Microcontrollers -UNIT I

Dr.Y.Narasimha Murthy Ph.D

MICROCONTROLLERS-UNIT-I

1.1 Introduction: Any electronic system built around a single chip computer or microcontroller to perform a specific dedicated task /application is known as an embedded system. Software is programmed in the on-chip ROM of the microcontroller. These microcontrollers are also popularly known as embedded processors or computers. Due to this reason some people also call the embedded system as the combination of hardware, software designed for a dedicated application. Actually the software program that runs the application is hidden (Embed) inside the system. Hence the name Embedded system. For low end embedded systems operating system is not required. But for those high end real-time embedded systems operating system is also included .Such operating systems are popularly known as Real Time operating Systems. In today's world, embedded systems are everywhere -- homes, offices, cars, factories, hospitals, and consumer electronics. For Example a typical present day contains about ten microcontrollers. The modern houses contain as many as 150 microcontrollers and on average a consumer interacts with nearly 300 microcontrollers a day. General embedded systems that are used in our day to day life are , Cell phone , Digital camera , Microwave Oven, MP3 player, Portable digital assistant, Bank ATMs , & antilock brake system in vehicles ,computer key boards ,Mouse ,Printers etc.Due to the unprecedented growth in embedded system technology and the market demand ,there is also a great increase in the design and manufacturing of innumerable number of Microcontrollers in the market. Because without a suitable microcontroller it is impossible to design any embedded system. Hence various manufacturers like INTEL ,Motorola,Texas Instruments, Microchip, Analog Devices, Philips ,Atmel, Samsung ,ARM etc have their share in the market.1.2 Characteristics of an Embedded System : The important characteristics of an embedded system are given below.

Speed : Should be of high speed Power : Low power dissipation Accuracy : The output of the system must be very accurate Adaptability: Embedded system must be highly adaptable and accessible to different conditions and situations. Reliability : The results must be reliable over a long period of time. Size and weight : As far as possible small in size and low weightAn embedded system must perform the operations at a high speed so that it can be readily used for real time applications and its power consumption must be very low and the size of the system should be as for as possible small and the readings must be accurate with minimum error. The system must be easily adaptable for different situations.

1.3 Types of Embedded Systems: Based on their functionality and performance Embedded systems are classified into following four categories.(i).Stand-alone embedded systems(ii).Real-time embedded systems ( Hard real-time systems & Soft real-time systems)(iii).Networked embedded systems and (iv).Mobile Embedded systems.

Based on the performance of the Microcontroller they are also classified into (i) Small scaled embedded system (ii) Medium scaled embedded system and (iii) Large scaled embedded system.This classification can be shown by the figure.1.1

Fig.1.1 Classification of Embedded systemsStand alone Embedded systems : A stand-alone embedded system works by itself. It is a self-contained device which do not require any host system like a computer. It accepts either analog or digital signals through its input ports and calibrates and processes the signals and finally outputs the resulting data to the connected output device . Temperature measurement systems, Video game consoles, MP3 players, digital cameras, and microwave ovens are the examples for this category.

Real-time embedded systems : An embedded system which gives the required output in a specified time or which strictly follows the time deadlines for completion of a task is known as a Real time embedded system. i.e a Real Time embedded system not only gives the correct output but delivers the output in the specified time also.Real Time Embedded systems are again two types(a) Soft real time system and (b) Hard real time system.Soft Real-Time system: A Real time system in which ,the violation of time constraints will cause only the degraded quality, but the system can continue to operate normally without any destruction is known as a Soft real time system. In soft real-time systems, the design focus is to offer a guaranteed bandwidth to each real-time task and to distribute the resources to the tasks. Examples are Microwave oven , Washing machine , TV remote etc.

Hard Real-Time system : A Real time system in which ,the violation of time constraints will cause critical failure and loss of life or property, damage or catastrophe is known as a Hard Real time system.These systems usually interact directly with physical hardware instead of through human being.For example, Deadline in a missile control , Delayed alarm during a Gas leakage , car airbag control system , A delayed response in pacemakers ,Failure in RADAR functioning etc.Networked embedded systems: The networked embedded systems are related to a network with network interfaces to access the resources. The connected network can be a Local Area Network (LAN) or a Wide Area Network (WAN), or the Internet. The connection can be either wired or wireless.The networked embedded system is the fastest growing area in embedded system applications. The embedded web server is such a system where all embedded devices are connected to a web server and can be accessed and controlled by any web browser.Home security system is an example of a LAN networked embedded system where all sensors (e.g. motion detectors, light sensors, or smoke sensors) are wired and running on the TCP/IP protocol.Mobile Embedded systems: The portable embedded devices like mobile and cellular phones, digital camaras,MP3 players, PDA (Personal Digital Assistants) are the example for mobile embedded systems. The basic limitation of these devices is the limitation of memory and other resources.Small scale embedded system : An embedded system supported by a single 816 bit Microcontroller with on-chip RAM and ROM designed to perform simple tasks is a Small scale embedded system. For example , a simple toy or display system is an example for Small Scale embedded system.Medium scale embedded system : An embedded system supported by 1632 bit Microcontroller /Microprocessor with external RAM and ROM that can perform more complex operations is a Medium scale embedded system. Large scale embedded system: An embedded system supported by 32-64 bit multiple chips which can perform distributed jobs is considered as a Large scale embedded system.The various Embedded systems and their applications are given in Table 1.1

S.NoEmbedded SystemApplication

1Home AppliancesDishwasher, washing machine, microwave, Top-set box, security system , HVAC system, DVD, answering machine, garden sprinkler systems etc..

2Office Automation Fax, copy machine, smart phone system, modern, scanner, printers.

3SecurityFace recognition, finger recognition, eye recognition, building security system , airport security system, alarm system.

4AcademiaSmart board, smart room, OCR, calculator, smart cord.

5InstrumentationSignal generator, signal processor, power supplier,Process instrumentation,

6TelecommunicationRouter, hub, cellular phone, IP phone, web camera

7Automobile Fuel injection controller, anti-locking brake system, air-bag system, GPS, cruise control.

8Entertainment MP3, video game, Mind Storm, smart toy.

9Aerospace Navigation system, automatic landing system, flight attitude controller, space explorer, space robotics.

10Industrial automation Assembly line, data collection system, monitoring systems on pressure, voltage, current, temperature, hazard detecting system, industrial robot.

11PersonalPDA, iPhone, palmtop, data organizer.

12Medical CT scanner, ECG , EEG , EMG ,MRI, Glucose monitor, blood pressure monitor, medical diagnostic device.

13Banking & Finance ATM, smart vendor machine, cash register ,Share market

14Miscellaneous:Elevators, tread mill, smart card, security door etc.

1.4 Embedded system-Block Diagram approach An Embedded system is a combination of hardware and software. The hardware system is built with a microprocessor or microcontroller .A microcontroller is an integrated chip which comes with built-in memory, I/O ports, timers, and other components. Most embedded systems are built on microcontrollers, which run faster than a custom-built system with a microprocessor, because all components are integrated within that single chip. Application software and operating systems are the main components of the software system. But all the embedded systems do not use the operating system. The systems with high end applications only use operating system. To use the operating system the embedded system should have large memory capability. So, This is not possible in low end applications like remote systems, digital cameras, MP3 players, robo toys etc. Since an Embedded system constitutes both hardware and software components, the architecture can be understood with block diagram approach where the hardware and software aspects are shown separately. As shown in Fig 1.2 an embedded system consists of Embedded controller along with necessary electronic hardware components like sensors , drivers ,buffers etc along with communication interfaces and I/O devices. The application software is loaded into the on chip ROM of the microcontroller.

