WILLINGDON COLLEGE SANGLI

ELECTRONICS (B. Sc.-II)

Digital Electronics

Moodle

2

E L E C T R O N I C S

8085 Microprocessor and Advanced Microprocessors

Moodle developed

By

Dr. S. R. Kumbhar

Department of Electronics

Willingdon College Sangli

For B. Sc. II Electronics, & Physics Students

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CHAPTER - I

INTRODUCTION TO MICROPROCESSOR AND

ORGANIZATION OF 8085

The evolution of microprocessor generation wise is explained as follows:

First Generation:

The first microprocessor introduced in the market was INTEL 4004, a 4 bit in 1971 by scientist

Faggin. This microprocessor was designed to use in calculators. It was not powerful and inadequate

for general purpose computing. Its successor is 4040 microprocessor.

In 1972 Intel introduced the 8008, the first 8-bit microprocessor with the development of LSI

technology with 45 instructions. Intel introduced its successor i.e. 8080. Simultaneously Intel’s

competitions inspired by the early design of 8080 introduced new microprocessors. Motorola

introduced 6800 microprocessor.

Second Generation:

In 1974 the successors to 8080 and 6800 were introduced. The Z80 and 8085 from INTEL,

6809 from motorola. In 1976 8085 was introduced which was 8-bit microprocessor. The

development of microprocessors has been directed towards a complete microcomputer system

with CPU, ROM, RAM, clock, I/O ports, all in single package.

Third Generation :

In 1978 Intel introduced its high performance 16 bit microprocessor the 8086 now called iPAX

86. Its memory was 64 KB and number of I/O ports has grown up to 256 along with full

arithmetic execution. The 8086 was followed by Z8000 in 1979, Motorola 68000 in 1980, all these

were of 16 bit microprocessors. The other microprocessor of this generation is 80286

Fourth Generation :

In 1981 first 32 bit microprocessor introduced which was known as 80386. It can address

physical memory up to 4 GB. Other Examples are HP- 32 and 80486. Further the all Pentium

series has been grouped in the fourth generation or sometimes Pentium series are called

generation five.

What is microprocessor?

Definition of microprocessor:

The microprocessor is programmable logic device that has computing and decision-making ability

similar to that of central processing unit (CPU) of the computer. Its physical size is very small so the name given

microprocessor .

The microprocessor is a multipurpose programmable logic device that reads binary instructions from a

storage device called a memory, accepts as input data and process data according to those instructions and

provides results as output. The microprocessor is programmable in fact it can be instructed to perform given

tasks within its capability.

Generation wise Evolution of Microprocessor

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Microprocessor is semiconductor device manufactured by using the LSI or VLSI technique. It includes

ALU, register arrays and control circuits on the single chip. Microprocessor is an electronic device which is

useful only when interfaced with memories and several other input and output devices.

CPU

Block diagram of microprocessor

Microprocessor is semiconductor device manufactured by using the LSI or VLSI technique. It includes

ALU, register arrays and control circuits on the single chip. The microprocessor is capable of performing

various computing functions and making decisions to change the sequence of program execution. In large

computers CPU is implemented on one or two circuit boards to perform these computing functions. The

microprocessor can be divided into three segments. Microprocessor is the heart of the microcomputer, which

contains the following main blocks.

1) Arithmetic logic unit 2) Control unit 3) Memory unit or register array

Details of the microprocessor blocks

1) Arithmetic Logic Unit (ALU): The arithmetic and logic unit is capable of performing arithmetic operations such as addition, subtraction, multiplication, division as well as logical operations such as AND, OR, NOT, etc. The results are stored either in memory or register.

2) Control unit : The control unit provides the necessary timing and control signals to all the operations in the microcomputer. It controls the flow of data between the microprocessor and memory peripherals.

3) Register array : This area of the microprocessor consists of various registers. These registers are primarily used to store data temporarily during execution of the program. Some of the registers are accessible to the user through instructions. Some of the common storage devices are IC memories, Magnetic tape, magnetic disks etc.

Microcomputer mainly contains following blocks:

1) Input device (keyboard) 2) Microprocessor unit (MPU)

3) Program memory (ROM) 4) Data memory (RAM)

5) Output devices (display)

1) Input device (keyboard) :

The instructions as well as data prepared for particular program are entered through input devices

like keyboard.

