilizes embedded computers in novel ways. In recent years, hardware such

as microprocessors, microcontrollers, and FPGA chips have become much cheaper. So when

implementing a new form of control, it's wiser to just buy the generic chip and write your own custom

software for it. Producing a custom-made chip to handle a particular task or set of tasks costs far more

time and money. Many embedded computers even come with extensive libraries, so that "writing your

own software" becomes a very trivial task indeed.

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From an implementation viewpoint, there is a major difference between a computer and

an embedded system. Embedded systems are often required to provideReal-Time response.

A Real-Time system is defined as a system whose correctness depends on the timeliness of its

response. Examples of such systems are flight control systems of an aircraft, sensor systems in

nuclear reactors and power plants. For these systems, delay in response is a fatal error. A more

relaxed version ofReal-Time Systems, is the one where timely response with small delays is

acceptable. Example of such a system would be the Scheduling Display System on the railway

platforms. In technical terminology,Real-Time Systems can be classified as:

a.Hard Real-Time Systems - systems with severe constraints on the timeliness of the response.

b. Soft Real-Time Systems - systems which tolerate small variations in response times.

c. Hybrid Real-Time Systems - systems which exhibit both hard and soft constraints on its

performance.

Why Study Embedded Systems?

Embedded systems are playing important roles in our lives every day, even though they might

not necessarily be visible. Some of the embedded systems we use every day control the menu

system on television, the timer in a microwave oven, a cell phone, an MP3 player or any other

device with some amount of intelligence built-in. In fact, recent poll data shows that embedded

computer systems currently outnumber humans in the USA. Embedded systems is a rapidly

growing industry where growth opportunities are numerous.

Processors embedded in to a system

Microprocessor, Microcontroller and System on Chip

Microprocessor:

Microprocessor is the Central Processing Unit (CPU) of embedded system. It does arithmetic and

logic operations of the digital binary data.Very old embedded systems circuit/board was

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generally made up of separate microprocessor (8085), I/P interface, O/P interface, memory,

clock and timing devices, power supply devices, and analog/linear devices. (little more to be

added)

Microcontroller:In the early days of embedded systems, engineers have built embedded systems with separate set

of devices connected on a printed circuit board. The complexity involved in manufacturing and

re-engineering was very high with many Integrated Circuits and other components on-board.

Also the advance in technology has enabled processor manufacturers to add one device after one

into single IC. It started with adding I/O interface and memory, now we see lot more functions

inside the processor chip. These microprocessors with all the additional support built-in are

called microcontrollers.

To define, Microcontroller is an Integrated Circuit device with CPU, memory, I/O interface and

any other logic and analog function on a single chip. (have to include a circuit and different types

of micro controllers used)

Microcontrollers versus Microprocessors

Microcontroller differs from a microprocessor in many ways. First and the most important is its

functionality. In order for a microprocessor to be used, other components such as memory, or

components for receiving and sending data must be added to it. In short that means that

microprocessor is the very heart of the computer. On the other hand, microcontroller is designed

to be all of that in one. No other external components are needed for its application because all

necessary peripherals are already built into it. Thus, we save the time and space needed to

construct devices.

System On Chip (SOC)

Even though MCU holds most of the functions, it still lacks in few special analog functions and

application specific functions. The idea of putting entire system (all the semiconductor IC

functions) on a single chip is called System on chip. On a printed circuit board, you see a single

IC accompanied with few discrete and passive components.

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If we look at the recent microcontrollers released in the market, most of them are very close to

System On Chip. The concept of SOC is well ticking in the market.

SOC is a common sense solution, that means, why we have to go for a complex board when we

can put everything into a single IC. SOC saves board space, ease manufacturing, and score

higher in reliability over non-SOC solutions. It's drawback is, the manufacturer profits from this

product only if it's used in millions. (need a block diagram)

Embedded hardware and software devices

Programming: machine language, assembly language, and C programming

Assembly and machine language

The microcontroller is the one, which decides what need to be done, what need not be

done, and how to be done. Basic rule we need to keep in mind while "instructing" the

microcontroller is - microcontroller is like a very intelligent child. The child (controller) would

do exactly what was told it to do - nothing more nothing less. If the instruction is ambiguous then

the behavior of the microcontroller would go haywire.