Fig.1.2 Block diagram of an embedded systemTo understand the architecture of an embedded system thoroughly the hardware and software details are considered in detail in the following sections.1.4.1 Hardware Architecture:Coming to the hardware details of the embedded system, it consists of the following important blocks. CPU(Central Processing Unit) RAM and ROM I/O Devices Communication Interfaces Sensors etc. (Application specific circuitry)This hardware architecture is shown in the following block diagram 1.3

. Fig.1.3 Hardware ArchitectureCentral Processing Unit : A CPU is composed of an Arithmetic Logic Unit (ALU), a Control Unit (CU), and many internal registers that are connected by buses. The ALU performs all the mathematical operations (Add, Sub, Mul, Div), logical operations (AND, OR), and shifting operations within CPU .The timing and sequencing of all CPU operations are controlled by the CU, which is actually built of many selection circuits including latches and decoders .The CU is responsible for directing the flow of instruction and data within the CPU and continuously running program instructions step by step.

The CPU works in a cycle of fetching an instruction, decoding it, and executing it, known as the fetch-decode-execute cycle. The cycle begins when an instruction is fetched from a memory location pointed to by the PC to the IR via the data bus.For embedded system design, many factors impact the CPU selection, e.g., the maximum size (number of bits) in a single operand for ALU (8, 16, 32, 64 bits), and CPU clock frequency for timing tick control, i.e. the number of ticks (clock cycles) per second in measures of MHz. Memory : Embedded system memory can be either on-chip or off-chip. On chip memory access is much fast than off-chip memory, but the size of on-chip memory is much smaller than the size of off-chip memory. Usually, it takes at least two I/O ports as external address lines plus a few control lines such as R/W and ALE control lines to enable the extended memory. Generally the data is stored in RAM and the program is stored in ROM.I/O Ports : The I/O ports are used to connect input and output devices. The common input devices for an embedded system include keypads, switches, buttons, knobs, and all kinds of sensors (light, temperature, pressure, etc). The output devices include Light Emitting Diodes (LED), Liquid Crystal Displays (LCD), printers, alarms, actuators, etc. Some devices support both input and output, such as communication interfaces including Network Interface Cards (NIC), modems, and mobile phones.Communication Interfaces :To transfer the data or to interact with other devices ,the embedded devices are provided the various communication interfaces like RS232, RS422, RS485 ,USB, SPI(Serial Peripheral Interface ) ,SCI (Serial Communication Interface) ,Ethernet etc.Application Specific Circuitry : The embedded system some times receives the input from a sensor or actuator. In such situations certain signal conditioning circuitry is needed. This hardware circuitry may contain ADC , Op-amps ,DAC etc. Such circuitry will interact with the embedded system to give correct output.Power supply: Most of the embedded systems now days work on battery operated supplies. Because low power dissipation is always required. Hence the systems are designed to work with batteries. 1.4.2 Software architecture: To design an efficient embedded system , both hardware and software aspects are equally important. The software of an embedded system is mainly aimed at accessing the hardware resources properly. The software of an embedded system means both operating system and application software. But every embedded system need not contain the operating system.For low end applications, operating system is not needed. In such cases the designer has to write the necessary software routines to access the hardware. The architecture of the software in an embedded system can be shown in the figure1.4.

. Fig.1.4 Software ArchitectureThe central part or nucleus of the operating system is the Kernel .A kernel connects the application software to the hardware of an embedded system. The other important components of the OS are Device manager, Communication software, Libraries and File system. The kernel will take care of task scheduling, priorities, memory management etc.It manages the tasks to achieve the desired performance of the system. It schedules the tasks and provide inter process communication between different tasks.The device manager manages the I/O devices through interrupts and device drivers. The device drivers provide the necessary interface between the application and the hardware. A device driver is a specific type of software developed to allow interaction with hardware devices. This constitutes an interface for communicating with the device, through the specific system bus or communications subsystem that the hardware is connected to, providing commands to receiving data from the device, and on the other end, the requisite interfaces to the operating system and software applications.The communication software provides necessary protocols to make the embedded system network enabled. This software integrates the upper layer protocols such as TCP/IP stack with the operating system. Application programming interface is used by the designer to write the application software.The API provides the function calls to access the operating system services.Application Specific software : It sits above the O.S. The application software is developed according to the features of the development tools available in the OS.These development tools provide the function calls to access the services of the OS. These function calls include, creating a task ,to read the data from the port and write the data to the memory etc.The various function calls provided by an operating system arei.To create ,suspend and delete tasks. ii. To do task scheduling to providing real time environment.iii.To create inter task communication and achieve the synchronization between tasks.iv.To access the I/O devices.vi.To access the communication protocol stack .The designer develops the application software based on these function calls.Communication Software: To connect to the external world through internet ,the embedded system need a communication interface. The communication software include the Ethernet interface and the TCP/IP protocol suit .Now a days even small embedded systems like mobile phones ,PDAs are network enabled through these TCP/IP support. The TCP/IP protocol suite is shown in the diagram below. Communication Software: To connect to the external world through internet ,the embedded system need a communication interface. The communication software include the Ethernet interface and the TCP/IP protocol suit .Now a days even small embedded systems like mobile phones ,PDAs are network enabled through these TCP/IP support. The TCP/IP protocol suite is shown in the Fig1.5 below. Application layer

Transport Layer TCP/UDP

IP Layer

Data Link Layer

Physical Layer

Fig 1.5 The TCP/IP protocol suiteThis suite consists of different layers like, application layer, Transport layer , IP layer etc.TCP means Transmission Control Protocol. It ensures that the data is delivered to the application layer without any errors. The UDP (User Datagram protocol) provides a connectionless service for error control and flow of data. This TCP/IP protocol suite helps to understand the working of communication software packages.1.5 Embedded system Development Tools : Basically the embedded tools are divided into two types( i).Hardware Development tools and (ii) Software Development tools.1.5.1 Hardware development tools : Hardware tools for embedded development include development or evaluation boards for specific processors, like Friendly ARMs Mini2440, Pandaboard , Beagleboard and Craneboard etc..In addition to this various othes devices like Digital multimeters ,Logic Analyzers , Spectrum Analyzers and Digital CROs etc.are also required in embedded design.The digital multimeter is used to measure voltages, currents and to check the continuity in the circuits in an embedded systems. Because the embedded system also contains some application specific circuitry which some times require debugging.The Logic analyzer is used to check the timings of the signals ,and their correctness.The Spectrum analyzer is helpful to to analyze the signals in the frequency domain.The digital CRO helps to display the output waveforms and also to store a portion of the waveforms etc.1.5.2 Software development tools /testing tools : The software development tools include the operating system development suite ,cross platform development tools, ROM emulator ,EPROM programming and In circuit Emulator (ICE) etc. The operating system development suite consists of API calls to access the OS services.This suite can run on either Windows or UNIX/Linux systems.Under the cross platform tools,the compiler generates the object code for the source code developed in high level languages like C and C++ or Java etc.For LINUX systems a number of GNU tools are available.The EPROM programmer is used to in circuit programming by burning the code in the memory of the target system. The instruction set Simulator (ISS) software creates the virtual version of the processor on the PC.Assembler and Compiler: The binary code obtained by translating an assembly language program using an assembler is smaller and runs faster than the binary code obtained by translating a high level language using a compiler since the assembly language gives the programmer complete control over the functioning of a processor. The advantage of using a high level language is that a program written in a high level language is easier to understand and maintain than a program written in assembly language. Hence time critical applications are written in assembly language while complex applications are written in a high level language.Cross compilation tools are very important for successful product development. Selection of these tools should be made based upon the embedded system itself as well as features to test and debug software remotely. The cross-platform development tools should be compatible with the host machine. Depending upon CPU family used for the target system, the toolset must be capable of generating code for the target machine. In the case of GNU development tools, we need to have a number of things to work together to generate executable code for the target. At least one of the following tools must be available on the machine.Cross compilerCross assemblerCross linkerCross debuggerCross-compiled libraries for the target host.Operating system-dependent libraries and header files for the target systemSimulator: A simulator is software tool that runs on the host and simulates the behavior of the targets processor and memory. The simulator knows the target processors architecture and instruction set. The program to be tested is read by the simulator and as instructions are executed the simulator keeps track of the values of the target processors registers and the targets memory. Simulators provide single step and breakpoint facilities to debug the program.

