88043168 16 doc of automatic load sharing of transformers

7/27/2019 88043168 16 Doc of Automatic Load Sharing of Transformers

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AUTOMATIC LOAD SHARING OF TRANSFORMERS


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CONTENTS

S.No. Contents Page

1. Abbreviations

2. Abstract

3. Introduction

5. Block diagram

6. Block diagram Description

7. Schematic

8. Schematic Description (pin to pin connectivity)

9.

10.

Hardware components

a. Microcontroller (max 50 pages)b. MAX 232c. Power supplyd. ADCe. Current sensorf. LCDg. Relay

Circuit description

11. Software components

a. About Keilb. Embedded C

12. Source Code OUT SIDE

12. Conclusion or Synopsis

13. Bibliography


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ABBREVIATIONS

Symbol Name

ACC AccumulatorB B register

PSW Program status word

SP Stack pointer

DPTR Data pointer 2 bytes

DPL Low byte

DPH High byte

P0 Port0

P1 Port1

P2 Port2

P3 Port3

IP Interrupt priority control

IE Interrupt enable control

TMOD Timer/counter mode control

TCON Timer/counter control

T2CON Timer/counter 2 control

T2MOD Timer/counter mode2 control

TH0 Timer/counter 0high byte

TL0 Timer/counter 0 low byte

TH1 Timer/counter 1 high byte


TH2 Timer/counter 2 high byte


SCON Serial control

SBUF Serial data buffer

PCON Power control


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Figure Locations

S.No. Figure Page No.

1Components of Typical Linear PowerSupply

2 An Electrical Transformer

3 Bridge Rectifier

4 Bridge Rectifier Positive Cycle

5 Bridge Rectifier Negative Cycle

6 Three terminal voltage Regulator

7 Functional Diagram of Microcontroller

8 Pin Diagram of Microcontroller

9 Oscillator connections

10 External clock drive connections

11 A register12 B register

13 RAM

14 RAM Allocation

15 Register Banks

16 PSW

17 DPTR

18 SP

19 PORT 0

20 TL0 and TH0

21 DB9

22 Connecting Microcontroller to PC

23

Types of SIM Structures

24Smart Card Pin-out

25Smart Card Reader

26LCD

27MAX 232 Pin-out

28 MAX 232 Operating circuit

29MAX 232 Logic output

30Relay

31Project


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32New Project

33Select Target device

34Select device for Target

35Copy 8051 startup code

36Source group 1

37New file

38Opened new file

39File Save

40Add files to the source group

41 Adding files to the source group

42Compilation

43After Compilation

44Build

45Selecting the Ports to be visualized

46Start Debugging

47


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CHAPTER 1

1. INTRODUCTION

1.1 EMBEDDED SYSTEMS:

Embedded systems are designed to do some specific task, rather than be a

general-purpose computer for multiple tasks. Some also have real time performance

constraints that must be met, for reason such as safety and usability; others may have low

or no performance requirements, allowing the system hardware to be simplified to reduce

costs.

An embedded system is not always a separate block - very often it is physically

built-in to the device it is controlling. The software written for embedded systems is often

called firmware, and is stored in read-only memory or flash convector chips rather than a

disk drive. It often runs with limited computer hardware resources: small or no keyboard,

screen, and little memory.

Wireless communication has become an important feature for commercial productsand a popular research topic within the last ten years. There are now more mobile phone

subscriptions than wired-line subscriptions. Lately, one area of commercial interest has

been low-cost, low-power, and short-distance wireless communication used for \personal

wireless networks." Technology advancements are providing smaller and more cost

effective devices for integrating computational processing, wireless communication, and

a host of other functionalities. These embedded communications devices will be

integrated into applications ranging from homeland security to industry automation and

monitoring. They will also enable custom tailored engineering solutions, creating a

revolutionary way of disseminating and processing information. With new technologies

and devices come new business activities, and the need for employees in these

technological areas. Engineers who have knowledge of embedded systems and wireless

communications will be in high demand. Unfortunately, there are few adorable


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environments available for development and classroom use, so students often do not learn

about these technologies during hands-on lab exercises. The communication mediums

were twisted pair, optical fiber, infrared, and generally wireless radio.

INTRODUCTION TO THE PROJECT:

In this project AUTOMATIC LOAD SHARING OF TRANSFORMERS we

are using two transformers, one is main transformer (TF1) and the next is backup

transformer (TF2). Here the load is directly connected to the secondary of the main

transformer as well as backup transformer; here two transformers are connected through

the relay.

The transfers switch senses when utility power is interrupted, and starts up the

transformer TF2 which acts as a backup transformer. If the utility power remains absent,

the transfer switch disconnects the load from the utility and connects it to the

Transformer TF2, restoring electricity to the load. The transfer switch continues to

monitor utility power, and when it is restored, switches the load from the Transformer

TF2 back to the Main transformer TF1. Once the Transformer TF2 is disconnected, it

goes through a cool-down routine and is automatically shut down.

The objective of the present project is to satisfy the above needs with an extent.

The present system is designed around two transformers. One transformer (TF1) is used

as the main supply and the other transformer (TF2) is used in the place of the generator

(for demo purpose). These two transformers are connected with the relay which is

controlled by the embedded controller. The loads are connected to the main line (TF1)

and as well as to the TF2.

Initially TF1 is connected to the load, the loads run with this power. Due to any

reason this power is interrupted, then it is identified by the controller and it immediately

switches ON to the TF2 through the relay. The controller continuously monitors the TF1


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(main line). When it finds the power on it again switches the loads connection to the main

line. All the status of the transformers will be displayed on LCD.

BLOCK DIAGRAM:

BLOCK DIAGRAM EXPLANATION:

The above block diagramgives the overview of the project in the pictorial form

with the help of the block diagram we will create pre model of the project and the analyze

AT

89C51MICRO

CONTROLLER

POWER SUPPLY LCD DISPLAY

MainTransformer

(TF1)

Sharingtransformer

(TF2)LOAD

RELAYADC

CURRENT

SENSOR


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the function of the project the explanation of the project with block diagram over view is

given as follows.

Power Supply Section:

This section is meant for supplying Power to all the sections mentioned above.It

basically consists of a Transformer to step down the 230V ac to 18V ac followed by

diodes. Here diodes are used to rectify the ac to dc. After rectification the obtained

rippled dc is filtered using a capacitor Filter. A positive voltage regulator is used to

regulate the obtained dc voltage.

Microcontroller Section:

This section forms the control unit of the whole project. This section basically

consists of a Microcontroller with its associated circuitry like Crystal with capacitors,

Reset circuitry, Pull up resistors (if needed) and so on. The Microcontroller forms the

heart of the project because it controls the devices being interfaced and communicates

with the devices according to the program being written.

Transformers:

In general, the ac line voltage present in your house wiring is not suitable for

electronic circuits. Most circuits require a considerably lower voltage, while a few require

higher voltages. The transformer serves to convert the ac line voltage to a voltage level

more appropriate to the needs of the circuit to be powered. At the same time, the

transformer provides electrical isolation between the ac line and the circuit being

powered, which is an important safety consideration. However, a line transformer is

generally large and heavy, and is rather expensive. Therefore, some power supplies

(notably for PCs) are deliberately designed to operate directly from the ac line without a

line transformer. The output of the transformer is still an ac voltage, but now of an

appropriate magnitude for the circuit to be powered.

ADC:


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Analog to digital (A/D, ADC) converters are electrical circuit devices that convert

continuous signals, such as voltages or currents, from the analog domain to the digital

domain where the signals are represented by numbers.

LCD Display Section:

This section is basically meant to show up the status of the project. This

project makes use of Liquid Crystal Display to display / prompt for necessary

information.

Sensors:

This part of the system consists of current sensor. These sensor sense various

parameters of load- current and are then sent to the Analog to Digital Converter.

Relay:

In this project Relays are used to the Trip the transformer. A relay is an

electrical switch that opens and closes under control of another electrical circuit. In the

original form, the switch is operated by an electromagnet to open or close one or many

sets of contacts.


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SCHEMATIC:

SCHEMATIC EXPLANATION:

Firstly, the required operating voltage for Microcontroller 89C51 is 5V.

Hence the 5V D.C. power supply is needed by the same. This regulated 5V is generated

by first stepping down the 230V to 18V and 12 V operating voltage for relays. Hence

another supply is required to generate 12V.

The step downed a.c voltage is being rectified by the Bridge Rectifier. The

diodes used are 1N4007. The rectified a.c voltage is now filtered using a C filter. Now

the rectified, filtered D.C. voltage is fed to the Voltage Regulator. This voltage regulator

allows us to have a Regulated Voltage which is +5V. We are using two voltage regulators

i.e., 7805 and 7812. These voltage regulators regulate 5v for microcontroller and 12v for

relays. The rectified; filtered and regulated voltage is again filtered for ripples using an


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electrolytic capacitor 100F. Now the output from this section is fed to 40th

pin of 89c51

microcontroller to supply operating voltage.