2) Microprocessor unit (MPU) :

1) MPU process system data and required control signals are generated to control the system.

2) All processing and data flow is done in the system with MPU chip.

3) Program memory (ROM)

ALU

Control Unit

Register array

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1) It contains permanently stored program known as monitor program.

2) It has address bus, chip select and read signal line.

4) Data memory (RAM)

1) It is used to store data. It is a temporary storage device.

2) Reading and writing of data in memory, so bi-directional bus is required.

5) Output devices (display)

stored data is displayed on the seven segment as well as screen.

Address bus Data bus Control bus

Microprocessor based system with architecture

This system includes the microprocessor, input, output and memory units. The

components are organized along the common path called a bus. The entire group of

components is also referred to as a system or a microcomputer system. Microprocessor is one

component of the microcomputer. Microcomputer is one complete computer system similar to

other computer system. The input and output devices are called peripheral devices. The various

components of the microcomputer system is shown in the following figure.

Keyboard

Keyboard Interface

MPU

Display Interface

Address decoder

Power supply connected to all the devices

Display

Clock

RAM

ROM

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CPU System bus

Memory

1) INPUT: The input section transfers data and instructions in binary from the outside world to the

microprocessor. It includes such devices such as keyboard, teletype and analog to digital converters.

The input devices convert information into suitable binary form ( which the computer understand)

and send it to the processing unit. Some other input devices are scanner, floppy drive, magnetic tape,

mouse, etc. The input devices like mouse and joystic do not require typing. They allow user to select

one or more items on the screen.

2) OUTPUT : Output unit of computer is that part which returns the information to the person in

desired form. Most commonly used output devices are printer, plotter, visual display, CRT, etc. The

output section transfers data from the microprocessor to the output devices.

3) SYSTEM BUS : The system bus is communication path between the microprocessor and

peripherals. System bus is nothing but a group of wires used to carry bit of information from one

device to other. In fact there are several buses in the system. All the peripherals share the same bus.

However the microprocessor communicate the device one at a time. The timing is provided by the

control unit of the microprocessor.

4) MEMORY : The function of the memory is to store the information. The information may be

program, data or result. It is like a page of the notebook. There is page for a fixed number of binary

numbers on each line. However, these pages are made up of semiconductor material. Typically each

line has space for eight binary bits. There is 8-bit register that can store eight binary bits. Thus the

memory is nothing but several registers arranged in sequence. These registers are always grouped

together in powers of two. For example group of 210 i.e. 1024 8-bits registers on a semiconductor

chip is known as 1 K byte. Suppose a microcomputer has a 32 K memory. It means that 32768

memory locations are available and each location is able to store 1 byte. These different memory

locations are identified by hexadecimal numbers called address.

Types of memories

There are two types of memories

a) semiconductor memories

b) Magnetic memories

ALU

Control Unit

Register array

Input Output

ROM R/WM

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Semiconductor memories are faster, lighter and consumes less power while magnetic memories are

slower but much cheaper than semiconductor memories.

Memory

Semiconductor memory Magnetic memory

RAM ROM Floppy disk Hard disk Tape based

PROM

Static Dynamic EPROM

EEPROM

1) RAM: It is a read / write memory, as the information can be read and written into it during normal

operation. In a random access memory any memory location can be accessed in a random fashion

without regard to any other location. The access time is same for each location. RAM is volatile

memory.

a) Static RAM: A static RAM is essentially an array of flip-flops one per bit. Data written into it is

stored as long as power is maintained. Static RAM is asynchronous means uncloaked during read

and write operation. Data is transferred without a clock signal. Static RAM is costlier and

consumes more power, they do not need refreshing circuits. They have fast speed.

b) Dynamic RAM: A dynamic RAM provides temporary data storage using simplified small

storage cell which required data refreshing to maintain the information. It looses its information

within short time even the power is on. They needs to refresh during the process.

2) (ROM (Read Only Memory): ROM means Read only memory. It is non-volatile memory. It has

also random access property. It is used for permanent storage. The contents of the ROM is decided

by the manufacturer. These contents are permanently stored in the ROM at the time of

manufacturing. The user can’t write in a ROM .