Fig 1.6 Cross compilationEmulator : Another important tool is the ICE(In-Circuit Emulator),which emulates the CPU. An emulator is a hardware tool that helps in testing and debugging the program on the target. The targets processor is removed from the circuit and the emulator is connected in its place. The emulator drives the signals in the circuit in the same way as the targets processor and hence the emulator appears to be the processor to all other components of the embedded system. Emulators also provide features such as single step and breakpoints to debug the program.Software emulators are software tools that can emulate a particular CPU. Using a software emulator one can debug the code and find out CPU register values, stack pointers and other information without having a real CPU. Software emulators are useful when we dont have the real hardware available for testing and debugging and want to see how the CPU will behave when a program is run on it.INTEL 8051 MICROCONTROLLERS1.6 IntroductionIn earlier days the process and control operations were totally implemented by the Microprocessors only. But since the last decade the situation is drastically changed and a new device called Microcontroller has emerged in the market . The advent and development of Microcontrollers is so drastic that it is impossible to find any electronic gadget without the use of a microcontroller in the todays electronic markets. This microcontroller changed the embedded system design so simple that the embedded market has become one of the most sought after for not only entrepreneurs but for design engineers also. A Microcontroller is a single chip computer or A CPU with all the peripherals like RAM, ROM, I/O Ports, Timers , ADCs etc... on the same chip. Or it is also defined as aSingle chip computer.For ex: Motorolas 6811, Intels 8051, Zilogs Z8 and Micro chips PIC 16X etc are few Microcontrollers available in the market. In addition to these people there are innumerable number of vendors like Atmel, Philips, AMD, Infineon (formerly Siemens)Samsung etc in the market.Let us now see how a microcontroller is different from a microprocessor by considering the differences between them.A Microprocessor is basically a CPU built into a single VLSI chip . It is a general-purpose device and additional external circuitry is added to work it as a microcomputer. The microprocessor contains arithmetic and logic unit (ALU), Instruction decoder , control unit, Instruction register, Program counter (PC), clock circuit (internal or external), reset circuit (internal or external) and registers. But the microprocessor has no on chip I/O Ports, Timers , Memory etc. For example, Intel 8085 is 8-bit microprocessor and Intel 8086/8088 is 16-bit microprocessor. The block diagram of a Microprocessor is shown in Fig 1.7.

Fig 1.7.Block diagram of a MicroprocessorA microcontroller is a highly integrated single chip, which consists of on chip CPU (Central Processing Unit), RAM (Random Access Memory), EPROM/PROM/ROM (Erasable Programmable Read Only Memory), I/O (input/output) serial and parallel, timers, interrupt controller. For example, Intel 8051 is 8-bit microcontroller and Intel 8096 is 16-bit microcontroller. The block diagram of Microcontroller is shown in Fig.1.8.

Fig.1.8.Block diagram of Microcontroller Let us now compare the two devices. The Table 1.2 gives some of the salient differences between Microprocessors and Microcontroller.S.NoMicroprocessorMicrocontroller

1A microprocessor is a general purpose device which is called a CPU A microcontroller is a dedicated chip which is also called single chip computer

2A microprocessor do not contain onchip I/OPorts, Timers, Memories etc..A microcontroller includes RAM, ROM, serial and parallel interface, timers, interruptcircuitry (in addition to CPU) in a single chip.

3Microprocessors are most commonly used as the CPU in microcomputer systemsMicrocontrollers are used in small, minimum component designs performing control-oriented applications.

4

Microprocessor instructions are Microcontroller instructions are both bit mainly nibble or byte addressableaddressable as well as byte addressable.

5Microprocessor instruction sets are mainly intended for catering to large volumes of data.Microcontrollers have instruction sets catering to the control of inputs and outputs.

6Microprocessor based system design is complex and expensiveMicrocontroller based system design is rather simple and cost effective

7The Instruction set of microprocessor is complex with large number of instructions.The instruction set of a Microcontroller is very simple with less number of instructions. For, ex: PIC microcontrollers have only 35 instructions.

8A microprocessor has zero status flag A microcontroller has no zero flag.

1.7 Computer Architectures : There are two popular computer architectures in use. One is the Van Neumann architecture and the other is the Harvard architecture.The first architecture was developed at Princeton advanced Institute (USA) and after the chief scientist name Von Neumann it is named as Von Neumann architecture or Princeton architecture. The second architecture was developed by a group at Harvard University and it is known as Harvard architecture.

1.7.1Von Neumann architectureThe Von Neumann architecture refers a specific type of computer model where the instruction and data are stored in the same memory. The instruction code is executed serially and and need more clock cycles. As shown in the Fig 1.14 the heart of the von Neumann computer is the central processing unit or CPU which consists of the Arithmetic Logic Unit(ALU) and control unit.The CPU interacts with I/O and memory subsystems and executes a stream of instructions that process the data stored in memory

Fig.1.9 Von Neumann architecture

. The basic advantage of von Neumann architecture is that it simplifies the microcontroller chip design because only one memory is accessed. For microcontrollers, its biggest asset is that the contents of RAM (random-access memory) can be used for both variable (data) storage as well as program instruction storage. An advantage for some applications is the program counter stack contents that are available for access by the program. This allows greater flexibility in developing software, primarily in the areas of real-time operating systems.1.7.2Harvard architecture In this computer model separate memories are used for both instruction and data and separate buses are also used for fetching the instruction code and data. Due to this ,a kind of parallelism is achieved in the execution of instructions by the processor

Fig.1.10. Harvard architecture

The Harvard architecture executes instructions in less number of instruction cycles than the Von Neumann architecture. This is because a much greater amount of instruction parallelism is possible in the Harvard architecture. Parallelism means that fetches for the next instruction can take place during the execution of the current instruction, without having to either wait for a "dead" cycle of the instruction's execution or stop the processor's operation while the next instruction is being fetched.