Example: In a bread toaster, the sequence of operations is,

a) Turn on the heater

b) Check whether the bread is properly roasted or not (by checking the temperature or set time)

c) If bread is not yet completely toasted properly again go to step (b)

d) Stop the heater as the bread is toasted properly.

Now how do you tell this sequence to a microcontroller inside a bread toaster? You should tell it

(microcontroller) in a way it understands. It is like speaking to a person who knows somelanguage, which you can't speak. The instant option left to you to speak to such person is to catch

hold of a translator, who knows both the languages and translate/convert your language to other's

language.

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The language what all microcontrollers understand is called machine language. Here is just a few

lines of machine language for Freescale's 6812 microcontroller.

CF0C00180B8000024D008018030FA008009600847FB1F033260EFE080009

7E080026EE4C008020E918030FA008004D008020DE23F000

Does this jumble of hexadecimal codes discourage you? Obviously you should be! Any way

don't get disheartened by this magic series of numbers. But make it very clear this (machine

language) is the native language of all the microcontrollers and you should "instruct" them only

in their language. Also this machine language is different for each microcontroller families

(8051, PIC, ARM etc.).

In the very early stages itself computer scientists/ chip designers noted this problem

instantaneously and came out with a solution. For each of the operation that microcontroller can

do (execute) they assigned an "English like" word so that programmer/ designer can easily

instruct the microcontroller. This is called assembly language.

Here below is table of assembly languages instructions for popular PIC16xx microcontroller. In

total it has only 35 instructions.

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Development/Classification of microcontrollers (Invisible)

Microcontrollers have gone through a silent evolution (invisible). The evolution can be rightly

termed as silent as the impact or application of a microcontroller is not well known to a common

user, although microcontroller technology has undergone significant change since early 1970's.

Development of some popular microcontrollers is given as follows.

Intel 4004 4 bit (2300 PMOS trans, 108 kHz) 1971

Intel 8048 8 bit 1976

Intel 8031 8 bit (ROM-less) .

Intel 8051 8 bit (Mask ROM) 1980

Microchip PIC16C64 8 bit 1985

Motorola 68HC11 8 bit (on chip ADC) .

Intel 80C196 16 bit 1982

Atmel AT89C51 8 bit (Flash memory) .

Microchip PIC 16F877 8 bit (Flash memory + ADC) .

We use more number of microcontrollers compared to microprocessors. Microprocessors are

primarily used for computational purpose, whereas microcontrollers find wide application in

devices needing real time processing / control. Applications of microcontrollers are numerous.

Starting from domestic applications such as in washing machines, TVs, airconditioners,

microcontrollers are used in automobiles, process control industries, cell phones, electrical

drives, robotics and in space applications.

Microcontroller Chips

Broad Classification of different microcontroller chips could be as follows:

Embedded (Self -Contained) 8 - bit Microcontroller

16 to 32 Microcontrollers

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Digital Signal Processors

Features of Modern Microcontrollers

Built-in Monitor Program Built-in Program Memory

Interrupts

Analog I/O

Serial I/O

Facility to Interface External Memory

Timers

Internal Structure of a Microcontroller

Fig. 2.1 Internal Structure of a Microcontroller

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BASIC 8051 ARCHITECTURE

8051 employs Harvard architecture. It has some peripherals such as 32 bit digital I/O, Timers

and Serial I/O. The basic architecture of 8051 is given in figure.

Fig 5.1 : Basic 8051 Architecture (can add another architectural design)

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Pin details of 8051

ALE/PROG: Address Latch Enable output pulse for latching the low byte of the address during

accesses to external memory. ALE is emitted at a constant rate of 1/6 of the oscillator frequency,for external timing or clocking purposes, even when there are no accesses to external memory.