3) PROM: PROM is a programmable ROM. The contents of the PROM are decided by the user. The

user can write permanent programs in PROM. But once the PROM is written the contents can’t be

changed forever.

4) EPROM (Erasable programmable Read only Memory): An EPROM is erasable PROM similar

to PROM, the contents of the EPROM are decided by the user. Here also the contents are

permanent but one can change the contents of EPROM by erasing it using high intensity short wave

ultraviolet light. The process of changing the content are not convenient because whole memory chip

is need to remove from the circuit.

5) EEPROM ( Electrically Erasable ) : This is similar to EPROM with difference that erasing is made

instead of ultraviolet light. The erasing is very simple.

Buses in the microcomputer

Address Bus: The address bus is group of 16 lines generally identified as A0 to A15. The address bus is

unidirectional. Bits flow in one direction only, from MPU to peripheral devices. The MPU uses the

address bus to perform the first function to identify a peripheral or a memory location. In computer

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system, each peripheral or memory location is identified by a binary number called address and the

address is the bus used to carry a 16-bit address. This is similar to postal address of a house. The number

of address lines of MPU determines the capacity to identify the different memory locations. The 8085

MPU with its 16 address lines is capable of addressing 216 = 65,536 (generally known as 64 K) memory

locations.

Address bus

Data bus

Control Bus

Data Bus: Data bus is group of 8 lines used for data flow. These lines are bi-directional. Data flow in

both directions between the MPU and peripheral devices. The MPU uses the data bus to perform second

function to transfer data. The eight data lines enable the MPU to manipulate 8-bit data ranging from 00

to FF (28 = 256 numbers). The largest number that can appear on the data bus is 1111 1111. The data

bus influences the microprocessor architecture considerably. It determines the word length and the

register size of a microprocessor. Thus the 8085 microprocessor is called an 8-bit microprocessor.

Control Bus: The control bus is comprised of various single lines that array synchronizing signals. The

MPU uses such lines to perform the third function to provide the timing and control signals. The term

Bus is related to control signals is somewhat confusing. These are not group of line like address or data

buses. But individual lines that that provides a pulse to indicate an MPU operation. The MPU generates

specific control signals for every operation (such as memory read or I/O Write) it performs. These

signals are used to identify a device type with which the MPU intends to communicate.

8085 microprocessor

A microprocessor is a central processing unit (CPU) of a microcomputer. It is the heart of the

microcomputer. The Intel 8085 is most popular 8-bit microprocessor in India. The Intel Corporation has

also developed a large number of general purpose and special purpose peripheral devices. These devices

are very valuable for the development of the microprocessor-based systems. The availability of the variety

of supporting systems is also important reason of popularity of Intel 8085 microprocessor.

Intel 8085 is an 8-bit microprocessor. It is 40 pin IC package fabricated on a single LSI chip. It

uses a single + 5 Vdc supply for the operation. It has 80 basic instructions and 246 opcodes. The fig

below shows the Block diagram of the microprocessor architecture. It consists of the following blocks.

1) Accumulator 2) Temporary register 3) Arithmetic and logic units.

4) General purpose registers 5) Instruction registers. 6) Flag registers.

7) Timing and control unit. 8) Program counter (PC) 9) Instruction decoder.

10) Stack pointer (SP). 11) Address buffer. 12) Address/Data buffer

13) Serial input / output 14) Interrupt control.

Accumulator:

A15 A0 MPU D7 D0

Memory Input Output

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Accumulator is 8-bit register that accumulates the data. In other words it stores intermediate answers during the

computer run operations. It is connected to 8-bit internal data bus. The bi-directional arrow between the accumulator and bus

indicates that accumulator can send and receive data. The accumulator is part of arithmetic and logic unit. This register is used

to store 8-bit data and to perform arithmetic and logic operations. The result of the operation is stored in the accumulator. It is

most busiest resistor.

1) Temporary register: One input to CPU is from accumulator and other is from the temporary register.

This is 8-bit register that stores the operands of arithmetic logic operations. During the Execution of

the arithmetic or logic instruction, operand is copied temporally in this register.