Distinguish between von Neumann Architecture and Harvard ArchitectureVon Neumann ArchitectureHarvard Architecture

This architecture is used in conventional processors found in PCs and Servers, and embedded systems with only control functions.This architecture is DSPs and other processors found in latest embedded systems and Mobile communication systems, audio, speech, image processing systems etc.

The data and program are stored in the same memoryThe data and program memories are stored in separate memories.

The code is executed serially and takes more clock cyclesThe code is executed in parallel and hence less number of clock cycles are required

There is no exclusive MultiplierIt has MAC (Multiply Accumulate)

There is no barrel shifter present in this architecture. In this architecture , the Barrel Shifter helps in shifting and rotating operations of the data

The programs can be optimized in lesser sizeThe program tend to grow big in size

1.8 RISC & CISC Processor architectureBased on the performance and the type of instruction set it handles ,there are two types of processor architectures. They are RISC processors and CISC processors.RISC means Reduced Instruction Set Computer. These RISC processors use only simple instructions that can be executed within one clock cycle. As each instruction requires only one clock cycle to execute, the entire program will execute in approximately the same amount of time . These RISC "reduced instructions" require less transistors of hardware space than the complex instructions, leaving more room for general purpose registers. Because all of the instructions execute in a uniform amount of time (i.e. one clock), pipelining is possible. The RISC architecture is based on Load/Store concept. In the RISC processors ,the control unit is hardwired i.e RISC architectures are not micro programmed. The code generated by the compiler is directly executed by the hardware. and it is not interpreted by the micro programming. The number of instruction formats is also (addressing modes) is also less. Many times less than four.The RISC architecture has a larger register file organization.RISC uses the concept of Instruction pipelining.

CISC stands for Complex Instruction Set Computers. The primary goal of CISC architecture is to complete a task in as few lines of assembly as possible. This is achieved by building processor hardware that is capable of understanding and executing a series of operations. For example Intel (x86 ) and AMD CPU's are based on CISC architecturesOne of the primary advantages of CISC Architecture is that the compiler has to do very little work to translate a high-level language statement into assembly. Because the length of the code is relatively short, very little RAM is required to store instructions. The emphasis is put on building complex instructions directly into the hardware.Distinguish between CISC and RISC processors architecturesS.NoCISCRISC

1Stands for Complex Instruction Set ComputerStands for Reduced Instruction Set Computer

2A typical CISC instruction set contains nearly 120-350 instructionsCISC Instruction set contains around 35 instructions

3The main emphasis is on hardware The main emphasis on software

4Includes multi-clock complex instructionsSingle-clock, reduced instruction only

5Most of the instructions are memory basedInstructions are register based

6Unified cache for Data and InstructionsSeparate instruction ,data caches are used.

7Lower clock rate 33-50M.Hz with CPI between 2-15Higher clock rate 50-150.Hz with CPI < 1.5

8Lower MIPSHigher MIPS

9CPU control is microcoded control memoryCPU control is Hardwired

10More number of addressing modes (8-24)Only 3-5 simple addressing modes.

11INTELX86 , Pentium processors are examples for CISCIBM Power PC is an example for RISC

12Memory-to-memory:"LOAD" and "STORE"incorporated in instructionsRegister to register:"LOAD" and "STORE"are independent instructions

13This architecture do not support Pipelining conceptPipelining is the main feature of the RISC processors

14Hard to designEasy to design

1.9. EVOLUTION OF MICROCONTROLLERS The first microcontroller TMS1000 was introduced by Texas Instruments in the year 1974. In the year 1976, Motorola designed a Microprocessor chip called 6801 which replaced its earlier chip 6800 with certain add-on chips to design a computer. This paved the way for the new revolution in the history of chip design and gave birth to a new entity called Microcontroller. Later the Intel company has produced its first Microcontroller 8048 with a CPU and 1K bytes of EPROM, 64 Bytes of RAM an 8-Bit Timer and 27 I/O pins in 1976. Later in the year 1980 the most popular microcontroller 8051 was released with 4K bytes of ROM,128 Bytes of RAM , a serial port, two 16-bit Timers , and 32 I/O pins. The 8051 family has many additions and improvements over the years and became the most acclaimed tool for todays circuit designers. INTEL introduced a 16 bit microcontroller 8096 in the year 1982 . Later INTEL introduced 80c196 series of 16-bit Microcontrollers for mainly industrial applications. Microchip, another chip design company from US is popular for PIC microcontrollers and has introduced an 8-bit Microcontroller PIC 16C64 in the year 1985.The 32-bit microcontrollers have been developed by IBM and Motorola. MPC 505 is a 32-bit RISC controller of Motorola. The 403 GA is a 32 -bit RISC embedded controller of IBM.In recent times ARM company (Advanced RISC machines) has developed and introduced 32 bit controllers for high-end application devices like mobiles , Ipods etc. 1.10 TYPES OF MICROCONTROLLERS :Microcontrollers are classified on the basis of internal bus width, architecture, memory and instruction set as 4-bit, 8-bit, 16-bit and 32-bit microcontrollers.4-bit Microcontrollers: These 4-bit microcontrollers are small size, minimum pin count and low cost controllers which are widely used for low end applications like LED & LCD display drivers ,portable battery chargers etc. Their power consumption is very low. The popular 4-bit controllers are Renasa M34501 which is a 20 pin DIP chip with 4kB of ROM,256 Bytes of RAM,2-Counters and 14 I/O Pins. Similarly ATAM862 series from ATMEL is also a 4-bit microcontroller.8-bit Microcontrollers : These are the most popular and widely used microcontrollers .Nearly 55% of the CPUs sold in the world market today are 8-bit microcontrollers only. The 8-bit microcontroller has 8-bit internal bus and the ALU performs all the arithmetic and logical operations on a byte instruction. The well known 8-bit microcontroller is 8051 which is designed by Intel in the year 1980 for the use in embedded systems. Other 8-bit microcontrollers are Intel 8031/8052 and Motorola MC68HC11 and AVR Microcontrollers, Microchips PIC microcontrollers 12C5XX ,16C5X and 16C505 etc.16-bit Microcontrollers : When the microcontroller performs 16-bit arithmetic and logical operations at an instruction, it is said to be a 16-bit microcontroller. The internal bus width of these microcontrollers is of 16-bit. These microcontrollers are having increased memory size and speed of operation when compared to 8-bit microcontrollers. They are most suitable for programming in High level languages like C or C++ .They find applications in disk drivers, modems, printers, scanners and servo motor control. Examples of 16-bit microcontrollers are Intel 8096 family and Motorola MC68HC12 and MC68332 families, The performance and computing capability of 16 bit microcontrollers are enhanced with greater precision as compared to the 8-bit microcontrollers.32-Bit Microcontrollers :These microcontrollers used in high-end applications like Automotive control, Communication networks, Robotics, Cell phones ,GPRS & PDAs etc..For example PIC32, ARM 7, ARM9 ,SHARP LH79520 , ATMEL 32 (AVR) ,TIs TMS 320F2802x/2803x etc .are some of the popular 32-bit microcontrollers.1.11 Overview of Intel 8051 MicrocontrollersThe 8051 microcontroller is a very popular 8-bit microcontroller introduced by Intel in the year 1981 and it has become almost the academic standard now a days. The 8051 is based on an 8-bit CISC core with Harvard architecture. Its 8-bit architecture is optimized for control applications with extensive Boolean processing. It is available as a 40-pin DIP chip and works at +5 Volts DC. The salient features of 8051 controller are given below. It has 4 KB on chip program memory (ROM or EPROM)) and 128 bytes of on chip data memory(RAM). It has an 8-bit data bus and 16-bit address bus for internal operations. 32 general purpose registers each of 8 bits Two -16 bit timers T0 and T1 Five Interrupts (3 internal and 2 external). Four Parallel ports each of 8-bits (PORT0, PORT1,PORT2,PORT3) with a total of 32 I/O lines. One 16-bit program counter and One 16-bit DPTR ( data pointer) One 8-bit stack pointer One Microsecond instruction cycle with 12 MHz Crystal. One full duplex serial communication port.1.12 Block Diagram and Architecture of 8051 MicrocontrollerThe architecture of the 8051 microcontroller chip can be understood from its block diagram. It has Harward architecture with RISC (Reduced Instruction Set Computer) concept. The block diagram of 8051 microcontroller is shown in fig1.9 below.It consists of an 8-bit ALU, one 8-bit PSW(Program Status Register), A and B registers , one 16-bit Program counter , one 16-bit Data pointer register(DPTR),128 bytes of RAM and 4kB of ROM and four parallel I/O ports each of 8-bit width.

