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KEEP DISTANCE WARNING SYSTEM

CHAPTER 1

INTRODUCTION

1.1 INTRODUCTION:

Most of the accidents in high-ways are happening due to the drowsiness

of drivers, most often this symptom is found in long distance truck drivers, these

drivers may take some time to recognize the opposite vehicle, which is running at low

speed, resulting accident. This kind of accidents may happen due to the drunken

drivers also. So to increase safety an electronic warning system is essential to alert the

drivers. This kind of system can be installed in all types of vehicles, especially in

commercial vehicles, so that accident rate can be minimized. The warning system

designed here can be called as driver attention system, which raises an alarm and

energizes the display board automatically when the following vehicle came near to the

in-front running vehicle.

The system is designed with 89S52 microcontroller, the task is quite

simple, the following vehicle detection circuit is designed with infrared sensors, when

the IR signal is interrupted due to the following vehicle, the circuit generates high

signal and it is fed to microcontroller. It offers several advantages over conventional

multi-chip systems. There is a cost and space advantage as extra chip costs and

printed circuit board and connectors required to support multi-chip systems are

eliminated.

SOFTWARE REQUIREMENT:

1. KEIL MICRO VISION-3

2. MICRO FLASH

HARDWARE REQUIREMENT:

1. MICROCONTROLLER (89S52)

2. LM324.

3. ULTRASONIC SENSOR

4. LCD DISPLAY

DEPT.OF DECS, JNNCE SHIVAMOGGA




1.2 AIM:

The aim of this system is to alert the following vehicle whenever it

came very close to the ahead vehicle; thereby to some extent accidents can be

avoided .Many accidents at High-ways are taking place due to the close running of vehicles, all of sudden, if the in front vehicle driver reduces the speed or applied

breaks, then it is quite difficult to the following vehicle driver to control his vehicle,

resulting accidents avoid this kind of accident, the warning system, which contains

alarm and display system can arrange at rear side of each and every vehicle.

1.3 BLOCK DIAGRAM:

FIG (1.3) BLOCK DIAGRAM


AT 89S52

MICRO

-CONTROLLE

R

LCD DISPLAY

POWER

SUPPLY

ULTRASONIC

SENSOR

BUZZER




1.4 FLOW CHART:

FIG (1.4) FLOWCHART

1.5 SUMMARY:

Hence, the circuit is designed such that whenever the infrared signal is

interrupted, logic high signal is generated and it is fed to microcontroller, on receipt of

this signal, the controller activates the display board and alarm automatically.


Object detection is displayed on LCD

START

Initialization of hardware’s

Initialization of LCD

Initialization of ultrasonic

STOP

STOP

If object

is

detected

Buzzer on

NO




CHAPTER 2

DESCRIPTION OF MICROCONTROLLER

2.1 INTRODUCTION :

The 8051 is an original member of the 8051 family. There are two other

members in the 8051 family of microcontrollers. They are 8052 and 8031. All the

three microcontrollers will have the same internal architecture, but they differ in the

following aspects.

8031 has 128 bytes of RAM, two timers and 6 interrupts.

89S51 has 4KB ROM, 128 bytes of RAM, two timers and 6interrupts.

89S52 has 8KB ROM, 128 bytes of RAM, three timers and

8 interrupts.

Of the three microcontrollers, 89S51 is the most preferable. Microcontroller

supports both serial and parallel communication.

In the concerned project 89S52 microcontroller is used. Here microcontroller

used is AT89S52, which is manufactured by ATMEL laboratories.

The AT89S52 provides the following standard features: 8Kbytes of Flash,

256 bytes of RAM, 32 I/O lines, three 16-bit timer/counters, six-vector two-level

interrupt architecture, a full duplex serial port, on-chip oscillator, and clock circuitry.

In addition, the AT89S52 is designed with static logic for operation down to zero

frequency and supports two software selectable power saving modes. The Idle Mode

stops the CPU while allowing the RAM, timer/counters, serial port, and interrupt

system to continue functioning. The Power down Mode saves the RAM contents but

freezes the oscillator, disabling all other chip functions until the next hardware reset.

The Idle Mode stops the CPU while allowing the RAM, timer/counters,

serial port and interrupt system to continue functioning. The Power down Mode

saves the RAM contents but freezes the oscillator disabling all other chip functions

until the next hardware reset.

By combining a versatile 8-bit CPU with Flash on a monolithic chip, the

AT89C52 is a powerful microcomputer which provides a highly flexible and cost

effective solution to many embedded control applications.





CHAPTER-3

POWER SUPPLY

3.1 INTRODUCTION:

Power supplies are designed to convert high voltage AC mains electricity to a

suitable low voltage supply for electronics circuits and other devices.

Fig (3.1) Block Diagram of a Regulated Power Supply System

3.2 TRANSFORMER:

A transformer steps down high voltage AC mains to low voltage AC. Here we

are using a center-tap transformer whose output will be sinusoidal with 24V peak to

peak.

.

Fig( 3.2) Output Waveform of transformer

The low voltage AC output is suitable for lamps, heaters and special AC motors. It is

not suitable for electronic circuits unless they include a rectifier and a smoothing

capacitor. The transformer output is given to the rectifier circuit.





3.3 RECTIFIER:

A rectifier converts AC to DC, but the DC output is varying. There are several

types of rectifiers; here we use a bridge rectifier.