The microcontroller 89C51 with Pull up resistors at Port0 and crystal

oscillator of 11.0592 MHz crystal in conjunction with couple of capacitors of is placed at

18th

& 19th

pins of 89c51 to make it work (execute) properly.

The LCD is interfaced to Microcontroller. The data pins of LCD are connected to

Port 0. The control pins of LCD are connected to Port 2 as shown in schematic.

One of the port 3 pin is connected to the secondary side of transformer one

through bridge rectifier and regulator to check the status of the transformer one i.e

whether the power supply is on or off to the transformer one through relay. Relay is used

to on & off the transformer occurred it is connected to P2.0 microcontroller.

The data pins of ADC are connected to the port 1 of microcontroller. What ever

the data in ADC from C.T (current transformer) will be converted from analog to digital

and fed it to the microcontroller.


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CHAPTER III

HARDWARE Components

The Hardware components used in this project are

Regulated Power Supply MicrocontrollerTransformers Relay ADC Current sensor LCD

3.1 MICROCONTROLLER 89C51


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INTRODUCTION:

A Micro controller consists of a powerful CPU tightly coupled with memory

RAM, ROM or EPROM), various I / O features such as Serial ports, Parallel Ports,

Timer/Counters, Interrupt Controller, Data Acquisition interfaces-Analog to Digital

Converter (ADC), Digital to Analog Converter (ADC), everything integrated onto a

single Silicon Chip.

It does not mean that any micro controller should have all the above said features

on chip, Depending on the need and area of application for which it is designed, The ON-CHIP features present in it may or may not include all the individual section said above.

Any microcomputer system requires memory to store a sequence of instructions

making up a program, parallel port or serial port for communicating with an external

system, timer / counter for control purposes like generating time delays, Baud rate for

the serial port, apart from the controlling unit called the Central Processing Unit.

Advantages of microcontrollers:

1. If a system is developed with a microprocessor, the designer has to go for externalmemory such as RAM, ROM or EPROM and peripherals and hence the size of the PCB

will be large enough to hold all the required peripherals. But, the micro controller has

got all these peripheral facilities on a single chip so development of a similar system with

a micro controller reduces PCB size and cost of the design.

One of the major differences between a micro controller and a microprocessor is

that a controller often deals with bits , not bytes as in the real world application, for


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example switch contacts can only be open or close, indicators should be lit or dark and

motors can be either turned on or off and so forth.

COMPARISON

What is the difference between a microprocessor and microcontroller?

By microprocessor is meant the general-purpose microprocessors such as Intels

x86 Family(8086,80286,80386,80486,and the Pentium)or Motorolas680x0

family(68000,68010,68020,68030,68040.etc.).These microprocessors contain no RAM,

no ROM, and no I/O PORTS on the chip itself. For this reason, they are commonly

referred as general-purpose microprocessors.

(A)General purpose Micro processor System

CPU RAM ROM

I/O TIMER Serial

RAM

CPU

General

purpose

Micro

Processor

ROMI/OPORT TIMER

SERIALCOM

PORT

Address bus

Data bus


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COM

PORT

(b)Microcontroller

A system designer using a general-purpose microprocessor such as the Pentium or

the 68040 must add RAM, ROM, I/O PORTS, and TIMERS externally to make them

functional. Although the addition of external RAM, ROM and I/O PORTS makes these

systems bulkier and much expensive, they have the advantage of versatility such that the

designer can decide on the amount of RAM, ROM and I/O PORTS needed to fit the task

at hand this is not the case with microcontrollers.

A microcontroller has a CPU (a microprocessor) in addition to a fixed amount of

RAM, ROM, I/OPORTS, and a timer all on a single chip. In other words, the processor,

RAM, ROM, I/OPORTS, and timer are all embedded together on one chip; therefore, the

designer cant add any external memory, I/O, or timer to it. The fixed amount of on-chip

ROM, RAM, and number of I/O PORTS in microcontrollers makes them ideal for many

applications in which cost and space are critical. In many applications, for example a TV

remote control, there is no need for the computing power of a 486 or even an 8086

microprocessor. In many applications, the space it takes, the power it consumes, and the

price per unit are much more critical considerations than the computing power. These

applications most often require some I/O operation to read signal and turn on and off

certain bits. For these reason some call these processors IBP (itty-bitty processors).

In 1981,Intel corporation introduced an 8 bit microcontroller called the 8051.This

microcontroller had 128 bytes of RAM,4K bytes of on-chip ROM, two timers, one serial

port, and 4 ports(each 8-bits wide)all on single chip. At that time it was also referred to as

a system on a chip.

The 8051 is an 8-bit processor, meaning that the CPU can work on only 8-bits of

data at a time. Data larger than 8-bits has to be broken into 8-bit pieces to be processed


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by the CPU. The 8051 can have a maximum of 64K bytes of ROM; many manufacturers

have put only 4Kbytes on chip.

A Micro controller consists of a powerful CPU tightly coupled with memory,

various I/O interfaces such as serial port, parallel port timer or counter, interrupt

controller, data acquisition interfaces-Analog to Digital converter, Digital to Analog

converter, integrated on to a single silicon chip. If a system is developed with a

microprocessor, the designer has to go for external memory such as RAM, ROM,

EPROM and peripherals. But controller is provided all these facilities on a single chip.

Development of a Micro controller reduces PCB size and cost of design. One of the

major differences between a Microprocessor and a Micro controller is that a controller

often deals with bits not bytes as in the real world application. Intel has introduced a

family of Micro controllers called the MCS-51.

The Major Features:

Compatible with MCS-51 products 4k Bytes of in-system Reprogrammable flash memory Fully static operation: 0HZ to 24MHZ Three level programmable clock 128 * 8bit timer/counters Six interrupt sources Programmable serial channel Low power idle power-down modes


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o Why AT 89C51? :

The system requirements and control specifications clearly rule out the use of 16,

32 or 64 bit micro controllers or microprocessors. Systems using these may be earlier to

implement due to large number of internal features. They are also faster and more reliable

but, the above application is satisfactorily served by 8-bit micro controller. Using an

inexpensive 8-bit Microcontroller will doom the 32-bit product failure in any competitive

market place.

Coming to the question of why to use AT89C51 of all the 8-bit microcontroller

available in the market the main answer would be because it has 4 Kb on chip flashmemory which is just sufficient for our application. The on-chip Flash ROM allows the

program memory to be reprogrammed in system or by conventional non-volatile memoryProgrammer. Moreover ATMEL is the leader in flash technology in todays market place

and hence using AT 89C51 is the optimal solution.

AT89C51 MICROCONTROLLER ARCHITECTURE

The 89C51 architecture consists of these specific features:

Eightbit CPU with registers A (the accumulator) and B Sixteen-bit program counter (PC) and data pointer (DPTR) Eight- bit stack pointer (PSW) Eight-bit stack pointer (Sp) Internal ROM or EPROM (8751) of 0(8031) to 4K (89C51) Internal RAM of 128 bytes:

1. Four register banks, each containing eight registers2. Sixteen bytes, which maybe addressed at the bit level3. Eighty bytes of general- purpose data memory

Thirtytwo input/output pins arranged as four 8-bit ports:p0-p3 Two 16-bit timer/counters: T0 and T1


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Full duplex serial data receiver/transmitter: SBUF Control registers: TCON, TMOD, SCON, PCON, IP, and IE Two external and three internal interrupts sources. Oscillator and clock circuits.

Functional block diagram of micro controller

The 89C51 oscillator and clock:

The heart of the 89C51 circuitry that generates the clock pulses by which all the

internal all internal operations are synchronized. Pins XTAL1 And XTAL2 is provided

for connecting a resonant network to form an oscillator. Typically a quartz crystal and

capacitors are employed. The crystal frequency is the basic internal clock frequency of

the microcontroller. The manufacturers make 89C51 designs that run at specific

minimum and maximum frequencies typically 1 to 16 MHz.


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Oscillator and timing circuit

Types of memory:

The 89C51 have three general types of memory. They are on-chip memory,

external Code memory and external Ram. On-Chip memory refers to physically existing

memory on the micro controller itself. External code memory is the code memory that

resides off chip. This is often in the form of an external EPROM. External RAM is the

Ram that resides off chip. This often is in the form of standard static RAM or flash

RAM.