Fig 1.9 Block Diagram of 8051 microcontroller8051 has 8-bit ALU which can perform all the 8-bit arithmetic and logical operations in one machine cycle. The ALU is associated with two registers A & BA and B Registers : The A and B registers are special function registers which hold the results of many arithmetic and logical operations of 8051.The A register is also called the Accumulator and as its name suggests, is used as a general register to accumulate the results of a large number of instructions. By default it is used for all mathematical operations and also data transfer operations between CPU and any external memory. The B register is mainly used for multiplication and division operations along with A register. MUL AB ; Multiply the contents of A and B, put the answer in AB DIV AB ; Divide A by B, put quotient in A and remainder in BIt has no other function other than as a location where data may be stored.The R registers: The "R" registers are a set of eight registers that are named R0, R1, etc. up to and including R7. These registers are used as auxillary registers in many operations. The "R" registers are also used to temporarily store values.1.13 Program Counter(PC) : 8051 has a 16-bit program counter .The program counter always points to the address of the next instruction to be executed. After execution of one instruction the program counter is incremented to point to the address of the next instruction to be executed.It is the contents of the PC that are placed on the address bus to find and fetch the desired instruction.Since the PC is 16-bit width ,8051 can access program addresses from 0000H to FFFFH ,a total of 6kB of code.1.14 Memory organization : The 8051 has two types of memory and these are Program Memory and Data Memory. Program Memory (ROM) is used to permanently save the program being executed, while Data Memory (RAM) is used for temporarily storing data and intermediate results created and used during the operation of the microcontroller. The 8051 microcontroller has 128 bytes of Data Memory (Internal RAM ) and 4kB of Program Memory(on chip ROM) .The RAM is also known as Data memory and the ROM is known as program memory. The program memory is also known as Code memory .This Code memory holds the actual 8051 program that is to be executed. In 8051 this memory is limited to 64K .Code memory may be found on-chip, as ROM or EPROM. It may also be stored completely off-chip in an external ROM or, more commonly, an external EPROM. The 8051 has only 128 bytes of Internal RAM but it supports 64kB of external RAM. As the name suggests, external RAM is any random access memory which is off-chip. Since the memory is off-chip it is not as flexible interms of accessing, and is also slower. For example, to increment an Internal RAM location by 1,it requires only 1 instruction and 1 instruction cycle but to increment a 1-byte value stored in External RAM requires 4 instructions and 7 instruction cycles. So, here the external memory is 7 times slower.1.14.1 Internal RAM OF 8051 :This Internal RAM is found on-chip on the 8051 .So it is the fastest RAM available, and it is also the most flexible in terms of reading, writing, and modifying its contents. Internal RAM is volatile, so when the 8051 is reset this memory is cleared. The 128 bytes of internal RAM is organized as below.(i) Four register banks (Bank0,Bank1, Bank2 and Bank3) each of 8-bits (total 32 bytes). By default always the s Bank 0 is selected. The remaining Banks are selected with the help of RS0 and RS1 bits of PSW Register.(ii) 16 bytes of bit addressable area and(iii) 80 bytes of general purpose area (Scratch pad memory) as shown in the diagram below. This area is also utilized by the microcontroller as a storage area for the operating stack.

Fig.1.10 Internal RAM OrganisationThe 32 bytes of RAM from address 00 H to 1FH are used as working registers organized as four banks .Each bank has 8 registers and they are named as R0-R7 .Each register can be addressed by its name or by its RAM address. For EX : MOV A, R5 or MOV A, 05H1.14.2 Internal ROM (On chip ROM): The 8051 microcontroller has 4kB of on chip ROM but it can be extended up to 64kB.This ROM is also called program memory or code memory. The CODE segment is accessed using the program counter (PC) for opcode fetches and by DPTR for data. The external ROM is accessed when the EA(active low) pin is connected to ground or the contents of program counter exceeds 0FFFH.When the Internal ROM address is exceeded the 8051 automatically fetches the code bytes from the external program memory.

1.15 Data Types and DirectivesGenerally while using a processor / microcomputer or Microcontroller the programmer should always know the data types supported by that device.There are different types of data like integer, character ,float ,signed char ,unsigned char ,short integer and long integer etc. Also the programmer should know the bit size supported by the device.For example 8051 microcontroller is an 8-bit controller .So care must be taken while using the data during the programming. So, a good understanding of C data types for 8051 help programmers to create smaller hex files. As 8051 is an 8-bit microcontroller, the character data type is the most natural choice for many applications. The unsigned char is an 8-bit data type takes a value in the range of 0 255 (00H FFH) and it is the most widely used data type for 8051. The 8051 supports un signed character data between 0 to 255 and signed character data between -128 to +127.This type of data can be used in situations like setting a counter value and ASCII characters, instead of signed char. It is important to specify the keyword unsigned in front of the char else compiler will use the signed char as the default. The unsigned int is a 16-bit data type that takes a value in the range of 0 to 65535 (0000 FFFFH). This is used to define 16-bit variables such as memory addresses, set counter values of more than 256. Since registers and memory accesses are in 8-bit chunks, the misuse of int variables will result in a larger hex file. However, for 8051 programming, do not use unsigned int in place where unsigned char will do the job. Also in situations where there is no need for signed data we should use unsigned int instead of signed int. The 8 bit keyword is a widely used 8051 C data types which is used to access single-bit addressable register. It allows access to the single bits of the SFR registers. It is a clear fact that some of the SFRs are bit addressable. Among the SFRs that are widely used are also bit addressable ports P0-P3.Directives are special instructions that provide information to the assembler but do not generate any code.They have no influence on the operation of the microcontroller. Some of them are obligatory part of every program while some are used only to facilitate or speed up the operation. Directives are written in the column reserved for instructions. There is a rule allowing only one directive per program line.They are messages to the assembler, nothing else.Under Data Directives DB is the most widely used directive.