2) Arithmetic and logic units (ALU) : The arithmetic and logic unit perform the following arithmetic and

logic operations;

1) Addition , Subtraction

2) Logic AND, OR, NOR, NOT, EXCLUSIVE OR

3) Increment, Decrement

4) Left shift, right shift

5) Clear

3) General purpose registers: (B, C, D, E, H and L registers)

General purpose registers is prominent part of microprocessor. The microprocessor registers can

temporarily store a word of data. Some registers are special purpose registers. The general purpose

registers are shown in fig. shown below.

The GPR’s are Reg B, Reg C, Reg D, Reg E, GPR are available for any use in programmers

imagination

Architecture of 8085 microprocessor

Accumulator A

Status flags

Reg. B Reg. C

Reg. D Reg. E

Reg. H Reg. L

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The registers are small memories within the CPU. They are used by the microprocessor for

temporary storage and manipulation of data and instructions. Data remain in the registers till they are

sent to the memory or input output devices. The are six 8-bit registers which are used for general

purpose as desired by the programmer. The 8-bit registers are B, C, D, E, H and L. These registers

can handle 8-bit data but to handle 16 bit data two eight bit registers can act in pair. The combination

of two 8-bit registers is called a register pair or extended registers. The valid register pair in Intel 8085

and B-C, D-E and H-L pairs. B-C and D-E pairs are used to handle data where as H-L pair is used

to address the memories.

4) Instruction register.: Instruction register is an 8-bit register. Instructions of microprocessor will be

discussed later consists of some operation code (opcode). This code is 8-bit. Thus 8-bit code can

accommodate 256 instructions. These opcodes of the instruction are stored in the instruction register

before being decoded and before instruction being executed.

5) Flag registers: The status resister

Status resister makes the difference between a simple calculator and computer. It is used to store

certain results of tests performed during the execution of program. Status resister is also called flag

resister. There is a set of five registers which acts as a status flag. Each of these registers hold one bit that

indicate certain condition which arise during the arithmetic and logic operations. The following status flag

have been provided in Intel 8085

1) Sign (S) flag: Sign flag is set to 1 in the result of arithmetic OR logic operation is negative. This

negative result is indicated by most significant bit (MSB). When result is negative MSB is 1 and

when result is positive. MSB is 0. Thus when result is positive, sign flag contains zero i.e. it

resets.

2) Zero (Z) flag: The zero status flag is set to 1 if the result of arithmetic OR logic operation is

zero. For non zero result it is reset to zero.

3) Carry (CY) flag: The carry status flag hold carry resulting from the execution of arithmetic

operation. If there is carry from the addition or borrow from subtraction. Carry flag is set to 1

otherwise 0.

7 6 5 4 3 2 1 0 Bit No.

Undefined bit

The status register length depends on the number of status bits used by a particular

microprocessor. Fig. below shows the status work of a typical microprocessor.

4) Auxiliary carry (AY) flag: The auxiliary carry flag holds carry from bit 4 to 5 resulting from the

execution of an arithmetic operation. The flag is used only internally for BCD operations and is

not available for the programmer.

5) Parity (P) flag: After an arithmetic or logic operation, if the result has an even number of 1’s the

flag is set. If it has odd number of 1’s the flag is reset to 0.

7) Timing and control unit. The timing and control unit synchronizes all the microprocessor

operations with the clock. It also generates the control signals necessary for the communication

S Z X AC X P X CY

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between the microprocessor and peripherals. Thus it generates timing and control signals which are

necessary for the execution of instructions. As this unit controls the entire operations of the

microprocessor and peripherals connected to it. This unit acts as brain of the computer.

8) Program counter (PC): PC is one of the most impotent resister in the microprocessor. Program

counter is a 16-bit register. It sequences the execution of instructions. This register acts as memory

pointer. As memory locations have a 16-bit address, so this register is also a 16-bit register. The

function of program counter is to point to the memory address from which the next byte is to be

fetched. When the instruction is being fetched the program counter is incremented by one to the

point to the next memory location. Thus the program counter is like some one pointing a finger at a

list of instructions, saying do this first, do this second, do this third, etc. This is why program counter

is also called pointer.

9) Instruction decoder: Instruction to the microprocessor is in machine code or opcode. Instruction

register holds this machine code and feed to the instruction decoder, where instruction is decoded

and executed.