(However, one ALE pulse is skipped during each access to external Data Memory.) This pin is

also the program pulse input (PROG) during EPROM programming.

PSEN: Program Store Enable is the read strobe to external Program Memory. When the device

is executing out of external Program Memory, PSEN is activated twice each machine cycle

(except that two PSEN activations are skipped during accesses to external Data Memory). PSEN

is not activated when the device is executing out of internal Program Memory.

EA/VPP: When EA is held high the CPU executes out of internal Program Memory (unless the

Program Counter exceeds 0FFFH in the 80C51). Holding EA low forces the CPU to execute out

of external memory regardless of the Program Counter value. In the 80C31, EA must be

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externally wired low. In the EPROM devices, this pin also receives the programming supply

voltage (VPP) during EPROM programming.

XTAL1: Input to the inverting oscillator amplifier.

XTAL2: Output from the inverting oscillator amplifier.

Port 0: Port 0 is an 8-bit open drain bidirectional port.

Port 1: Port 1 is an 8-bit bidirectional I/O port with internal pullups.

Port 2: Port 2 is an 8-bit bidirectional I/O port with internal pullups.

Port 3: Port 3 is an 8-bit bidirectional I/O port with internal pullups.

Port Pin Alternate Function

P3.0 RxD (serial input port)

P3.1 TxD (serial output port)

P3.2 INT0 (external interrupt 0)

P3.3 INT1 (external interrupt 1)

P3.4 T0 (timer 0 external input)

P3.5 T1 (timer 1 external input)

P3.6 WR (external data memory write strobe)

P3.7 RD (external data memory read strobe)

VCC: Supply voltage

VSS: Circuit ground potential

Various features of 8051 microcontroller are given as follows.

8-bit CPU

16-bit Program Counter

8-bit Processor Status Word (PSW)

8-bit Stack Pointer

Internal RAM of 128bytes

Special Function Registers (SFRs) of 128 bytes

32 I/O pins arranged as four 8-bit ports (P0 - P3)

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Two 16-bit timer/counters : T0 and T1

Two external and three internal vectored interrupts

One full duplex serial I/O

8051 Instructions

8051 has about 111 instructions. These can be grouped into the following categories

1. Arithmetic Instructions

2. Logical Instructions

3. Data Transfer instructions

4. Boolean Variable Instructions

5. Program Branching Instructions

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PIC MICROCONTROLLERS

PIC stands for Peripheral Interface Controller given by Microchip Technology to identify its

single-chip microcontrollers. These devices have been very successful in 8-bit microcontrollers.

The main reason is that Microchip Technology has continuously upgraded the device

architecture and added needed peripherals to the microcontroller to suit customers' requirements.

The development tools such as assembler and simulator are freely available on the internet at

www.microchip.com .

The architectures of various PIC microcontrollers can be divided as follows.

Low - end PIC Architectures :

Microchip PIC microcontrollers are available in various types. When PIC microcontroller was

first available from General Instruments in early 1980's, the microcontroller consisted of a

simple processor executing 12-bit wide instructions with basic I/O functions. These devices are

known as low-end architectures. They have limited program memory and are meant for

applications requiring simple interface functions and small program & data memories. Some of

the low-end device numbers are

12C5XX

16C5X

16C505

Mid range PIC Architectures

Mid range PIC architectures are built by upgrading low-end architectures with more number of

peripherals, more number of registers and more data/program memory. Some of the mid-range

devices are

16C6X

16C7X

16F87X

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CPU Architecture: The CPU uses Harvard architecture with separate Program and Variable

(data) memory interface. This facilitates instruction fetch and the operation on data/accessing of

variables simultaneously.