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a) Code memory

Code memory is the memory that holds the actual 89C51 programs that is to be

run. This memory is limited to 64K. Code memory may be found on-chip or off-chip. It

is possible to have 4K of code memory on-chip and 60K off chip memory

simultaneously. If only off-chip memory is available then there can be 64K of off chip

ROM. This is controlled by pin provided as EA

b) Internal RAM

The 89C51 have a bank of 128 of internal RAM. The internal RAM is found on-

chip. So it is the fastest Ram available. And also it is most flexible in terms of reading

and writing. Internal Ram is volatile, so when 89C51 is reset, this memory is cleared. 128

bytes of internal memory are subdivided. The first 32 bytes are divided into 4 register

banks. Each bank contains 8 registers. Internal RAM also contains 128 bits, which are

addressed from 20h to 2Fh. These bits are bit addressed i.e. each individual bit of a byte

can be addressed by the user. They are numbered 00h to 7Fh. The user may make use of

these variables with commands such as SETB and CLR.

FLASH MEMORY:

Flash memory (sometimes called "flash RAM") is a type of constantly-powered

non volatile that can be erased and reprogrammed in units of memory called blocks. It is

a variation of electrically erasable programmable read-only memory (EEPROM) which,

unlike flash memory, is erased and rewritten at the byte level, which is slower than flash

memory updating. Flash memory is often used to hold control code such as the basic

input/output system (BIOS) in a personal computer. When BIOS needs to be changed

(rewritten), the flash memory can be written to in block (rather than byte) sizes, making it

easy to update. On the other hand, flash memory is not useful as random access memory

(RAM) because RAM needs to be addressable at the byte (not the block) level.

Flash memory gets its name because the microchip is organized so that a section

of memory cells are erased in a single action or "flash." The erasure is caused by Fowler-


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Nordheim tunneling in which electrons pierce through a thin dielectric material to remove

an electronic charge from afloating gate associated with each memory cell. Intel offers a

form of flash memory that holds two bits (rather than one) in each memory cell, thus

doubling the capacity of memory without a corresponding increase in price.

Flash memory is used in digital cellular phones, digital cameras, LAN switches,

PC Cards for notebook computers, digital set-up boxes, embedded controllers, and other

devices.

Memory Types


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Memory Type Features

FLASH Low-cost, high-density, high-speed

architecture; low power; high reliability

ROM

Read-Only Memory

Mature, high-density, reliable, low cost;

time-consuming mask required, suitable

for high production with stable code

SRAM

Static Random-Access Memory

Highest speed, high-power, low-density

memory; limited density drives up cost

EPROM

Electrically Programmable Read-Only

Memory

High-density memory; must be exposed

to ultraviolet light for erasure

EEPROM or E

Electrically Erasable Programmable

Read-Only Memory

Electrically byte-erasable; lower

reliability, higher cost, lowest density

DRAM

Dynamic Random Access Memory

High-density, low-cost, high-speed,

high-power

Technical Overview of Flash Memory

Flash memory is a nonvolatile memory using NOR technology, which allows the

user to electrically program and erase information. Intel Flash memory uses memory

cells similar to an EPROM, but with a much thinner, precisely grown oxide between the

floating gate and the source (see Figure 2). Flash programming occurs when electrons are

placed on the floating gate. The charge is stored on the floating gate, with the oxide layer


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allowing the cell to be electrically erased through the source. Intel Flash memory is an

extremely reliable nonvolatile memory architecture.

Pin diagram of AT89C51

Pin Description:

VCC: Supply voltage.

GND: Ground.


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Port 0:

Port 0 is an 8-bit open-drain bi-directional I/O port. As an output port, each pin

can sink eight TTL inputs. When 1s are written to port 0 pins, the pins can be used as

high impedance inputs. Port 0 may also be configured to be the multiplexed low order

address/data bus during accesses to external program and data memory. In this mode P0

has internal pull-ups. Port 0 also receives the code bytes during Flash programming, and

outputs the code bytes during program verification. External pull-ups are required during

program verification.

Port 1:

Port 1 is an 8-bit bi-directional I/O port with internal pull-ups. The Port 1 output

buffers can sink/source four TTL inputs. When 1s are written to Port 1 pins they are

pulled high by the internal pull-ups and can be used as inputs. As inputs, Port 1 pins that

are externally being pulled low will source current (IIL) because of the internal pull-ups.

Port 1 also receives the low-order address bytes during Flash programming and

verification.

Port 2:




are externally being pulled low will source current (IIL) because of the internal pull-ups.

Port 2 emits the high-order address byte during fetches from external program memory

and during accesses to external data memories that use 16-bit addresses (MOVX

@DPTR). In this application, it uses strong internal pull-ups when emitting 1s. During

accesses to external data memories that use 8-bit addresses (MOVX @ RI), Port 2 emits

the contents of the P2 Special Function Register. Port 2 also receives the high-order

address bits and some control signals during Flash programming and verification.


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Port 3:




are externally being pulled low will source current (IIL) because of the pull-ups.

Port 3 also serves the functions of various special features of the AT89C51 as listed

below:

Port 3 also receives some control signals for Flash programming and verification

Tab 6.2.1 Port pins and their alternate functions

RST:

Reset input. A high on this pin for two machine cycles while the oscillator is

running resets the device.

ALE/PROG:

Address Latch Enable output pulse for latching the low byte of the address during

accesses to external memory. This pin is also the program pulse input (PROG) during

Flash programming. In normal operation ALE is emitted at a constant rate of 1/6the

oscillator frequency, and may be used for external timing or clocking purposes. Note,

however, that one ALE pulse is skipped during each access to external Data Memory.


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If desired, ALE operation can be disabled by setting bit 0 of SFR location 8EH. With the

bit set, ALE is active only during a MOVX or MOVC instruction. Otherwise, the pin is

weakly pulled high. Setting the ALE-disable bit has no effect if the microcontroller is in

external execution mode.

PSEN:

Program Store Enable is the read strobe to external program memory. When the

AT89C51 is executing code from external program memory, PSEN is activated twice

each machine cycle, except that two PSEN activations are skipped during each access to

external data memory.

EA/VPP:

External Access Enable (EA) must be strapped to GND in order to enable the

device to fetch code from external program memory locations starting at 0000H up to

FFFFH.

Note, however, that if lock bit 1 is programmed, EA will be internally latched on

reset.

EA should be strapped to VCC for internal program executions. This pin also receives the

12-volt programming enable voltage (VPP) during Flash programming, for parts that

require 12-volt VPP.

XTAL1:

Input to the inverting oscillator amplifier and input to the internal clock operating

circuit.

XTAL2:

Out put from the inverting oscillator amplifier.

Oscillator Characteristics:

XTAL1 and XTAL2 are the input and output, respectively, of an inverting

amplifier which can be configured for use as an on-chip oscillator, as shown in Figs

6.2.3. Either a quartz crystal or ceramic resonator may be used. To drive the device from

an external clock source, XTAL2 should be left unconnected while XTAL1 is driven as


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shown in Figure 6.2.4.There are no requirements on the duty cycle of the external clock

signal, since the input to the internal clocking circuitry is through a divide-by-two flip-

flop, but minimum and maximum voltage high and low time specifications must be

observed.

Oscillator Connections External Clock Drive Configuration

Notes:

1. Under steady state (non-transient) conditions, IOL must be externally limited as

follows:

Maximum IOL per port pin: 10 mA Maximum IOL per 8-bit port: Port 0: 26 mA Ports 1, 2, 3: 15 mA Maximum total IOL for all output pins: 71 mA If IOL exceeds the test condition, VOL may exceed the related specification.

Pins are not guaranteed to sink current greater than the listed test conditions.

2. Minimum VCC for Power-down is 2V.

Types of memory:

The 8051 have three general types of memory. They are on-chip memory,

external Code memory and external Ram. On-Chip memory refers to physically existing


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memory on the micro controller itself. External code memory is the code memory that

resides off chip. This is often in the form of an external EPROM. External RAM is the

Ram that resides off chip. This often is in the form of standard static RAM or flash

RAM.

b) Code memory

Code memory is the memory that holds the actual 8051 programs that is to be run.

This memory is limited to 64K. Code memory may be found on-chip or off-chip. It is

possible to have 4K of code memory on-chip and 60K off chip memory simultaneously.

If only off-chip memory is available then there can be 64K of off chip ROM. This is

controlled by pin provided as EA

b) Internal RAM

The 8051 have a bank of 128 bytes of internal RAM. The internal RAM is found

on-chip. So it is the fastest Ram available. And also it is most flexible in terms of reading

and writing. Internal Ram is volatile, so when 8051 is reset, this memory is cleared. 128

bytes of internal memory are subdivided. The first 32 bytes are divided into 4 register

banks. Each bank contains 8 registers. Internal RAM also contains 128 bits, which are

addressed from 20h to 2Fh. These bits are bit addressed i.e. each individual bit of a byte

can be addressed by the user. They are numbered 00h to 7Fh. The user may make use of

these variables with commands such as SETB and CLR.