10) Stack pointer (SP) : This important register is always used by a programmer. The stack pointer is a 16-bit register. This

register controls the portion of memory known as stack.

The stack is a portion of memory set aside primarily for saving return address. The

saving address is required for subroutines. The program can locate the stack anywhere in the

memory, but once they have set up a stack they no longer use the portion of memory for

program and data. Thus the stack becomes the special space in the memory used for starting the

return address for the subroutine calls. The programmer pushes information on to the stack in

order and pops information from the stack in reverse order. The last information stored is first

to take off. This kind of operation is called LIFO (Last in first out) operation like program

counter. Stack pointer automatically points to the next available location. The stack pointer

decrements after it is used. Stack operation is two byte operation. The stack pointer starts at a

memory location initially set by a programmer. This process is called stack pointer initialization.

If stack pointer is not initialized then it starts at a random memory location.

Instruction Register : The instruction register holds the instruction, which the microprocessor is currently

executing. The instruction register is loaded by starting the microprocessor on a fetch cycle called instruction

cycle. The instruction register is connected to the microprocessors internal data bus, which only receives the

data. The instruction register can not place data on the internal bus. Its output drives instruction decoder.

Initially instruction is fetched, then the program counter points to the next instruction in the memory and when

instruction is fetched a copy of it is taken from instruction memory location. This copy is placed on the

microprocessors internal data bus and carried to the instruction decoder. The length of instruction register

varies from one microprocessor to another.

Temporary data register : This is an 8-bit register that states the operands of arithmetic and logic operations.

During the execution of arithmetic or logic instruction operand is copied temporarily in this register. The

need of the temporary registers arises because the ALU has no data storage of its own. The ALU is

constructed entirely on combinational logic. The temporary registers are essential to the internal

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functioning of microprocessor. ALU’s temporary data registers can not be used by programmer so it do

not become a part of programming model.

Microprocessors internal Data Bus : ALU and all registers are connected on the common 8-bit data bus and

all internal microprocessor data transfer takes place on this bus. Internal bus is called data bus although the

control signal plays vital role of controlling the use of bus. This is a bi-directional communication bus. It

places data or receives data from the device and CPU. The internal data bus may be connected to an

external data bus when microprocessor needs to change data with external memory.

Address buffer and Address/Data buffer: At the bottom right are two buffer registers called the

address buffer and the address data buffer. The contents of the stack pointer or program counter can

be loaded into the address buffer and address data buffer. The output of these buffers then drives

the external address bus and address data bus. Memory and input output chips are connected these

buses. In this way CPU can send the address of desired data to the memory or input output chips.

The 8-bit internal data bus is also connected to address data buffer. The bi-directional arrow

indicates that the address data buffer can send or receive data from 8-bit internal data bus.

10) Serial input / output: Serial data is input to the microprocessors or though SID and serial data is

taken out at SOD.

11) Interrupt control: The interrupt requests are of two types, Maskable and Non-maskable interrupts.

A maskable interrupt can be ignored or delayed by the microprocessor if it performing a critical task.

However the processor has to respond a non maskable request immediately. The maskable interrupt

is somewhat like telephone that can be kept of the hook when one is not interested in receiving any

calls. The non-maskable interrupt is like smoke detector that requires immediate attention when it is

set off. The interrupt process allows the microprocessor to respond to these external requests for

attention or serves on the demand basis and leaves the microprocessor free to perform other tasks.

1) The 8085 interrupt process is controlled by the interrupt enable flip-flop, which is internal to the

processor and can be set or reset by using software instructions. If the input of INTR goes high

the microprocessor is interrupted. This is maskable interrupt and can be disabled.

2) There are three RESET instructions namely RST 7.5, RST 6.5 and RST 5.5. These are maskable

interrupts are enable under program control with two instructions. EI (enable Interrupt) and

SIM ( Set Interrupt Mask)

3) TRAP: Trap is a non-maskable interrupt known as NMI and is analogous to smoke detector. It

has highest priorities among the interrupt signals. It needn’t to be enabled an it can’t be disabled.