Fig 16.1 CPU Architecture of PIC microcontroller

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PIC16F84:

The PIC16F84A belongs to the mid-range family of the PICmicro microcontroller devices. A

block diagram of the device is shown in Figure 1-1. The program memory contains 1K words,

which translates to 1024 instructions, since each 14-bit program memory word is the same width

as each device instruction. The data memory (RAM) contains 68 bytes. Data EEPROM is 64

bytes. There are also 13 I/O pins that are user-configured on a pin-to-pin basis. Some pins are

multiplexed with other device functions. These functions include:

External interrupt

Change on PORTB interrupt

Timer0 clock input

The pin diagram and pin descriptions are given below.

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The figure shows the block diagram of PIC 16F84.

A number of important new additions have appeared. The addition of an EEPROM memory

gives the valuable capability of being able to store data values even when the chip is powered

down. There are now two digital input/output ports. These are Port A, with five pins, and Port B,

with eight. Importantly, there is the addition of an interrupt capability. This can be seen

externally on pin 6, where bit 0 of Port B is shared with the external interrupt input. We will also

see that there are three further internal interrupt sources, generated by the peripherals. Overall,

we have a microcontroller that, while only modestly more complex than the 12F508, has proved

incredibly diverse and useful in small applications.

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The Program Development Process

The process of writing in assembler needs to be placed in the broader context of project

development. The possible stages in the development process for the program of a simple

embedded system project are shown in Fig. 4.3. The programmer writes the program, called the

source code, in Assembler language.

This is then assembled by the cross-assembler running on the host computer. If the programmer

has access to a simulator then he/she may choose to test the program by simulation. This is likely

to lead to program errors being discovered, which will require alteration to the original source

code. When satisfied with the program, the developer will then download it to the program

memory of the microcontroller itself, using either a stand-alone programmer linked to the host

computer or a programming facility designed into the embedded system itself. He/ she will then

test the program running in the actual hardware. Again, this may lead to changes being required

in the source code.Clearly, to develop even a simple project, a selection of different software tools is beneficial.

These are usually bundled together into what is called an Integrated Development Environment

(IDE).

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4.2 The PIC 16 Series Instruction Set, with a Little More on the ALU

4.2.1 More on the PIC 16 Series ALU

Before looking at the 16 Series instruction set, it is worth taking a more detailed look at the ALU

(Fig. 4.4). Understanding this will aid in understanding the instruction set. Looking at this, we

see that the ALU can operate on data from two sources. One is the W (or Working) register. The

other is eithera literalvalue ora value from a data memory (whose memory locations Microchip

calls register files). A literal value is a byte of data associated with a particular instruction that

the programmer embeds in the program.

Thus, we can expect to see some instructions that call on data memory and others that require

literal data to be specified whenever they are used. Examples of all are coming! The data that the

instruction operates on, or uses, is called the operand. Operands can be data or addresses. Wewill see that some types of instructions always need an operand to be specified with them, others

do not.

Once an instruction has been executed, where is the result stored? For many instructions

Microchip offer a choice, whereby the result can eitherbe held in the W registerorstored back

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in data memory. Which one is used is fixed by certain instructions; in others it is determined by

the state of a special dbit, which is specified within the instruction.

4.2.2 The PIC 16 Series Instruction Setan Introduction

Instruction set given in the table is divided into six columns, and each of the 35 instructions gets

one line. The first column gives the actual mnemonic, together with the code specifying the type

of operand it acts on. There are four such operand codes:

ffor file (i.e., memory location in RAM), a 7-bit number

b for bit, to be found within a file also specified, a 3-bit number

d for destination, as described above, a single bit

kfor literal, an 8-bit number if data or 11-bit if address.

The second column summarizes what the instruction does. The third column shows how many

instruction cycles the instruction takes to execute. The fourth column gives the actual 14-bit

opcode of each instruction. This is the code that the cross-assembler produces, as it converts the

original program in Assembler language to machine code. It is interesting to see here how the

operand codes, listed above, become embedded within the opcode. The fifth column shows

which bits in the Status register (Fig. 2.3) are affected by each instruction.

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THE PIC 16 SERIES INSTRUCTION SET

Table A1.

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Let us have a look at five example instructions, to see how the information is presented.

