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Microprocessor and Microcontroller UNIT I 8085 MICROPROCESSOR 1 1 Introduction Microcomputer The term microcomputer is generally synonymous with personal computer or a computer that depends on a microprocessor Microcomputers are designed to be used by individuals whether in the form of PCs workstations or notebook computers A microcomputer contains a CPU on a microchip the microprocessor a memory system typically ROM and RAM a bus system and I O ports typically housed in a motherboard Microprocessor A silicon chip that contains a CPU In the world of personal computers the terms microprocessor and CPU are used interchangeably A microprocessor sometimes abbreviated P is a digital electronic component with miniaturized transistors on a single semiconductor integrated circuit IC One or more microprocessors typically serve as a central processing unit CPU in a computer system or handheld device Microprocessors made possible the advent of the microcomputer At the heart of all personal computers and most working stations sits a microprocessor Microprocessors also control the logic of almost all digital devices from clock radios to fuel injection systems for automobiles Three basic characteristics differentiate microprocessors Instruction set The set of instructions that the microprocessor can execute Bandwidth The number of bits processed in a single instruction Clock speed Given in megahertz MHz the clock speed determines how many instructions per second the processor can execute In both cases the higher the value the more powerful the CPU For example a 32 bit microprocessor that runs at 50MHz is more powerful than a 16 bit microprocessor that runs at 25MHz 1 Dr N Karuppiah Dr S Ravivarman In addition to bandwidth and clock speed microprocessors are classified as being either RISC reduced instruction set computer or CISC complex instruction set computer 1 2 8085 Microprocessor The Intel 8085 is an 8 bit microprocessor introduced by Intel in 1977 It was binary compatible with the more famous Intel 8080 but required less supporting hardware thus allowing simpler and less expensive microcomputer systems to be built The 5 in the model number came from the fact that the 8085 requires only a 5 Volt V power supply rather than the 5 V 5 V and 12 V supplies the 8080 needed The main features of 8085 P are It is an 8 bit microprocessor It is manufactured with N MOS technology It has 16 bit address bus and hence can address up to 216 65536 bytes 64KB memory locations through A0 A15 The first 8 lines of address bus and 8 lines of data bus are multiplexed AD0 AD7 Data bus is a group of 8 lines D0 D7 It supports external interrupt request A 16 bit program counter PC A 16 bit stack pointer SP Six 8 bit general purpose register arranged in pairs BC DE HL It requires a signal 5V power supply and operates at 3 2 MHZ single phase clock It is enclosed with 40 pins DIP Dual in line package 1 3 8085 Architecture own functions The various units of a microprocessor are listed below 8085 consists of various units as shown in Fig 1 and each unit performs its Accumulator Arithmetic and logic Unit General purpose register Program counter Stack pointer Temporary register Flags Instruction register and Decoder 2 Microprocessor and Microcontroller Timing and Control unit Interrupt control Address buffer and Address Data buffer Address bus and Data bus Accumulator Accumulator is nothing but a register which can hold 8 bit data Accumulator aids in storing two quantities The data to be processed by arithmetic and logic unit is stored in accumulator It also stores the result of the operation carried out by the Arithmetic and Logic unit The accumulator is also called an 8 bit register The accumulator is connected to Internal Data bus and ALU arithmetic and logic unit The accumulator can be used to send or receive data from the Internal Data bus Arithmetic and Logic Unit There is always a need to perform arithmetic operations like and to perform logical operations like AND OR NOT etc So there is a necessity for creating a separate unit which can perform such types of operations These operations are performed by the Arithmetic and Logic Unit ALU ALU performs these operations on 8 bit data But these operations cannot be performed unless we have an input or data on which the desired operation is to be performed So from where do these inputs reach the ALU For this purpose accumulator is used ALU gets its Input from accumulator and temporary register After processing the necessary operations the result is stored back in accumulator General Purpose Registers Apart from accumulator 8085 consists of six special types of registers called General Purpose Registers These general purpose registers are used to hold data like any other registers The general purpose registers in 8085 processors are B C D E H and L Each register can hold 8 bit data Apart from the above function these registers can also be used to work in pairs to hold 16 bit data They can work in pairs such as B C D E and H L to store 16 bit data The H L pair works as a memory pointer A memory pointer holds the address of a particular memory location They can store 16 bit address as they work in pair 3 Dr N Karuppiah Dr S Ravivarman Fig 1 1 8085 Architecture Program Counter and Stack Pointer Program counter is a special purpose register Consider that an instruction is being executed by processor As soon as the ALU finished executing the instruction the processor looks for the next instruction to be executed So there is a necessity for holding the address of the next instruction to be executed in order to save time This is taken care by the program counter A program counter stores the address of the next instruction to be executed In other words the program counter keeps track of the memory address of the instructions that are being executed by the microprocessor and the memory address of the next instruction that is going to be executed Microprocessor increments the program whenever an instruction is being executed so that the program counter points to the memory address of the next instruction that is going to be executed Program counter is a 16 bit register stack is nothing but the portion of RAM Random access memory Stack pointer is also a 16 bit register which is used as a memory pointer A So does that mean the stack pointer points to portion of RAM 4 Microprocessor and Microcontroller Yes Stack pointer maintains the address of the last byte that is entered into

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