Special Function registered memory:

Special function registers are the areas of memory that control specific

functionality of the 8051 micro controller.

a) Accumulator (0E0h)

As its name suggests, it is used to accumulate the results of large no of

instructions. It can hold 8 bit values.

B) B register (0F0h)


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The B register is very similar to accumulator. It may hold 8-bit value. The b

register is only used by MUL AB and DIV AB instructions. In MUL AB the higher byte

of the product gets stored in B register. In div AB the quotient gets stored in B with the

remainder in A.

c) Stack pointer (81h)The stack pointer holds 8-bit value. This is used to indicate where the next value

to be removed from the stack should be taken from. When a value is to be pushed onto

the stack, the 8051 first store the value of SP and then store the value at the resulting

memory location. When a value is to be popped from the stack, the 8051 returns the

value from the memory location indicated by SP and then decrements the value of SP.

d) Data pointer

The SFRs DPL and DPH work together work together to represent a 16-bit value

called the data pointer. The data pointer is used in operations regarding external RAM

and some instructions code memory. It is a 16-bit SFR and also an addressable SFR.

e) Program counter

The program counter is a 16 bit register, which contains the 2 byte address, which

tells the 8051 where the next instruction to execute to be found in memory. When the

8051 is initialized PC starts at 0000h. And is incremented each time an instruction is

executes. It is not addressable SFR.

f) PCON (power control, 87h)

The power control SFR is used to control the 8051s power control modes.

Certain operation modes of the 8051 allow the 8051 to go into a type of sleep mode

which consume much lee power.


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g) TCON (timer control, 88h)

The timer control SFR is used to configure and modify the way in which the

8051s two timers operate. This SFR controls whether each of the two timers is running

or stopped and contains a flag to indicate that each timer has overflowed. Additionally,

some non-timer related bits are located in TCON SFR. These bits are used to configure

the way in which the external interrupt flags are activated, which are set when an

external interrupt occurs.

h) TMOD (Timer Mode, 89h)

The timer mode SFR is used to configure the mode of operation of each of the

two timers. Using this SFR your program may configure each timer to be a 16-bit timer,

or 13 bit timer, 8-bit auto reload timer, or two separate timers. Additionally you may

configure the timers to only count when an external pin is activated or to count events

that are indicated on an external pin.

i) TO (Timer 0 low/high, address 8A/8C h)

These two SFRs taken together represent timer 0. Their exact behavior depends

on how the timer is configured in the TMOD SFR; however, these timers always count

up. What is configurable is how and when they increment in value.

j) T1 (Timer 1 Low/High, address 8B/ 8D h)


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These two SFRs, taken together, represent timer 1. Their exact behavior depends

on how the timer is configured in the TMOD SFR; however, these timers always count

up. What is

Configurable is how and when they increment in value.

k) P0 (Port 0, address 90h, bit addressable)

This is port 0 latch. Each bit of this SFR corresponds to one of the pins on a micro

controller. Any data to be outputted to port 0 is first written on P0 register. For e.g., bit 0

of port 0 is pin P0.0, bit 7 is pin p0.7. Writing a value of 1 to a bit of this SFR will send a

high level on the corresponding I/O pin whereas a value of 0 will bring it to low level.

l) P1 (port 1, address 90h, bit addressable)

This is port latch1. Each bit of this SFR corresponds to one of the pins on a

micro controller. Any data to be outputted to port 0 is first written on P0 register. For

e.g., bit 0 of port 0 is pin P1.0, bit 7 is pin P1.7. Writing a value of 1 to a bit of this SFR

will send a high level on the corresponding I/O pin whereas a value of 0 will bring it to

low level

m) P2 (port 2, address 0A0h, bit addressable) :

This is a port latch2. Each bit of this SFR corresponds to one of the pins on a

micro controller. Any data to be outputted to port 0 is first written on P0 register. For e.g.,

bit 0 of port 0 is pin P2.0, bit 7 is pin P2.7. Writing a value of 1 to a bit of this SFR will

send a high level on the corresponding I/O pin whereas a value of 0 will bring it to low

level.

n) P3(port 3,address B0h, bit addressable) :

This is a port latch3. Each bit of this SFR corresponds to one of the pins on a

micro controller. Any data to be outputted to port 0 is first written on P0 register. For e.g.,

bit 0 of port 0 is pin P3.0, bit 7 is pin P3.7. Writing a value of 1 to a bit of this SFR will

send a high level on the corresponding I/O pin whereas a value of 0 will bring it to low

level


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o) IE (interrupt enable, 0A8h):

The Interrupt Enable SFR is used to enable and disable specific interrupts. The

low 7 bits of the SFR are used to enable/disable the specific interrupts, where the MSB

bit is used to enable or disable all the interrupts. Thus, if the high bit of IE is 0 all

interrupts are disabled regardless of whether an individual interrupt is enabled by setting

a lower bit.

p) IP (Interrupt Priority, 0B8h)

The interrupt priority SFR is used to specify the relative priority of each interrupt.

On 8051, an interrupt maybe either low or high priority. An interrupt may interrupt

interrupts. For e.g., if we configure all interrupts as low priority other than serial

interrupt. The serial interrupt always interrupts the system, even if another interrupt is

currently executing. However, if a serial interrupt is executing no other interrupt will be

able to interrupt the serial interrupt routine since the serial interrupt routine has the

highest priority.

q) PSW (Program Status Word, 0D0h)

The program Status Word is used to store a number of important bits that are set

and cleared by 8051 instructions. The PSW SFR contains the carry flag, the auxiliary

carry flag, the parity flag and the overflow flag. Additionally, it also contains the register

bank select flags, which are used to select, which of the R register banks currently in

use.


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r) SBUF (Serial Buffer, 99h)

SBUF is used to hold data in serial communication. It is physically two registers.

One is writing only and is used to hold data to be transmitted out of 8051 via TXD. The

other is read only and holds received data from external sources via RXD. Both mutually

exclusive registers use address 99h.

I/O ports:

One major feature of a microcontroller is the versatility built into the input/output

(I/O) circuits that connect the 8051 to the outside world. The main constraint that limits

numerous functions is the number of pins available in the 8051 circuit. The DIP had 40

pins and the success of the design depends on the flexibility incorporated into use of

these pins. For this reason, 24 of the pins may each used for one of the two entirely

different functions which depend, first, on what is physically connected to it and, then, on

what software programs are used to program the pins.

PORT 0

Port 0 pins may serve as inputs, outputs, or, when used together, as a bi directional

low-order address and data bus for external memory. To configure a pin as input, 1 must

be written into the corresponding port 0 latch by the program. When used for interfacing

with the external memory, the lower byte of address is first sent via PORT0, latched

using Address latch enable (ALE) pulse and then the bus is turned around to become the

data bus for external memory.

PORT 1

Port 1 is exclusively used for input/output operations. PORT 1 pins have no dual

function. When a pin is to be configured as input, 1 is to be written into the

corresponding Port 1 latch.


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PORT 2

Port 2 maybe used as an input/output port. It may also be used to supply a high

order address byte in conjunction with Port 0 low-order byte to address external memory.

Port 2 pins are momentarily changed by the address control signals when supplying the

high byte a 16-bit address. Port 2 latches remain stable when external memory is

addressed, as they do not have to be turned around (set to 1) for data input as in the case

for Port 0.

PORT 3

Port 3 may be used to input /output port. The input and output functions can be

programmed under the control of the P3 latches or under the control of various special

function registers. Unlike Port 0 and Port 2, which can have external addressing functions

and change all eight-port b se, each pin of port 3 maybe individually programmed to be

used as I/O or as one of the alternate functions. The Port 3 alternate uses are:

Pin Alternate Use SFR

P3.0 - RXD Serial data input SBUF

P3.1 - TXD Serial data output SBUF

P3.2 - INTO 0 External interrupt 0 TCON.1

P3.3 - INTO 1 External interrupt 1 TCON.3

P3.4 - T0 External Timer 0 input TMOD

P3.5 T1 External timer 1 input TMOD

P3.6 - WR External memory write pulse -

P3.7 - RD External memory read pulse -


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TIMERS

On-chip timing/counting facility has proved the capabilities of the microcontrollerfor implementing the real time application. These includes pulse counting, frequency

measurement, pulse width measurement, baud rate generation, etc,. Having sufficient

number of timer/counters may be a need in a certain design application. The 8051 has

two timers/counters. They can be used either as timers to generate a time delay or as

counters to count events happening outside the microcontroller. Let discuss how these

timers are used to generate time delays and we will also discuss how they are been used

as event counters.

PROGRAMMING 8051 TIMERS

The 8051 has timers: Timer 0 and Timer1.they can be used either as timers or as

event counters. Let us first discuss about the timers registers and how to program the

timers to generate time delays.