Assembler programming does not have to be case sensitive.

clrwthis clears the value in the W register to zero. There are no operands to specify. Column

5 tells us that the Status register Z bit is affected by the instruction. As the result of this

instruction is always zero, the bit is always set to 1. No other Status register bits are affected.

clrf fthis clears the value of a memory location, symbolized as f. It is up to the programmer

to specify a value forf. Again, because the result is zero, the Status registerZbit is affected.

addwf f,dthis adds the contents of the W register to the contents of a memory location

symbolized by f. It is up to the programmer to specify a value for f. There is a choice of where

the result is placed, as discussed above. This is determined by the value of the operand bit d.

Because of the different values that the result can take, all three condition code bits, i.e. Z, the

Carry bit C, and the Digit Carry bit DC are affected by the instruction.

bcf f,bthis instruction clears a single bit in a memory location. Both the bit and the location

must be specified by the programmer. The bit numberb will take a value from 0 to 7, to identify

any one of the 8 bits in a memory location. No Status register flags are affected, even though it is

possible to imagine that the result of the instruction could be to set a memory location to zero.

addlw kThis instruction adds the value of a literal, whose value kmust be specified by the

programmer, to the value held in the W register. The result is stored in the W register; there is no

choice. Like addwf, all condition code bits can be affected by this instruction.

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ASSEMBLY LANGUAGE PROGRAMMING

Introduction

The ability to communicate is of great importance in any field. However, it is only possible if

both communication partners know the same language, i.e follow the same rules during

communication. Using these principles as a starting point, we can also define communication

that occurs between microcontrollers and man . Language that microcontroller and man use to

communicate is called "assembly language". The title itself has no deeper meaning, and is

analogue to names of other languages , ex. English or French. More precisely, "assembly

language" is just a passing solution. Programs written in assembly language must be translated

into a "language of zeros and ones" in order for a microcontroller to understand it. "Assembly

language" and "assembler" are two different notions. The first represents a set of rules used in

writing a program for a microcontroller, and the other is a program on the personal computer

which translates assembly language into a language of zeros and ones. A program that is

translated into "zeros" and "ones" is also called "machine language".

The process of communication between a man and a microcontroller

Physically, "Program" represents a file on the computer disc (or in the memory if it is read in a

microcontroller), and is written according to the rules of assembler or some other language for

microcontroller programming. Man can understand assembler language as it consists of alphabet

signs and words. When writing a program, certain rules must be followed in order to reach a

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desired effect. A Translator interprets each instruction written in assembly language as a series

of zeros and ones which have a meaning for the internal logic of the microcontroller.

Lets take for instance the instruction "RETURN" that a microcontroller uses to return from a

sub-program. When the assembler translates it, we get a 14-bit series of zeros and ones which the

microcontroller knows how to interpret.

Example:

RETURN 00 0000 0000 1000

Similar to the above instance, each assembler instruction is interpreted as corresponding to series

of zeros and ones. The place where this translation of assembly language is found, is called an

"execution" file. We will often meet the name "HEX" file. This name comes from a hexadecimalrepresentation of that file, as well as from the suffix "hex" in the title, ex. "test.hex". Once it is

generated, the execution file is read in a microcontroller through a programmer.

An Assembly Language program is written in a program for text processing (editor) and is

capable of producing an ASCII file on the computer disc or in specialized surroundings such as

MPLAB,which will be explained later.

3.1 Representing numbers in assembler

In assembly language MPLAB, numbers can be represented in decimal, hexadecimal or binary

form. We will illustrate this with a number 240:

.240 decimal

0xF0 hexadecimal

b'11110000' Binary

Decimal numbers start with a dot, hexadecimal with 0x, and binary start with b with the number

itself under quotes '.