It is high level and edge sensitive meaning that the input should go high and stay high to

acknowledged. It can’t be acknowledged until it makes a transition from high to low or low to

high

TRAP Highest priority

RST 7.5 2nd highest priority

RST 6.5

RST 5.5

INTR Lowest priority

Buffer : Buffer is logic circuit that amplifies the current or power. It has one input line one output line. The logic level is same as that of the output. Logic 1 input provides logic 1 output ( opposite of inverter).

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The buffer is primarily used to increase the driving capability of the logic circuit. It is known as driver. Fig. below shows the typical example of tri-state buffer. It is also known as line driver or line receiver. This device is commonly used for driving address bus.

1.1 Buffer Tri state Buffer

Define the following:

1) Instruction cycle : An instruction cycle is defined as the time required to complete the execution of the instruction. The 8085 instruction cycle consists of one to five machine cycles.

2) Machine cycle : Machine cycle is defined as the time required to complete any operation of accessing either memory or I/O which is the subpart of an instruction. In 8085, the machine cycle may consists of three to 6 T states. 4) T State : The subdivision of an operation, which is performed in one clock period is called as T-

state.

Clock T T1 T2 T3 T1 T2 T3 T4

Machine Cycle 1 Machine cycle 2 Instruction Cycle

Pin 1 an 2 (X1, X2) :

These terminals are to connected to external crystal oscillator which drives the internal circuitry

of the microprocessor to produce a suitable clock for the operation of the microprocessor. The 8085 has

on chip oscillator with all required circuitry except for crystal L.c. tank or RC network that controls the

frequency. The crystal oscillator can provide the frequency to the microprocessor. The frequency of the

oscillator may range from 3 MHz to 5 MHz. This frequency is used to generate a clock for driving the

various peripheral devices.

Pin 3 (RESET OUT) :

This pin carries the REST OUT signal 1 when high it indicated that CPU is being reset, i.e.

program counter, instruction register and son are being reset to zero. The RESET OUT signal goes to

peripheral chips. When you first power up the whole system includes the 8085 and peripheral chips are

reset or initialized. After the RESET OUT goes low the processing begins.

Pin 4 and 5 (SOD and SID):

SOD stands for serial out. The data in the accumulator is converted into serial form using some

special instructions. This serial data comes out if pin 4 which can be connected to serial output device.

But accumulator accepts parallel data and if the peripheral device output data is serial than it is accepted

by pin number 5 as serial input data.

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Pin Out Diagram Of 8085

Pin 6 to 11 ( Interrupts control) :

These pins are part of control unit. The 8085 has 5 interrupts for interrupts requests. A priority

exists among the interrupt pins. Some are more important than others. The order of priority of interrupts

is as follows.

TRAP (highest priority)

RST 7.5

RST 6.5

RST 5.5

INTR (lowest priority)

If two or more interrupts goes high at the same time, the 8085 will service then in order of their

importance. Pin 11 however is an output pin with a signal called an interrupt acknowledge (INTA). It is

interrupt acknowledge sent by the microprocessor when INTR is received.

Pin 12 to 19 ( AD0 to AD7):

These pins carry a lower 8 address bits or 8 data bits. The lower half of the address bus is

multiplexed with the data bus to keep the pin count at 40. Initially these 8 lines will carry the lower

address and then data flows after the address is carried by the address bus. These lines are multiplexed

because it reduces the number of pins as well as the space. After receiving the ALE signal the data lines

take over the control. The same bus is used for both address as well as data. These lines are bi-directional.

1 40

2 39

3 38

4 37

5 36

6 35

7 34

8 33

9 8085 32

10 31

11 30

12 29

13 28

14 27

15 26

16 25

17 24

18 23

19 22

20 21

X1

X2

RESET OUT

SOD

SID

TRAP

RST 7.5

RST 6.5

RST 5.5

INTR

INTA

AD0

AD1

A. AD2

AD3

AD4

AD5

AD6

B. AD7

C.

D. Vss

Vcc

HOLD

HLDA

CLK

RESET IN

READY

IO/M

S1

RD

WR

ALE

SO

A15

A14

A13

A12

A11

A10

A9

A8

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Pin 20 : It is ground pin

Pin 21 to 28 ( A8 – A15):

These pins carry the higher 8 address lines. These lines and unidirectional.