BASIC RIGISTERS OF THE TIMER

Both Timer 0 and Timer 1 are 16 bits wide. Since the 8051 has an 8-bit

architecture, each 16-bit timer is accessed as two separate registers of low byte and high

byte.

TIMER 0 REGISTERS

The 16-bit register of Timer 0 is accessed as low byte and high byte. the low

byte register is called TL0(Timer 0 low byte)and the high byte register is referred to as

TH0(Timer 0 high byte).These register can be accessed like any other register, such as


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A,B,R0,R1,R2,etc.for example, the instruction MOV TL0, #4Fmoves the value 4FH

into TL0,the low byte of Timer 0.These registers can also be read like any other register.

D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1

TH0 TL0

TIMER 1 REGISTERS

Timer 1 is also 16-bit register is split into two bytes, referred to as TL1 (Timer

1 low byte) and TH1(Timer 1 high byte).these registers are accessible n the same way as

the register of Timer 0.

TMOD (timer mode) REGISTER

Both timers 0 and 1 use the same register, called TMOD, to set the various timer

operation modes. TMOD is an 8-bit register in which the lower 4 bits are set aside for


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Timer 0 and the upper 4 bits for Timer 1.in each case; the lower 2 bits are used to set the

timer mode and the upper 2 bits to specify the operation.

MODES:

M1, M0:

M0 and M1 are used to select the timer mode. There are three modes: 0, 1, 2.Mode 0 is

a 13-bit timer, mode 1 is a 16-bit timer, and mode 2 is an 8-bit timer. We will concentrate

on modes 1 and 2 since they are the ones used most widely. we will soon describe the

characteristics of these modes, after describing the reset of the TMOD register.

GATE Gate control when set. The timer/counter is enabled only

while the INTx pin is high and the TRx control pin is.

set. When cleared, the timer is enabled.

C/T Timer or counter selected cleared for timer operation

(Input from internal system clock).set for counter

operation (input TX input pin).


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M 1 Mode bit 1

M0 Mode bit 0

M1 M0 MODE Operating Mode

0 0 0 13-bit timer mode

8-bit timer/counter THx with TLx as

5-bit prescaler.

0 1 1 16-bit timer mode

16-bit timer/counters THx with TLx are

cascaded; there is no prescaler

1 0 2 8-bit auto reload

8-bit auto reload timer/counter;THx

Holds a value that is to be reloaded into

TLx each time it overflows.

1 1 3 Split timer mode.

TMOD Register

C/T (clock/timer):

This bit in the TMOD register is used to decide whether the timer is used as a delay

generator or an event counter. If C/T=0, it is used as a timer for time delay generation.

The clock source for the time delay is the crystal frequency of the 8051.this section is

concerned with this choice. The timers use as an event counter is discussed in the next

section.

EX: Find the values of TMOD to operate as timers in the following modes.

(a)Mode 1 Timer 1(b)Mode 2 Timer 0.Mode 2 Timer 1


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(c)Mode 0 Timer 1

Solution:

(a)TMOD is 00010000 = 10H

The gate control bit and C/T bit are made 0, and the unused timer(Timer 0 bit is also 0)

(b) TMOD is 01010010=52H

(c) TMOD is 00000000H = 00H

Note: 8051 Timers use 1/12 of XTAL frequency, regardless of machine cycle time.

Clock source for timer

As you know, every timer needs a clock pulse to tick. What is the source of the clock

pulse for the 8051 timers? If C/T=0, the crystal frequency attached to the 8051 is the

source of the clock for the timer. This means that the size of the crystal frequency

attached to the 8051 also decides the speed at which the 8051 timer ticks. The frequency

for the timer is always 1/12th

the frequency of the crystal attached to the 8051.

Although various 8051-based systems have an XTAL frequency of 10MHz,we will

concentrate on the XTAL frequency of 11.0592MHz.the reason behind such an odd

number has to do with the baud rate for serial communication of the 8051.XTAL

=11.0592MHz allows the 8051 system to communicate with the IBM PC with no errors.

Gate

The other bit of the TMOD register id the GATE bit. Notice in the TMOD register in the

below example figure that both Timers 0 and 1 have the GATE bit. What is its purpose?

Every timer has a means of starting and stopping. Some timers do this by software, some

by hardware, and some have both software and hardware controls. the timers in the 8051


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have is achieved by the instructions SETB TR1 and CLR TR1 for Timer 1,and

SETB TR0 and CLR TR0 for Timer0.the SETB instruction starts it, and it is stopped

by the CLR instruction. These instructions start and stop the timers as long as GATE =0

in the TMOD register. The hardware way of starting and stopping the timer by an

external source GATE =0, meaning that no external hardware is needed to start and stop

the timers. in using software to start and stop the timer where GATE=0,all we need are

the instructions SETB TRx and CLR TRx.

Now that we have this basic understanding of the role of the TMOD register, we will

look at the timers modes and how they are programmed to create a time delay. Because

modes 1 and 2 are so widely used, we describe each of them in detail.

EX:

How are timers 0 and 1 started and stopped by instruction?

Solution:

The timers are started by using instructions to set timer start bits TR0 and

TR1,which are called timer run control bits. They can be cleared by clearing these bits.

When a timer counts to its maximum value, it sets a flag TF0 or TF1.at this point, it is

necessary to know more about the bits TF and TR for timers 0 and 1.while TMOD

controls the timer modes, another register called the TCON controls the timer/counter

operations. The lower four bits of TCON cater to interrupt functions, but the upper four

bits are for timer operations. The details of the TCON register are shown below.

MSB LSB

TF1 TR1 TF0 TR0 IE1 IT1 IE0 IT0


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BIT SYMBOL FUNCTION

TCON.7 TF1 Timer 1 overflows flag

TCON.6 TR1 Timer 1 run control bit

TCON.5 TF0 Timer 0 overflows flag

TCON.4 TR0 Timer 0 run control bit

Some assemblers dont allow the use of TF0, TR0, etc.in programs, but instead need the

use of these as bits of TON. For example, TF1 is TCON.7 and TR1 is TCON.6.

Mode 1 programming

The following are the characteristic and operations of mode 1:

1. it is a 16-bit timer: therefore, it allows values of 0000 to FFFFH to be loaded intothe timers registers TL and TH.

2. After TH and TL are loaded with 16-bit initial value, the timer must be started.This is done by SETB TR0 for Timer 0 and SET TR1 for Timer1.

3. After the timer is started, it starts to count up. it counts up until it reaches its limitof FFFFH. When it rolls over from FFFFH to 0000, it sets high a flag bit called

TF (timer flag).this timer flag can be monitored. When this timer flag is raised,

one option would be to stop the timer with the instructions CLR TR0 or CLR

TR1 for Timer 0 and Timer 1, respectively.again, it must be noted that each

timer has its own timer flag: TF0 for Timer 0, and TF1 for Timer 1.

4. After the timer reaches its limit its limit and rolls over, in order to repeat theprocess the registers TH and TL must be reloaded with the original value and TF

must be reset to 0


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. _

C/T =0 TR TF goes high overflow

When FFFF0 flag

XTAL

oscillat

or/12 TH TL TF


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Steps to program in mode 1

1. Load the TMOD value register indicating which timer (Timer 0 or Timer 1) is tobe used and which timer mode (0 or 1) is selected.

2. Load register TL and TH with initial count values.3. Start the timer.4. Keep monitoring the timer flag (TF) with the JNB TFx, target instruction to see

if it is raised. Get out of the loop when TF becomes high.

5. Stop the timer.6. Clear the TF flag for the next round7. Go back to Step 2 to load TH and TL again.

EX: In the following program, we are creating a square wave of 50% duty cycle (with

equal portion high and low) on the P1.5 bit. Timer 0 is used to generate the time delay.

Analyze the program.

Solution:

MOV TMOD, # 01 ; Timer 0, mode 1 (16bit mode)

HERE: MOV TL0, #0F2H ; TL0 =F2H, the low byte

MOV TH0, #0FFH ; TH0 =FFH, the High byte

CPL P1.5 ;toggle P1.5

ACALL DELAY

SJMP HERE ; load TH , TL again

; ------------ delay using Timer 0

DELAY:

SETB TR0 ; start Timer 0

AGAIN: JNB TF0, AGAIN ; monitor Timer 0 flag until

; It rolls over


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CLR TR0 ; stop Timer 0

CLR TF0 ; clear Timer 0 flag

RET

Finding values to be loaded into the timer

Assuming that we know the amount of timer delay we need, the question is how to find

the values needed for the TH, TL registers. To calculate the values to be loaded into the

TL and TH registers.