3.2 Assembly language elements

Basic elements of assembly language are:

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Labels

Instructions

Operands

Directives

Comments

Labels

A Label is a textual designation (generally an easy-to-read word) for a line in a program, or

section of a program where the micro can jump to - or even the beginning of set of lines of a

program. It can also be used to execute program branching (such as Goto .......) and the program

can even have a condition that must be met for the Goto instruction to be executed. It isimportant for a label to start with a letter of the alphabet or with an underline "_". The length of

the label can be up to 32 characters. It is also important that a label starts in the first clumn.

Instructions

Instructions are already defined by the use of a specific microcontroller, so it only remains for us

to follow the instructions for their use in assembly language. The way we write an instruction is

also called instruction "syntax". In the following example, we can recognize a mistake in writing

because instructions movlp and gotto do not exist for the PIC16F84 microcontroller.

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Operands

Operands are the instruction elements for the instruction is being executed. They are usually

registers orvariables orconstants.

Comments

Comment is a series of words that a programmer writes to make the program more clear and

legible. It is placed after an instruction, and must start with a semicolon ";".

Directives

A directive is similar to an instruction, but unlike an instruction it is independent on the

microcontroller model, and represents a characteristic of the assembly language itself. Directives

are usually given purposeful meanings via variables or registers. For example, LEVEL can be a

designation for a variable in RAM memory at address 0Dh. In this way, the variable at that

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address can be accessed via LEVEL designation. This is far easier for a programmer to

understand than for him to try to remember address 0Dh contains information about LEVEL.

3.3 Writing a sample program

The following example illustrates a simple program written in assembly language respecting the

basic rules.

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When writing a program, beside mandatory rules, there are also some rules that are not written

down but need to be followed. One of them is to write the name of the program at the beginning,

what the program does, its version, date when it was written, type of microcontroller it was

written for, and the programmer's name.

Since this data isn't important for the assembly translator, it is written as comments. It should be

noted that a comment always begins with a semicolon and it can be placed in a new row or it can

follow an instruction.After the opening comment has been written, the directive must be

included. This is shown in the example above.

In order to function properly, we must define several microcontroller parameters such as: - type

of oscillator,

- whether watchdog timer is turned on, and

- whether internal reset circuit is enabled.

All this is defined by the following directive:

_CONFIG _CP_OFF&_WDT_OFF&PWRTE_ON&XT_OSC

When all the needed elements have been defined, we can start writing a program.

First, it is necessary to determine an address from which the microcontroller starts, following a

power supply start-up. This is (org 0x00). The address from which the program starts if an

interrupt occurs is (org 0x04). Since this is a simple program, it will be enough to direct the

microcontroller to the beginning of a program with a "goto Main" instruction.

The instructions found in the Main select memory bank1 (BANK1) in order to access TRISB

register, so that port B can be declared as an output (movlw 0x00, movwf TRISB).

The next step is to select memory bank 0 and place status of logic one on port B (movlw 0xFF,

movwf PORTB), and thus the main program is finished.

We need to make another loop where the micro will be held so it doesn't "wander" if an error

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occurs. For that purpose, one infinite loop is made where the micro is retained while power is

connected. The necessary "end" at the end of each program informs the assembly translator that

no more instructions are in the program.

3.4 Control directives

3.1 #DEFINE Exchanges one part of text for another

Syntax:

#define []

Description:

Each time appears in the program , it will be exchanged for .

Example:

#define turned_on 1

#define turned_off 0

Similar directives: #UNDEFINE, IFDEF,IFNDEF

3.2 INCLUDE Include an additional file in a program

Syntax:

#include

#include "file_name"

Description:

An application of this directive has the effect as though the entire file was copied to a place

where the "include" directive was found. If the file name is in the square brackets, we are dealing

with a system file, and if it is inside quotation marks, we are dealing with a user file. The

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directive "include" contributes to a better layout of the main program.

Example:

#include

#include "subprog.asm"

3.3 CONSTANT Gives a constant numeric value to the textual designation

Syntax:

Constant =

Description:

Each time that appears in program, it will be replaced with .