Pin 29 and 33 (SO and S1):

Pins 29 and 33 carry output signals known as status signals. Microprocessors sends these status

signals to distinguish the various types of operation given below

S0 S1 Operation

0 0 1 1

0 1 0 1

HALT WRITE READ

FETCH

Pin 30 (ALE) :

Each memory chip has its own memory address register (MAR) also called address latch). This

latch stores the incoming address from the address bus and address data bus. ALE stands for address

latch enable. The ALE signal comes out of pin 30 and goes to peripheral chips. The falling edge of the

ALE signal strobes(loads) the address on the address bus or address data bus into address latch of the

memory chips.

Pin 31 and 32 ( WR and RD)

The WR or RD determines whether a write or read is done. As these signals are active low. A

low WR means a write operation and low RD means a read operation. They are never low at the same

time. Only one process may happen at a time.

Pin 34 (IO/M):

A low IO/M indicates a memory operation and high IO/M means that on I/O instruction is

being executed. In other words a low IO/M signal enables the memory chip and a high IO/M enable the

I/O chips. Thus it is the status signal which distinguishes whether the address is for memory or I/O.

Pin 35 (READY) :

Some peripheral devices are slow . They are unable to run at a maximum speed of MPU. There is

one way to slow down the 8085 is with READY signal. This signal is issued by the microprocessor to

sense whether a peripheral device is ready to transfer

data or not. If the device is not ready, It will return a low READY signal to 8085. The microprocessor

waits till it goes high. When the peripheral device is ready, it will send a high READY signal to the

microprocessor. Then it can complete the data transfer.

Pin 36 ( RESET IN):

Pin 36 is an input carrying the RESET IN signal. The signal may come from the operator reset

button or other sources. When RESET IN is low, the CPU reset the program counter, instruction

register and other circuits. It also sends a high RESET OUT to pin 3. The CPU remain in the reset

condition until the RESET IN signal goes high. Then the data processing begins.

Pin 37 ( CLK): The CLK signal out of pin 37 is derived from the on chip oscillator. This is the system

clock available for the user. It can be used for other digital ICs or peripheral chips, so that it synchronizes

their timing.

Pins 38, 39 ( HLDA and HOLD):

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We can transfer the data from peripheral devices to microprocessor or vice versa using some

instructions. But in both cases accumulator is involved in receiving and sending the data. Data going

through the accumulator slows down the I/O transfer. In order to speed up the memory peripheral

transfer direct memory access (DMA) is used. The HOLD and HLDA signals are used in DMA

operations. With the DMA approach the large amount of data can be transferred in short time.

Pin 40 ( +Vcc): It is the power supply pin of 8085. It connects a source of + Vcc.

Questions

1. What is microprocessor ? Give evolutions of microprocessors.

a) HLDA b) READY c) RST 7.5

2. The invalid register pair for microprocessor is --------.

a) BC b) HL c) SP d) DE

3. Write a short note on evolution of microprocessor giving one example of each generation.

(M- 2002)

4. List any three primary functions of CPU of microcomputer.

5. Write the functions of the following units in the microprocessor.

a) ALU b) Stack pointer b) PC c) Instruction reg. and decoder.

6. Define the following terms with suitable example.

a) Instruction cycle b) Machine cycle c) T – State.

7. Describe in brief function of following pins in 8085 microprocessor

a) HOLD b) INTR c) RESETIN

8. The flag register of 8085 microprocessor contains the data 45 H interpret its meaning.

(M-2002)

9. Describe the following registers of 8085 microprocessor.

1) Program counter 2) Stack pointer 3) Accumulator

10. Draw the block diagram of microcomputer. Explain functions of each block in short.

(M-2003)

11. Explain the functions of the following pins

a) HOLD b) Reset In c) IO / M

12. What is flag register? Explain its use in ALU operations giving one example each.

13. In 8085 microprocessor, serial data from external device is received on ----- pin.

A) SID b) SOD c) HOLD d) READY

14. Explain the organization of ALU with simple block diagram.

15. Describe the following registers of 8085 microprocessor.

1) Program counter 2) Instruction register 3) Accumulator

16. Explain in brief the four primary functions of the CPU of the microcomputer.

17. Explain the sign and parity flags of 8085 with suitable example.

18. Explain the functions of the following pins.

a) HLDA b) Reset OUT c) IO / M