1. Divide the desired time delay by 1.085s.2. Perform 65536-n, where n is the decimal value we got in step 1.3. Convert the result of step 2 to hex, where yyxx is the initial hex value to be loaded

into the timers registers.

4. Set TL=xx and TH=yy.

Generating a large time delay

Size of the time delay depends on two factors; (a) the crystal frequency and (b) the

timers 16-bit register in mode 1.both of these factors are beyond the control of the 8051

programmer. We saw earlier that the largest time delay is achieved by making both TH

and TL zero. what if that is not enough?

See the below example.

Ex: AssumingXTAL = 2MHz,write a program to generate a pulse train 2 seconds period

on pin P2.4.USse Timer 1 in mode 1.

Solution:

MOV TMOD, #10H ; Timer 1, mode 1

REPT: MOV R0, #28 ; counter for multiple delays


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CPL P2.4 ; complement P2.4

BACK: MOV TL1, #00H ; load count value in TL1

MOV TH1, #00H ; load count value in th1

SETB TR1 ; start timer

AGAIN: JNB TF1, AGAIN ; stay until timer roller over

CLR TR1 ; stop timer

CLR TF1 ; clear timer flag

DJNZ R0, BACK ; if R0 is not zero, reload timer

SJMP REPT ; repeat for continuous pulse

generation

Calculation = 28x 35.75ms =1001 ms.

Mode 0

Mode 0 is exactly like mode 1 except that it is a 13-bit timer instead of 16-bit.the 13-bit

counter can hold values between 0000 to 1FFFH in TH-TL. Therefore, when the timer

reaches its maximum of 1FFH, it rolls over to 0000, and TF is raised.


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Mode 2 programming

The following are the characteristics and operations of mode 2.

1. It is 8-bit timer; therefore, it allows only values if 00 to FFH to be loaded into thetimers register TH.

2. After TH is loaded with the 8-bit value, the 8051 gives a copy of it to TL. Thenthe timer must be started. This is done by the instruction SETB TR0 for Timer 0

and SETB TR1 for Timer 1.this is just like mode 1.

3. After the timer is started, it started, it start to count up by incrementing the TLregister. It counts up until it reaches its limit of FFH. When it rolls over from FFH

to 00, it sets high the TF (timer flag).if we are using Timer 0, TF0 goes high; if

we are using Timer 1, and TF1 is raised.

Overflow


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flag

TR TF goes high

when FF---0

4. When the TL registers rolls from FFH to 0 and TF is set to 1, TL is reloadedautomatically with the original value kept by the TH register. To repeat the

process, we must simply clear TF and let it go without any need by the

programmer to reload the original value. This makes mode 2 an auto-reload, in

contrast with mode 1 in which the programmer has to reload TH and TL.

XTAL

Oscillator /12 TL

TH

TF


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It must be emphasized that mode 2 is an 8-bit timer. However, it has an auto-

reloading capability. In auto-reload TH is loaded with the initial count and a copy of

it is given to TL. This reloading leaves TH unchanged, still holding a copy of the

original value. This mode has many applications, including setting the baud rate in

serial communication.

Steps to program in mode 2

To generate a time delay using the timers mode 2 take the following steps.

1. Load the TMOD value register indicting which timer (Timer 0 or Timer 1) is tobe used, and select the timer mode(mode 2).

2. Load the TH registers with the initial count value.3. start the timer


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4. Keep monitoring the timer flag (TF) with the JNB TFx, target instruction to seewhether it is raised. get out of the loop when TF goes high.

5. clear the TF flag6. Go back to step 4, since mode 2 is auto-reload.

SERIAL COMMUNICATION:

Computers can transfer data in two ways: parallel and serial. In parallel

data transfers, often 8 or more lines (wire conductors) are used to transfer data to a device

that is only a few feet away. Examples of parallel transfers ae printers and hard disks;

each uses cables with many wire strips. Although in such cases a lot of data can be

transferred in a short amount of time by using many wires in parallel, the distance cannot

be great. To transfer to a device located many meters away, the serial method is used. In

serial communication, the data is sent one bit at a time, in contrast to parallel

communication, in which the data is sent a byte or more at a time. Serial communication

of the 8051 is the topic of this chapter. The 8051 has serial communication capability

built into it, there by making possible fast data transfer using only a few wires.

If data is to be transferred on the telephone line, it must be converted from 0s and

1s to audio tones, which are sinusoidal-shaped signals. This conversion is performed by

a peripheral device called a modem, which stands for modulator/demodulator.

Serial data communication uses two methods, asynchronous and synchronous.

The synchronous method transfers a block of data at a time, while the asynchronous

method transfers a single byte at a time.

In data transmission if the data can be transmitted and received, it is a duplex

transmission. This is in contrast to simplex transmissions such as with printers, in which

the computer only sends data. Duplex transmissions can be half or full duplex,

depending on whether or not the data transfer can be simultaneous. If data is transmitted

one way at a time, it is referred to as half duplex. If the data can go both ways at the

same time, it is full duplex. Of course, full duplex requires two wire conductors for the

data lines, one for transmission and one for reception, in order to transfer and receiv e

data simultaneously.


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Asynchronous serial communication and data framing

The data coming in at the receiving end of the data line in a serial data transfer is

all 0s and 1s; it is difficult to make sense of the data unless the sender and receiver agree

on a set of rules, a protocol, on how the data is packed, how many bits constitute a

character, and when the data begins and ends.

Start and stop bits

Asynchronous serial data communication is widely used for character-oriented

transmissions, while block-oriented data transfers use the synchronous method. In the

asynchronous method, each character is placed between start and stop bits. This is called

framing. In the data framing for asynchronous communications, the data, such as ASCII

characters, are packed between a start bit and a stop bit. The start bit is always one bit,

but the stop bit can be one or two bits. The start bit is always a 0 (low) and the stop bit

(s) is 1 (high).

Data transfer rate

The rate of data transfer in serial data communication is stated in bps (bits per

second). Another widely used terminology for bps is baud rate. However, the baud and

bps rates are not necessarily equal. This is due to the fact that baud rate is the modem

terminology and is defined as the number of signal changes per second. In modems a

single change of signal, sometimes transfers several bits of data. As far as the conductor

wire is concerned, the baud rate and bps are the same, and for this reason we use the bps

and baud interchangeably.

The data transfer rate of given computer system depends on communication ports

incorporated into that system. For example, the early IBMPC/XT could transfer data at

the rate of 100 to 9600 bps. In recent years, however, Pentium based PCS transfer data at

rates as high as 56K bps. It must be noted that in asynchronous serial data

communication, the baud rate is generally limited to 100,000bps.

RS232 Standards

To allow compatibility among data communication equipment made by various

manufacturers, an interfacing standard called RS232 was set by the Electronics Industries

Association (EIA) in 1960. In 1963 it was modified and called RS232A. RS232B AND

RS232C were issued in 1965 and 1969, respectively. Today, RS232 is the most widely


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used serial I/O interfacing standard. This standard is used in PCs and numerous types of

equipment. However, since the standard was set long before the advert of the TTL logic

family, its input and output voltage levels are not TTL compatible. In RS232, a 1 is

represented by -3 to -25V, while a 0 bit is +3 to +25V, making -3 to +3 undefined. For

this reason, to connect any RS232 to a microcontroller system we must use voltage

converters such as MAX232 to convert the TTL logic levels to the RS232 voltage levels,

and vice versa. MAX232 IC chips are commonly referred to as line drivers.

RS232 pins

RS232 cable, commonly referred to as the DB-25 connector. In labeling, DB-25P

refers to the plug connector (male) and DB-25S is for the socket connector(female).

Since not all the pins are used in PC cables, IBM introduced the DB-9 Version of the

serial I/O standard, which uses 9 pins only, as shown in table.

DB-9 pin connector

1 2 3 4 5

6 7 8 9

(Out of computer and exposed end of cable)

Pin Functions:

Pin Description

1 Data carrier detect (DCD)

2 Received data (RXD)

3 Transmitted data (TXD)

4 Data terminal ready(DTR)

5 Signal ground (GND)6 Data set ready (DSR)

7 Request to send (RTS)

8 Clear to send (CTS)

9 Ring indicator (RI)


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Note: DCD, DSR, RTS and CTS are active low pins.

The method used by RS-232 for communication allows for a simple connection of three

lines: Tx, Rx, and Ground. The three essential signals for 2 way RS-232

Communications are these:

TXD: carries data from DTE to the DCE.

RXD: carries data from DCE to the DTE

SG: signal ground

8051 connection to RS232

The RS232 standard is not TTL compatible; therefore, it requires a line driver

such as the MAX232 chip to convert RS232 voltage levels to TTL levels, and vice versa.

The interfacing of 8051 with RS232 connectors via the MAX232 chip is the main topic.