DB (define byte)The DB directive is used to define the 8-bit data. When DB is used to define data, the numbers can be in decimal, binary, hex, or ASCII formats. For decimal, the D after the decimal number is optional, but using B (binary) and H (hexadecimal) for the others is required. Regardless of which is used, the assembler will convert the numbers into hex. To indicate ASCII, simply keep the characters in quotation marks ( ). The assembler will assign the ASCII code for the numbers or characters automatically. The DB directive is the only directive that can be used to define ASCII strings larger than two characters; therefore, it should be used for all ASCII data definitions.As an illustration let us consider the following assembly language code.

ORG 000HDATA1 : DB 27 ; Decimal ( 1B in Hex)DATA 2 : DB 01101101 B ; BINARY ( 6D in Hex)DATA 3 : DB 31H ; Hex NumberDATA 4 : DB 1589 ; ASCII NumberDATA 5 : DB My name is Murthy : ASCII Characters

Either single or double quotes can be used around ASCII strings. This can be useful for strings, which contain a single quote. DB is also used to allocate memory.

In addition to the data directives some directives like ORG ,EQU and END are also use in 8051 microcontroller assembly language programming. ORG (origin)The ORG directive is used to indicate the beginning of the address location. The number that comes after ORG can be either in hex or in decimal. If the number is not followed by H, it is decimal and the assembler will convert it to hex. Some assemblers use . ORG instead of ORG for the origin directive. EQU (equate)This directive is used to define a constant without occupying a memory location. The EQU directive does not set aside storage for a data item but associates a constant value with a data label so that when the label appears in the program, its constant value will be substituted for the label. The following example uses EQU for the counter constant and then the constant is used to load the R5 register.COUNT EQU 15.. ------------------ --------- MOV R5,# COUNT

While executing the instruction MOV R5, # COUNT, the register R5 will be loaded with the value 15 . The advantage of using this EQU directive is ,it becomes easy to change a constant value at different places in the program .Once the programmer uses this directive the assembler will automatically apply at all occurrences. i.e wherever it finds the word COUNT ,it is replaced with the data value 15END directiveEND directive is an important pseudo code that indicates to the assembler the end of the source (asm) file. The END directive is the last line of an 8051 program, meaning that in the source code anything after the END directive is ignored by the assembler. Some assemblers use . END ( dot before END) instead of simply END.

1.16 Flag bits and Program Status Register (PSW) : The 8051 has a 8-bit PSW register which is also known as Flag register. In the 8-bit register only 6-bits are used by 8051.The two unused bits are user definable bits.In the 6-bits four of them are conditional flags .They are Carry CY,Auxiliary Carry-AC, Parity-P,and Overflow-OV .These flag bits indicate some conditions that resulted after an instruction was executed.

The bits PSW3 and PSW4 are denoted as RS0 and RS1 and these bits are used th select the bank registers of the RAM location. The meaning of various bits of PSW register is shown below.CY PSW.7 Carry Flag.Set to 1 when there is a carry in any arithmetic operation.Else reset to 0AC PSW.6 Auxiliary Carry FlagFO PSW.5 Flag 0 available for general purpose .RS1 PSW.4 Register Bank select bit 1RS0 PSW.3 Register bank select bit 0OV PSW.2 Overflow flag--- PSW.1 User definable flagP PSW.0 Parity flag .set/cleared by hardware.The selection of the register Banks and their addresses are given below table 1.3. Table 1.3 Register banks and their addressesExample-1To explain the status of the flag bits let us consider an addition program shown below.And check the status of CY, AC and P flag after the addition.MOV A, #38HADD A, #2FH On execution 38 00111000 + 2F 00101111 --------------------------- 67 01100111After addition A=67H, CY=0CY = 0 since there is no carry beyond the D7 bitAC = 1 since there is a carry from the D3 to the D4 bitP = 1 since the accumulator has an odd number of 1s (it has five 1s)Example-2Let us consider the addition of 9CH and 64H in the following illustration

MOV A, # 9CHADD A, # 64H 9C 10011100 + 64 01100100 ------------------------------ After addition, 100 00000000 Here CY = 1 since there is a carry beyond the D7 bit , AC = 1 since there is a carry from the D3 to the D4 bit, P = 0 since the accumulator has an even number of 1s (it has zero 1s)

1.17 Stack Pointer Register (SP) : It is an 8-bit register which stores the address of the stack top. i.e the Stack Pointer is used to indicate where the next value to be removed from the stack should be taken from. When a value is pushed onto the stack, the 8051 first increments the value of SP and then stores the value at the resulting memory location. Similarly when a value is popped off the stack, the 8051 returns the value from the memory location indicated by SP, and then decrements the value of SP. Since the SP is only 8-bit wide it is incremented or decremented by two . SP is modified directly by the 8051 by six instructions: PUSH, POP, ACALL, LCALL, RET, and RETI. It is also used intrinsically whenever an interrupt is triggered.1.18 STACK in 8051 Microcontroller : The stack is a part of RAM used by the CPU to store information temporarily. This information may be either data or an address .The CPU needs this storage area as there are only limited number of registers. The register used to access the stack is called the Stack pointer which is an 8-bit register..So,it can take values of 00 to FF H.When the 8051 is powered up ,the SP register contains the value 07.i.e the RAM location value 08 is the first location being used for the stack by the 8051 controller There are two important instructions to handle this stack.One is the PUSH and the Other is the POP. The loading of data from CPU registers to the stack is done by PUSH and the loading of the contents of the stack back into aCPU register is done by POP.EX : MOV R6 ,#35 H MOV R1 ,#21 H PUSH 6 PUSH 1 In the above instructions the contents of the Registers R6 and R1 are moved to stack and they occupy the 08 and 09 locations of the stack.Now the contents of the SP are incremented by two and it is 0ASimilarly POP 3 instruction pops the contents of stack into R3 register.Now the contents of the SP is decremented by 1In 8051 the RAM locations 08 to 1F (24 bytes) can be used for the Stack.In any program if we need more than 24 bytes of stack ,we can change the SP point to RAM locations 30-7F H.this can be done with the instruction MOV SP,# XX.Data Pointer Register(DPTR) : It is a 16-bit register which is the only user-accessible. DPTR, as the name suggests, is used to point to data. It is used by a number of commands which allow the 8051 to access external memory. When the 8051 accesses external memory it will access external memory at the address indicated by DPTR. This DPTR can also be used as two 8-registers DPH and DPL.

During the programming this DPTR can be used similar to any other register.The following examples illustrates the usage of this register. MOV DPTR, #1576 H ; move the hex data 1576 into the DPTR register MOV DPL, #65H ; move the hex data 65 into lower byte of DPTR MOV A ,DPH ; The contents of higher byte of DPTR are copied to Accumulator

SPECIAL FUNCTION REGISTERS (SFRs) : In 8051 microcontroller there certain registers which uses the RAM addresses from 80h to FFh and they are meant for certain specific operations .These registers are called Special function registers (SFRs).Some of these registers are bit addressable also. The list of SFRs and their functional names are given below. In these SFRs some of them are related to I/O ports (P0,P1,P2 and P3) and some of them are meant for control operations (TCON,SCON, PCON..) and remaining are the auxillary SFRs, in the sense that they don't directly configure the 8051.S.NoSymbolName of SFRAddress (Hex)