Example:

Constant MAXIMUM=100

Constant Length=30

Similar directives: SET, VARIABLE

3.4 VARIABLE Gives a variable numeric value to textual designation

Syntax:

Variable=

Description:

By using this directive, textual designation changes with particular value.

It differs from CONSTANT directive in that after applying the directive, the value of textual

designation can be changed.

Example:

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variable level=20

variable time=13

Similar directives: SET, CONSTANT

3.5 SET Defining assembler variable

Syntax:

set

Description:

To the variable is added expression . SET directive is similar to EQU,

but with SET directive name of the variable can be redefined following a definition.

Example:

level set 0

length set 12

level set 45

Similar directives: EQU, VARIABLE

3.6 EQU Defining assembler constant

Syntax:

equ

Description:

To the name of a constant is added value

Example:

five equ 5

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six equ 6

seven equ 7

Similar instructions: SET

3.7 ORG Defines an address from which the program is stored in microcontroller

memory

Syntax:

org

Description:This is the most frequently used directive. With the help of this directive we define where some

part of a program will be start in the program memory.

Example:

Start org 000

movlw 0xFF

movwf PORTB

The first two instructions following the first 'org' directive are stored from address 00, and the

other two from address 10.

3.8 END End of program

Syntax:

end

Description:

At the end of each program it is necessary to place 'end' directive so that assembly translator

would know that there are no more instructions in the program.

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Example:

.

.

movlw 0xFF

movwf PORTB

end

3.9 IF Conditional program branching

Syntax:

if

Description:

If condition in was met, part of the program which follows IF directive

would be executed. And if it wasn't, then the part following ELSE or ENDIF directive would be

executed.

Example:

if level=100

goto FILL

else

goto DISCHARGE

endif

Similar directives: #ELSE, ENDIF

3.10 ELSE The alternative to 'IF' program block with conditional terms

Syntax:

Else

Description:

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Used with IF directive as an alternative if conditional term is incorrect.

Example:

If time< 50

goto SPEED UP

else goto SLOW DOWN

endif

Similar instructions: ENDIF, IF

3.11 ENDIF End of conditional program section

Syntax:

endif

Description:

Directive is written at the end of a conditional block to inform the assembly translator that it is

the end of the conditional block

Example:

If level=100

goto LOADS

else

goto UNLOADS

endif

Similar directives: ELSE, IF

3.12 WHILE Execution of program section as long as condition is met

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Syntax:

while.

endw

Description:

Program lines between WHILE and ENDW would be executed as long as condition was met. If a

condition stopped being valid, program would continue executing instructions following ENDW

line. Number of instructions between WHILE and ENDW can be 100 at the most, and number of

executions 256.

Example:

While i

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Syntax:

ifdef

Description:

If designation was previously defined (most commonly by #DEFINE instruction),

instructions which follow would be executed until ELSE or ENDIF directives are not would be

reached.

Example:

#define test

.

ifdef test ;how the test was defined

......; instructions from these lines would execute

endif

Similar directives: #DEFINE, ELSE, ENDIF, IFNDEF, #UNDEFINE

3.15 IFNDEF Execution of a part of the program if symbol was defined

Syntax:

ifndef

Description:

If designation was not previously defined, or if its definition was erased with

directive #UNDEFINE, instructions which follow would be executed until ELSE or ENDIF

directives would be reached.

Example:

#define test

..........

#undefine test

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..........

ifndef test ;how the test was undefined

..... .; instructions from these lines would execute

endif

Similar directives: #DEFINE, ELSE, ENDIF, IFDEF, #UNDEFINE

3.16 CBLOCK Defining a block for the named constants

Syntax:

Cblock [][:], [:]......

endc

Description:

Directive is used to give values to named constants. Each following term receives a value greater

by one than its precursor. If parameter is also given, then value given in

parameter is added to the following constant.Value of parameter is the starting value. If it is not given, it is considered to be zero.