The 8051 has two pins that are used specifically for transferring and receiving

data serially. These two pins are called TXD and RXD and a part of the port 3 group

(P3.0 and P3.1). pin 11 of the 8051 is assigned to TXD and pin 10 is designated as RXD.

These pins are TTL compatible; therefore, they require a line driver to make them RS232

compatible. One such line driver is the MAX232 chip.

MAX232 converts from RS232 voltage levels to TTL voltage levels, and viceversa. One advantage of the MAX232 chip is that it uses a +5V power source which, is

the same as the source voltage for the 8051. In the other words, with a single +5V power

supply we can power both the 8051 and MAX232, with no need for the power supplies

that are common in many older systems. The MAX232 has two sets of line drivers for

transferring and receiving data. The line drivers used for TXD are called T1 and T2,

while the line drivers for RXD are designated as R1 and R2. In many applications only

one of each is used.


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Embedded

Controller

RXD

TXD

TXD

RXD2

3

5

GND

MAX 232

Baud rate in the 8051

The 8051 transfers and receives data serially at many different baud rates. The

baud rate in the 8051 is programmable. The is done with the help of Timer1. The 8051

divides the crystal frequency by 12 to get the machine cycle frequency. In the case of

XTAL=11.0592MHZ, the machine cycle frequency is 921.6 KHz

(11.0592MHz/12=921.6KHz). the 8051s serial communication UART circuitry divides

the machine cycle frequency of 921.6khz divided by 32 once more before it is used by

Timer 1 to set the Baud rate. Therefore, 921.6 kHz divided by 32 gives 28,800 Hz. This

is the number well use to find the Timer 1 value to set baud rate.

Baud rate TH1(Decimal) TH1(Hex)

9600 -3 FD

4800 -6 FA

2400 -12 F4

1200 -24 E8

NOTE: XTAL=11...592MHz

SBUF register

SBUF is an 8-bit register used solely for serial communication in the 8051. for a

byte of data to be transferred via the TXD line, it must be placed in the SBUF register.

Similarly, SBUF holds the byte of data when it is received by the 8051s RXD line.

SBUF can be accessed like any other register in the 8051. Look at the following the

examples of how this register is accessed.


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MOV SBUF, #D ; load SBUF=44h, ASCII for D

MOV SBUF, A ; copy accumulator into SBUF

MOV A, SBUF ; copy SBUF into accumulator

The moment a byte is written into SBUF, it is framed with the start and stop bits

and transferred serially via the TXD pin. Similarly, when the bits are received serially

via RXD, the 8051 defames it by eliminating the stop and start bits, making a byte out of

the data received, and then placing it in the SBUF.

SCON (serial control) register

The SCON register is an 8-bit register used to program the start bit, stop bit, and

data bits of data framing, among other things.

The following describes various bits of the SCON register.

SM0 SM1 SM2 REN TB8 RB8 TI RI

SM0 SCON.7 Serial port mode specifier

SM1 SCON.6 Serial port mode specifier

SM2 SCON.5 Used for multiprocessor communication. (make it 0.)

REN SCON.4 Set/cleared by software to enable/disable reception

TB8 SCON.3 Not widely used

RB8 SCON.2 Not widely used

TI SCON.1 Transmit interrupt flag. Set by hardware at the beginning

Of the stop bit in mode 1. Must be cleared by software.

RI SCON.0 Receive interrupt flag. Set by hardware halfway through

the stop bit time mode 1. Must be cleared by software.

Note: Make SM2, TB8, and RB8=0.


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SM0, SM1

SM0 and SM1 are D7 and D6 of the SCON register, respectively. These two bits

determine the framing of data by specifying the number of bits per character and the start

and stop bits. They take the following combinations.

SM0 SM1 FUNCTION

0 0 Serial Mode 0

0 1 Serial Mode 1, 8-bit data, 1 stop bit, 1 start bit

1 0 Serial Mode 2

1 1 Serial Mode 3

Of the 4 serial modes, only mode 1 is of interest to us. In the SCON register,

when serial mode 1 is chosen, the data framing is 8 bits, 1 stop bit, and 1 start bit, which

makes it compatible with the COM port of IBM/compatible PCs. More importantly,

serial mode 1 allows the baud rate to be variable and is set by Timer 1 of the 8051. In

serial mode 1, for each character a total of 10 bits are transferred, where the first bit is the

start bit, followed by 8 bits of data, and finally 1 stop bit.

MODE 0

In mode 0, the data are transferred through the RXD pin, while clock pulses appear on the

TXD pin. The bout rate is fixed at 1/12 the quartz oscillator frequency. On transmit, the

least significant bit (LSB bit) is being sent/received first. (received).

TRANSMIT

Data transmission in form of pulse train automatically starts on the pin RXD at the

moment the data has been written to the SBUF register.In fact, this process starts after

any instruction being performed on this register. Upon all 8 bits have been sent, the bit TI

in the SCON register is automatically set.


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RECEIVE

Starts data receiving through the pin RXD once two necessary conditions are met: bit

REN=1 and RI=0 (both bits reside in the SCON register). Upon 8 bits have been

received, the bit RI (register SCON) is automatically set, which indicates that one byte is

received.

Since, there are no START and STOP bits or any other bit except data from the SBUF

register, this mode is mainly used on shorter distance where the noise level is minimal

and where operating rate is important. A typical example for this is I/O port extension by

adding cheap IC circuit ( shift registers 74HC595, 74HC597 and similar).


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Mode 1

In Mode1 10 bits are transmitted through TXD or received through RXD in the following

manner: a START bit (always 0), 8 data bits (LSB first) and a STOP bit (always 1) last.

The START bit is not registered in this pulse train. Its purpose is to start data receiving

mechanism. On receive the STOP bit is automatically written to the RB8 bit in the SCON

register.

TRANSMIT

A sequence for data transmission via serial communication is automatically started upon

the data has been written to the SBUF register. End of 1 byte transmission is indicated by

setting the TI bit in the SCON register.


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RECEIVE

Receiving starts as soon as the START bit (logic zero (0)) appears on the pin RXD. The

condition is that bit REN=1and bit RI=0. Both of them are stored in the SCON register.

The RI bit is automatically set upon receiving has been completed.

The Baud rate in this mode is determined by the timer 1 overflow time.

MODE 2

SM2

SM2 is the D5 bit of the SCON register. This bit enables the multiprocessing

capability of the 8051. For our applications, we will make SM2=0 since we are not using

the 8051 in a multi processor environment.

REN

The REN (receive enable) bit is D4 of the SCON register. The REN bit is also

referred to as SCON.4 since SCON is a bit-addressable register. When the REN bit is

high, it allows the 8051 to receive data on the RXD pin of the 8051. As a result if we

want the 8051 to both transfer and receive data, REN must be set to 1. By making

REN=0, the receiver is disabled. Making REN=1 or REN=0 can be achieved by the

instructions SETB SCON.4 and CLR SCON.4, respectively. Notice that these

instructions use the bit-addressable features of the register SCON. This bit can be used to

block any serial data reception and is an extremely important in the SCON register.


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TB8

TB8 (transfer bit 8) is bit D3 of SCON. It is used for serial modes 2 and 3. we

make TB8=0 since it is not used in our applications.

RB8

RB8 (receive bit 8) is bit D2 of the SCON register. In serial mode 1, this bit gets

a copy of the stop bit when an 8-bit data is received. This is (as is the case for TB8) is

rarely used anymore. In all our applications we will make RB8=0. Like TB8, the RB8

bit is also used in serial modes 2 and 3.

TRANSMIT

TI (transmit interrupt) is bit D1 of the SCON register. This is an extremely

important flag bit in the SCON register. When the 8051 finishes the transfer of the 8-bit

character, it raises the TI flag to indicate that it is ready to transfer another byte. The TI

bit is raised at the beginning of the stop bit.


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RECEIVE

RI (receive interrupt) is the D0 of the SCON register. This is another extremely

important flag bit in the SCON register. When the 8051 receives data serially via RXD,

it gets rid of the start and stop bits and places the byte in the SBUF register. Then it

raises the RI flag bit to indicate that a byte has been received and should be picked up

before it is lost. RI is raised halfway through the stop bit.

Mode 3

Mode 3 is the same as Mode 2 except the baud rate. In Mode 3 is variable and can be

selected.


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FLOW CHART:

TX Loop:

START

Initialization:

Load SFRs with Respective values

Copy data into Accumulator

(or) memory location

Copy into SBUF

IsTI = = 1

Clr TI

STOP

RX Loop:

START

Initialization:

Load SFRs with Respective values

`

Copy SBUF into

Memory

Is

RI = = 1

Clr RI

STOP


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BAUD RATE CALCULATION:

Internal timer stages are as fallows

XClkDivided

b 12

Divided

by 32

Divided

by X

UARTBaud Rate

Divided by X box can be replaced with T1 timer so that by changing the value of timer

we can obtain the required baud rate.