1ACC*Accumulator0E0

2B*B-Register0F0

3PSW*Program Status word register0DO

4SPStack Pointer Register81

5DPTRDPL Data pointer low byte82

DPHData pointer high byte83

6P0*Port 080

P1*Port 190

8P2*Port 20A

9P3*Port 30B

10IP*Interrupt Priority control0B8

11IE*Interrupt Enable control0A8

12TMODTmier mode register89

13TCON*Timer control register88

14TH0Timer 0 Higher byte8C

15TL0Timer 0 Lower byte8A

16TH1Timer 1Higher byte8D

17TL1Timer 1 lower byte8B

18SCON*Serial control register98

19SBUFSerial buffer register99

20PCONPower control register87

The * indicates the bit addressable SFRs Table 1.4 SFRs of 8051 Microcontroller1.19 Pin Diagram of 8051 Microcontroller The 8051 microcontroller is available as a 40 pin DIP chip and it works at +5 volts DC. Among the 40 pins , a total of 32 pins are allotted for the four parallel ports P0,P1,P2 and P3 i.e each port occupies 8-pins .The remaining pins are VCC, GND, XTAL1, XTAL2, RST, EA ,PSEN. XTAL1,XTAL2: These two pins are connected to Quartz crystal oscillator which runs the on-chip oscillator. The quartz crystal oscillator is connected to the two pins along with a capacitor of 30pF as shown in the circuit. If a source other than the crystal oscillator is used , it will be connected to XTAL1 and XTAL2 is left unconnected.

Fig 1.11 Pin Diagram of 8051 MicrocontrollerRST: The RESET pin is an input pin and it is an active high pin. When a high pulse is applied to this pin the microcontroller will reset and terminate all activities. Upon reset all the registers except PC will reset to 0000 Value and PC register will reset to 0007 value. (External Access): This pin is an active low pin. This pin is connected to ground when microcontroller is accessing the program code stored in the external memory and connected to Vcc when it is accessing the program code in the on chip memory. This pin should not be left unconnected.(Program Store Enable) : This is an output pin which is active low. When the microcontroller is accessing the program code stored in the external ROM ,this pin is connected to the OE (Output Enable) pin of the ROM.ALE (Address latch enable): This is an output pin, which is active high. When connected to external memory , port 0 provides both address and data i.e address and data are multiplexed through port 0 .This ALE pin will demultiplex the address and data bus .When the pin is High , the AD bus will act as address bus otherwise the AD bus will act as Data bus.P0.0- P0.7(AD0-AD7) : The port 0 pins multiplexed with Address/data pins .If the microcontroller is accessing external memory these pins will act as address/data pins otherwise they are used for Port 0 pins.P2.0- P2.7(A8-A15) : The port2 pins are multiplexed with the higher order address pins .When the microcontroller is accessing external memory these pins provide the higher order address byte otherwise they act as Port 2 pins.P1.0- P1.7 :These 8-pins are dedicated for Port1 to perform input or output port operations.P3.0- P3.7 :These 8-pins are meant for Port3 operations and also for some control operations like Read,Write,Timer0,Timer1 ,INT0,INT1 ,RxD and TxD1.20 PORT ORGANISATION :The 8051 microcontroller has four parallel I/O ports , each of 8-bits .So, it provides the user 32 I/O lines for connecting the microcontroller to the peripherals. The four ports are P0 (Port 0), P1(Port1) ,P2(Port 2) and P3 (Port3). Upon reset all the ports are output ports. In order to make them input, all the ports must be set i.e a high bit must be sent to all the port pins. This is normally done by the instruction SETB.Ex: MOV A,#0FFH ; A = FF MOV P0,A ; make P0 an input portPORT 0:Port 0 is an 8-bit I/O port with dual purpose. If external memory is used, these port pins are used for the lower address byte address/data (AD0-AD7), otherwise all bits of the port are either input or output.. Unlike other ports, Port 0 is not provided with pull-up resistors internally ,so for PORT0 pull-up resistors of nearly 10k are to be connected externally as shown in the fig.1.12.Dual role of port 0: Port 0 can also be used as address/data bus(AD0-AD7), allowing it to be used for both address and data. When connecting the 8051 to an external memory, port 0 provides both address and data. The 8051 multiplexes address and data through port 0 to save the pins. ALE indicates whether P0 has address or data. When ALE = 0, it provides data D0-D7, and when ALE =1 it provides address and data with the help of a 74LS373 latch.

Fig 1.12 Port 1 with external pull-up resistors.Port 1: Port 1 occupies a total of 8 pins (pins 1 through 8). It has no dual application and acts only as input or output port. In contrast to port 0, this port does not need any pull-up resistors since pull-up resistors connected internally. Upon reset, Port 1 is configured as an output port. To configure it as an input port , port bits must be set i.e a high bit must be sent to all the port pins. This is normally done by the instruction SETB. For Ex : MOV A, #0FFH ; A=FF HEX MOV P1,A ; make P1 an input port by writing 1s to all of its pinsPort 2 : Port 2 is also an eight bit parallel port. (pins 21- 28). It can be used as input or output port. As this port is provided with internal pull-up resistors it does not need any external pull-up resistors. Upon reset, Port 2 is configured as an output port. If the port is to be used as input port, all the port bits must be made high by sending FF to the port. For ex, MOV A, #0FFH ; A=FF hex MOV P2, A ; make P2 an input port by writing all 1s to itDual role of port 2 : Port2 lines are also associated with the higher order address lines A8-A15. In systems based on the 8751, 8951, and DS5000, Port2 is used as simple I/O port.. But, in 8031-based systems, port 2 is used along with P0 to provide the 16-bit address for the external memory. Since an 8031 is capable of accessing 64K bytes of external memory, it needs a path for the 16 bits of the address. While P0 provides the lower 8 bits via A0-A7, it is the job of P2 to provide bits A8-A15 of the address. In other words, when 8031 is connected to external memory, Port 2 is used for the upper 8 bits of the 16 bit address, and it cannot be used for I/O operations.PORT 3 : Port3 is also an 8-bit parallel port with dual function.( pins 10 to 17). The port pins can be used for I/O operations as well as for control operations. The details of these additional operations are given below in the table. Port 3 also do not need any external pull-up resistors as they are provided internally similar to the case of Port2 & Port 1. Upon reset port 3 is configured as an output port . If the port is to be used as input port, all the port bits must be made high by sending FF to the port. For ex, MOV A, #0FFH ; A= FF hex MOV P3, A ; make P3 an input port by writing all 1s to itAlternate Functions of Port 3 : P3.0 and P3.1 are used for the RxD (Receive Data) and TxD (Transmit Data) serial communications signals. Bits P3.2 and P3.3 are meant for external interrupts. Bits P3.4 and P3.5 are used for Timers 0 and 1 and P3.6 and P3.7 are used to provide the write and read signals of external memories connected in 8031 based systems