Example:

Cblock 0x02

First, second, third ;first=0x02, second=0x03, third=0x04

endc

cblock 0x02

first : 4, second : 2, third ;first=0x06, second=0x08, third=0x09

endc

Similar directives: ENDC

3.17 ENDC End of constant block definition

Syntax:

endc

Description:

Directive was used at the end of a definition of a block of constants so assembly translatorcould know that there are no more constants.

Similar directives: CBLOCK

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3.18 DB Defining one byte data

Syntax:

[]db [, ,.....,]

Description:

Directive reserves a byte in program memory. When there are more terms which need to beassigned a byte each, they will be assigned one after another.

Example:db 't', 00f, 'e', 's', 012

Similar instructions: DE, DT

3.19 DE Defining the EEPROM memory byte

Syntax:

[] de [, ,....., ]

Description:

Directive is used for defining EEPROM memory byte. Even though it was first intended onlyfor EEPROM memory, it could be used for any other location in any memory.

Example:org H'2100'de "Version 1.0" , 0

Similar instructions: DB, DT

3.20 DT Defining the data table

Syntax:

[] dt [, ,........., ]

Description:

Directive generates RETLW series of instructions, one instruction per each term.

Example:dt "Message", 0

dt first, second, third

Similar directives: DB, DE

3.21 _CONFIG Setting the configurational bits

Syntax:

_ _config or_ _config,

Description:

Oscillator, watchdog timer application and internal reset circuit are defined. Before usingthis directive, the processor must be defined using PROCESSOR directive.

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Example:_CONFIG _CP_OFF&_WDT_OFF&_PWRTE_ON&_XT_OSC

Similar directives: _IDLOCS, PROCESSOR

3.22 PROCESSOR Defining microcontroller model

Syntax:

Processor

Description:

Instruction sets the type of microcontroller where programming is done.

Example:processor 16F84

3.5 Files created as a result of program translation

As a result of the process of translating a program written in assembler language we get files

like:

Executing file (Program_Name.HEX)

Program errors file (Program_Name.ERR)

List file (Program_Name.LST)

The first file contains translated program which was read in microcontroller by programming. Its

contents can not give any information to programmer, so it will not be considered any further.

The second file contains possible errors that were made in the process of writing, and which

were noticed by assembly translator during translation process. Errors can be discovered in a

"list" file as well. This file is more suitable though when program is big and viewing the 'list' file

takes longer. The third file is the most useful to programmer. Much information is contained in

it, like information about positioning instructions and variables in memory, or error signalization.

Example of 'list' file for the program in this chapter follows. At the top of each page is stated

information about the file name, date when it was translated, and page number. First column

contains an address in program memory where a instruction from that row is placed. Second

column contains a value of any variable defined by one of the directives : SET, EQU,

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VARIABLE, CONSTANT or CBLOCK. Third column is reserved for the form of a translated

instruction which PIC is executing. The fourth column contains assembler instructions and

programmer's comments. Possible errors will appear between rows following a line in which the

error occurred.

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At the end of the "list" file there is a table of symbols used in a program. Useful element of 'list'

file is a graph of memory utilization. At the very end, there is an error statistic as well as the

amount of remaining program memory.

DEVELOPMENT SUPPORT

The PICmicro microcontrollers are supported with a full range of hardware and software

development tools:

Integrated Development Environment

- MPLAB IDE Software

Assemblers/Compilers/Linkers

- MPASMTM Assembler

- MPLAB C17 and MPLAB C18 C Compilers

- MPLINKTM Object Linker/

MPLIBTM Object Librarian

Simulators

- MPLAB SIM Software Simulator

Emulators

- MPLAB ICE 2000 In-Circuit Emulator

- ICEPIC In-Circuit Emulator

In-Circuit Debugger

- MPLAB ICD

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Device Programmers

- PRO MATE II Universal Device Programmer

- PICSTART Plus Entry-Level Development

Programmer

Low Cost Demonstration Boards

- PICDEMTM 1 Demonstration Board

- PICDEM 2 Demonstration Board

- PICDEM 3 Demonstration Board

- PICDEM 17 Demonstration Board

- KEELOQ Demonstration Board