Let XClk = 11.0592 Mhz

Baud Rate = (XClk / 12 / 16 / 2 / X )

For attaining 9600 baud Rate,

X can be calculated as

= 11.0592 x 106 / 12 / 16 / 2 / 9600

= 3

So set the 2s Complement of 3 in Timer 1 so that we can achieve 9600 baud rates.

Note: Assuming 8-bit Auto reload mode and 8-bit variable baud rate modes.

Doubling the baud rate in the 8051:

There are two ways to increase the baud rate to data transfer in the 8051.

1. Use a higher- frequency crystal.2. Change a bit in the PCON register, shown below.

SMOD -- -- -- GF1 GF0 PD IDL

Option1 is not feasible in many situations since the system crystal is fixed. More


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Importantly, it is not feasible because the new crystal may not be compatible with the

IBM PC serial COM ports baud rate. Therefore, we will explore option2. there is a

software way to double the baud rate of the 8051 while the crystal frequency is fixed.

This is done with the register called PCON (power control). The PCON register is an 8-

bit register. Of the 8 bits, some are unused, and some are used for the power control

capability of the 8051. The bit that is used for the serial communication is D7, the SMOD

(serial mode) bit. When the 8051 is powered up, D7 (SMOD) of this PCON register is

zero. We can set it to high by software and thereby double the baud rate. The following

sequence of instructions must be used to set high D7 of PCON, since it is not a bit-

addressable register:

MOV A, PCON ; place a copy a of PCON in ACC

SETB ACC.7 ; make D7=1MOV PCON, A ; changing any other bits

To see how the baud rate is double with this method, we show the role of the

SMOD bit (D7 bit of the PCON register), which can be 0 or 1. we discuss each case.

Baud rates for SMOD =0

When SMOD=0, the 8051 divides 1/12 of the crystal frequency by 32 and uses

that frequency for Timer 1 to set the baud rate. In the case of XTAL=11.0592MHZ we

have:

Machine cycle freq.=11.0592 MHz/12 = 921.6KHz

And

921.6 KHz/32 = 28,800 Hz since SMOD=0

This is the frequency used by Timer 1 to set the baud rate. This has been the basis

of all the examples so far since it is the default when the 8051 is powered up. The baud

rate for SMOD=0 was listed in above table.

Baud rates for SMOD = 1

With the fixed crystal frequency, we can double the baud rate by making

SMOD=1. When the SMOD bit (D7 of the PCON register) is set to 1, 1/12 of XTAL is

divided by 16 (instead of 32) and that is the frequency used by Timer 1 to set the baud

rate. In the case of XTAL=11.0592 MHz, we have:


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Machine cycle freq. = 11.0592 MHz / 12 = 921.6 KHz

And

921.6 KHz / 16 = 57,600 Hz since SMOD = 1

This is the frequency used by Timer 1 to set the baud rate.

Table: Baud Rate comparison for SMOD = 0 and SMOD = 1

TH1 (Decimal) (Hex) SMOD = 0 SMOD = 1

-3 FD 9,600 19,200

-6 FA 4,800 9,600

-12 F4 2,400 4,800

-24 E8 1,200 2,400

Note: XTAL=11.0592 MHz.

Table: shows that the values loaded into TH1 are the same for both cases; however, thebaud rates are doubled when SMOD=1.

INTERRUPTS:

Interrupts are hardware signals that are used to determine conditions that exist in

external and internal circuits. Any interrupt can cause the 8051 to perform a hardware

call to an interrupt handling subroutine that is located at a predetermined absolute

address in the program memory.

Five interrupts are provided in the 8051. Three of these are generated automatically

by the internal operations: Timer flag 0, Timer Flag 1, and the serial port interrupt (RI or

TI) Two interrupts are triggered by external signals provided by the circuitry that is

connected to the pins INTO 0 and INTO1. The interrupts maybe enable or disabled, given

priority or otherwise controlled by altering the bits in the Interrupt Enabled (IE) register,

Interrupt Priority (IP) register, and the Timer Control (TCON) register. . These interrupts

are mask able i.e. they can be disabled. Reset is a non maskable interrupt which has the

highest priority. It is generated when a high is applied to the reset pin. Upon reset, the

registers are loaded with the default values.


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Each interrupt source causes the program to do store the address in PC onto the

stack and causes a hardware call to one of the dedicated addresses in the program

memory. The appropriate memory locations for each for each interrupt are as follows:

Interrupt Address

RESET 0000

IE0 (External interrupt 0) 0003

TF0 (Timer 0 interrupt) 000B

IE1 (External interrupt 1) 0013

TF1 (Timer 1 interrupt) 001B

SERIAL 0023

Functions of micro controller in main unit are

1. Sends appropriate commands to ADC to start conversion,

2. Collects the output of ADC for every 10 sec and stores in buffer,

3. Sends command to the modem for dialing the base every 80 sec,

4. Sends data in a serial manner to MODEM,

5. After data gets transmitted sends command for disconnection.

6. Sends appropriate information to the display unit as per status

of the communication medium.


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RELAY:

A relay is an electrical switch that opens and closes under the control of another

electrical circuit. In the original form, the switch is operated by anelectromagnetto open

or close one or many sets of contacts. It was invented byJoseph Henryin1835. Because

a relay is able to control an output circuit of higher power than the input circuit, it can be

considered to be, in a broad sense, a form of an electricalamplifier.

Operation

When acurrentflows through thecoil, the resultingmagnetic fieldattracts an armature

that is mechanically linked to a moving contact. The movement either makes or breaks a

connection with a fixed contact. When the current to the coil is switched off, the armature

is returned by a force approximately half as strong as the magnetic force to its relaxed

position. Usually this is aspring, but gravity is also used commonly in industrial motor

starters. Most relays are manufactured to operate quickly. In a low voltage application,

this is to reduce noise. In a high voltage or high current application, this is to reduce

arcing.

If the coil is energized with DC, adiodeis frequently installed across the coil, to dissipate

the energy from the collapsing magnetic field at deactivation, which would otherwise

generate a spike of voltage and might cause damage to circuit components. Some

automotive relays already include that diode inside the relay case. Alternatively a contact

protection network, consisting of a capacitor and resistor in series, may absorb the surge.

If the coil is designed to be energized with AC, a small copper ring can be crimped to the

end of the solenoid. This "shading ring" creates a small out-of-phase current, which

increases the minimum pull on the armature during the AC cycle.[1]

By analogy with the functions of the original electromagnetic device, a solid-state relay is

made with athyristoror other solid-state switching device. To achieve electrical isolation

anoptocouplercan be used which is alight-emitting diode(LED) coupled with a photo

transistor.
http://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Electrical_circuithttp://en.wikipedia.org/wiki/Electrical_circuithttp://en.wikipedia.org/wiki/Magnethttp://en.wikipedia.org/wiki/Magnethttp://en.wikipedia.org/wiki/Magnethttp://en.wikipedia.org/wiki/Joseph_Henryhttp://en.wikipedia.org/wiki/Joseph_Henryhttp://en.wikipedia.org/wiki/Joseph_Henryhttp://en.wikipedia.org/wiki/1835http://en.wikipedia.org/wiki/1835http://en.wikipedia.org/wiki/1835http://en.wikipedia.org/wiki/Amplifierhttp://en.wikipedia.org/wiki/Amplifierhttp://en.wikipedia.org/wiki/Amplifierhttp://en.wikipedia.org/wiki/Current_%28electricity%29http://en.wikipedia.org/wiki/Current_%28electricity%29http://en.wikipedia.org/wiki/Current_%28electricity%29http://en.wikipedia.org/wiki/Coilhttp://en.wikipedia.org/wiki/Coilhttp://en.wikipedia.org/wiki/Coilhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Armature_%28electrical_engineering%29http://en.wikipedia.org/wiki/Armature_%28electrical_engineering%29http://en.wikipedia.org/wiki/Spring_%28device%29http://en.wikipedia.org/wiki/Spring_%28device%29http://en.wikipedia.org/wiki/Spring_%28device%29http://en.wikipedia.org/wiki/Arcinghttp://en.wikipedia.org/wiki/Arcinghttp://en.wikipedia.org/wiki/Flyback_diodehttp://en.wikipedia.org/wiki/Flyback_diodehttp://en.wikipedia.org/wiki/Flyback_diodehttp://en.wikipedia.org/wiki/#_note-0http://en.wikipedia.org/wiki/#_note-0http://en.wikipedia.org/wiki/#_note-0http://en.wikipedia.org/wiki/Thyristorhttp://en.wikipedia.org/wiki/Thyri

88043168 16 doc of automatic load sharing of transformers

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