S.No Port 3 bit Pin No Function

1P3.010RxD

2P3.111TxD

3P3.212

4P3.313

5P3.414T0

6P3.515T1

7P3.616

8P3.717

Table 1.5 Alternate functions of PORT 3 I/O Programming:I/O Programming mainly deals with the transfer of data between external device to microcontroller or vice versa through the ports. This concept of I/O programming also helps to understand the interfacing of peripheral devices to the microcontroller and parallel data transfer.8051 microcontroller has four I/O ports each of 8-bit.These ports can be accessed similar to any other register of the 8051 microcontroller. MOV P0 ,# 25H ; Move or copy the Hex data 25 into the Port0 ADD A,P1 ; Adds the contents of Port1 to the contents of Accumulator AAvery important feature of these ports is upon reset all the ports acts as output ports and to make them as input ports ,all the port pins must be made high. i.e a I or HIGH must be sent to each pin.This can be done by the following instruction. MOV P0 ,#OFFH ; All the port pins are made high.Or using the instruction SETB (ser bit) also a particular port pin can be made as out port.Ex: SETB P2.1 ; Makes the pin1 of port2 as out port.The following program continuously send out alternate values 55H and AAH to Port0BACK: MOV A,#55H ; copy the Hex value 55 to the Accumulator MOV P0, A ; send the hex value 55 to Port0 ACALL DELAY ; Call a delay program MOV A,#0AAH ; copy the Hex value AA to the Accumulator MOV P0,A ; send the hex value AA to Port0 ACALL DELAY ; Call a delay program SJMP BACK ; Go back to starting of the program.On execution ,the code sends alternately the values 55 and AA to Port 0.The following code will continuously send out to port 0 the alternating value 55H and AAH

MOV A,#55H ; copy the Hex value 55 to the Accumulator BACK: MOV P1,A ; end the hex value 55 to Port0

ACALL DELAY ; Call a delay program CPL A ; Complement A i.e A=AAH SJMP BACK ; repeat the program continuously.This program also sends the values 55H and AA H to the Port1 continuously .Bit Manipulation: Microcontroller always allows bit manipulations.i.e a single bit of a byte data can be either set (1) or reset (0) by using suitable instructions. This feature has many advantages in various real time applications. The instructions used to manipulate single bit data are given below. SETB ; bit Set the bit (bit = 1) CLR bit ; Clear the bit (bit = 0) CPL bit ; Complement the bit (bit = NOT bit) JB bit, target ; Jump to target if bit = 1 (jump if bit) JNB bit, target ; Jump to target if bit = 0 (jump if no bit) JBC bit, target ; Jump to target if bit = 1, clear bit(jump if bit, then clear)

Example: Generate a 50% duty cycle square wave on bit 1 of Port1 by using the bit manipulation instructions.The 50% duty cycle means that the on and off state (or the high and low portion of the pulse) have the same length. Therefore, we toggle P1.0 with a time delay in between each state.

HERE: SETB P1.1 ; set to high bit 1 of port 1 LCALL DELAY ; call the delay subroutine CLR P1.0 ; P1.1 = 0 LCALL DELAY ; call the delay subroutine SJMP HERE ; keep doing itThe same program can also implemented by usnin another bit manipulation instruction namely CPL HERE : CPL P11 ; set to high bit 1 of port 1 LCALL DELAY ; call the delay subroutine SJMP HERE ; keep doing continuously.8051P1.01.21Interrupts : An interrupt is an external or internal event that disturbs the microcontroller to inform it that a device needs its service. The program which is associated with the interrupt is called the interrupt service routine (ISR) or interrupt handler. Upon receiving the interrupt signal the Microcontroller , finish current instruction and saves the PC on stack. Jumps to a fixed location in memory depending on type of interrupt Starts to execute the interrupt service routine until RETI (return from interrupt)Upon executing the RETI the microcontroller returns to the place where it was interrupted. Get pop PC from stackThe 8051 microcontroller has FIVE interrupts in addition to Reset. They are Timer 0 overflow Interrupt Timer 1 overflow Interrupt External Interrupt 0(INT0) External Interrupt 1(INT1) Serial Port events (buffer full, buffer empty, etc) InterruptEach interrupt has a specific place in code memory where program execution (interrupt service routine) begins. External Interrupt 0: 0003 H Timer 0 overflow: 000B H External Interrupt 1:0013 H Timer 1 overflow: 001B H Serial Interrupt : 0023 HUpon reset all Interrupts are disabled & do not respond to the Microcontroller. These interrupts must be enabled by software in order for the Microcontroller to respond to them. This is done by an 8-bit register called Interrupt Enable Register (IE).Interrupt Enable Register :

EA : Global enable/disable. To enable the interrupts this bit must be set High. --- : Undefined-reserved for future use. ET2 : Enable /disable Timer 2 overflow interrupt. ES : Enable/disable Serial port interrupt. ET1 : Enable /disable Timer 1 overflow interrupt. EX1 : Enable/disable External interrupt1. ET0 : Enable /disable Timer 0 overflow interrupt. EX0 : Enable/disable External interrupt0Upon reset the interrupts have the following priority.(Top to down). The interrupt with the highest PRIORITY gets serviced first.1. External interrupt 0 (INT0)2. Timer interrupt0 (TF0)3. External interrupt 1 (INT1)4. Timer interrupt1 (TF1)5. Serial communication (RI+TI)Priority can also be set to high or low by 8-bit IP register.- Interrupt priority register

IP.7: reservedIP.6: reservedIP.5: Timer 2 interrupt priority bit (8052 only)IP.4: Serial port interrupt priority bitIP.3: Timer 1 interrupt priority bitIP.2: External interrupt 1 priority bitIP.1: Timser 0 interrupt priority bitIP.0: External interrupt 0 priority bit 1.20 Timer/Counter Modules in 8051 Microcontrollers : The 8051 microcontroller has two 16-bit timers Timer 0 (T0) and Timer 1(T1) which can be used either to generate accurate time delays or as event counters. These timers are accessed as two 8-bit registers TLO, THO & TL1 ,TH1 because the 8051 microcontroller has 8-bit architecture.TIMER 0 : The Timer 0 is a 16-bit register and can be treated as two 8-bit registers (TL0 & TH0) and these registers can be accessed similar to any other registers like A,B or R1,R2,R3 etcEx : The instruction Mov TL0,#07 moves the value 07 into lower byte of Timer0.Similarly Mov R5,TH0 saves the contents of TH0 in the R5 register.

TIMER 1 : The Timer 1 is also a 16-bit register and can be treated as two 8-bit registers (TL1 & TH1) and these registers can be accessed similar to any other registers like A,B or R1,R2,R3 etcEx : The instruction MOV TL1,#05 moves the value 05 into lower byte of Timer1.Similarly MOV R0,TH1 saves the contents of TH1 in the R0 register

TMOD Register : The various operating modes of both the timers T0 and T1 are set by an 8-bit register called TMOD register. In this TMOD register the lower 4-bits are meant for Timer 0 and the higher 4-bits are meant for Timer1.

GATE: This bit is used to start or stop the timers by hardware .When GATE= 1 ,the timers can be started / stopped by the external sources. When GATE= 0, the timers can be started or stopped by software instructions like SETB TR0 or SETB TR1C/T (clock/Timer) : This bit decides whether the timer is used as delay generator or event counter. When C/T = 0 ,the Timer is used as delay generator and if C/T=1 the timer is used as an event counter. The clock source for the time delay is the crystal frequency of 8051.M1,M0 (Mode) : These two bits are the timer mode bits. The timers of the 8051 can be configured in three modes.Mode0, Mode1 and Mode2.The selection and operation of the modes is shown below.S.No M0 M1 ModeOperation

100013-bit Timer mode8-bit Timer/counter. THx with TLx as 5-bit prescalar

201116-bit Timer mode.16-bit timer /counter without pre-scalar

31028-bit auto reload. THx contains a value that is to be loaded into TLx each time it overflows

4113Split timer mode