The Bridge rectifier is a circuit, which converts an ac voltage to dc voltageusing both half cycles of the input ac voltage. The circuit has four diodes connected

to form a bridge. The ac input voltage is applied to the diagonally opposite ends of the

bridge. The load resistance is connected between the other two ends of the bridge.For

the positive half cycle of the input ac voltage, diodes D1 and D3 conduct. The

conducting diodes will be in series with the load resistance R L and hence the load

current flows through R L. For the negative half cycle of the input ac voltage, diodes

D2 and D4 conduct.The conducting diodes D2 and D4 will be in series with the load

resistance R L and hence the current flows through R L in the same direction as in the

previous half cycle. Thus a bi-directional wave is converted into unidirectional.

FIG (3.3.1) Rectifier circuit

FIG (3.3.2) Rectifier output


IN0047

IN0047IN0047




Smoothing:

The smoothing block smoothes the DC from varying greatly to a small ripple

and the ripple voltage is defined as the deviation of the load voltage from its DC

value. Smoothing is also named as filtering. Filtering is frequently effected by

shunting the load with a capacitor. The action of this system depends on the fact that

the capacitor stores energy during the conduction period and delivers this energy to

the loads during the no conducting period. In this way, the time during which the

current passes through the load is prolonging Ted, and the ripple is considerably

decreased. The action of the capacitor is shown with the help of waveform.

Figure (3.3.3) Smoothing action of capacitor

Figure (3.3.4) Waveform of the rectified output smoothing

3.4 Voltage Regulator:

Voltage Regulator eliminates ripple by setting DC output to a fixed voltage.

Voltage regulator ICs are available with fixed (typically 5V, 12V and 15V) or





variable output voltages.Many of the fixed voltage regulator ICs has 3 leads (input,

output and high impedance). Zener diode is an example of fixed regulator which is

shown here.

Figure (3.4) Voltage Regulator

3.5 POWER SUPPLY CIRCUIT:

Fig (3.5) POWER SUPPLY CIRCUIT

3.5 SUMMARY:

We here by conclude that most of the digital logic circuits and processors need

a 5 volt power supply. To use these parts we include a regulated 5 volt source.

Unregulated power supply will be ranging from 9 volts to 24 volts DC . So,to make a

5 volt power supply, we use a LM7805 voltage regulator IC .





CHAPTER 4

ULTRASONIC SENSOR

4.1 INTRODUCTION :

Parallax's PING)))™ ultrasonic sensor provides a very low-cost and easy

method of distance measurement. This sensor is perfect for any number of

applications that require you to perform measurements between moving or stationary

objects. Naturally, robotics applications are very popular but you'll also find this

product to be useful in security systems or as an infrared replacement if so desired.

FIG (4.1) ULTRASONIC SENSOR

This measures distance using sonar; an ultrasonic (well above human hearing) pulse is

transmitted from the unit and distance-to-target is determined by measuring the time

required for the echo return. Output from the PING))) sensor is a variable-width pulse

that corresponds to the distance to the target.

PING)))™ Sensor Features:

• The PING))) has only has 3 connections, which include Vdd, Vss, and 1 I/O

pin.

• The 3-pin header makes it easy to connect using a servo extension cable, no

soldering required.

• Several sample codes are available using the Ping))) sensor.

List of technical specifications:

• Range - 2cm to 3m (~.75" to 10')

• Supply Voltage: 5V +/-10% (Absolute: Minimum 4.5V, Maximum 6V)

• Supply Current: 30 mA typ; 35 mA max

• 3-pin interface (power, ground, signal)

• 20 mA power consumption





• Narrow acceptance angle

• Simple pulse in / pulse out communication

• Indicator LED shows measurement in progress

• Input Trigger - positive TTL pulse, 2 µs min, 5 µs typ.

• Echo Pulse - positive TTL pulse, 115 µs to 18.5 ms

• Echo Hold-off - 750 µs from fall of Trigger pulse

• Burst Frequency - 40 kHz for 200 µs

• Size - 22 mm H x 46 mm W x 16 mm D (0.85 in x 1.8 in x 0.6 in)

Ultrasonic sensors (also known as transducers when they both send and receive)

work on a principle similar to radar or sonar which evaluate attributes of a target by

interpreting the echoes from radio or sound waves respectively. Ultrasonic sensors

generate high frequency sound waves and evaluate the echo which is received back by

the sensor. Sensors calculate the time interval between sending the signal and

receiving the echo to determine the distance to an object.

This technology can be used for measuring: wind speed and direction

(anemometer), fullness of a tank, and speed through air or water. For measuring speed

or direction a device uses multiple detectors and calculates the speed from the relative

distances to particulates in the air or water. To measure the amount of liquid in a tank,

the sensor measures the distance to the surface of the fluid. Further applications

include: humidifiers, sonar, medical ultrasonography, burglar alarms, and non-

destructive testing. Systems typically use a transducer which generates sound waves

in the ultrasonic range, above 20,000 hertz, by turning electrical energy into sound,

then upon receiving the echo turn the sound waves into electrical energy which can be

measured and displayed. The technology is limited by the shapes of surfaces and the

density or consistency of the material. For example foam on the surface of a fluid in a

tank could distort a reading.


http://en.wikipedia.org/wiki/Humidifier

http://en.wikipedia.org/wiki/Sonar

http://en.wikipedia.org/wiki/Medical_ultrasonography

http://en.wikipedia.org/wiki/Non-destructive_testing


http://en.wikipedia.org/wiki/Humidifier

http://en.wikipedia.org/wiki/Sonar

http://en.wikipedia.org/wiki/Medical_ultrasonography






Transducers:

An ultrasonic transducer is a device that converts energy into ultrasound , or

sound waves above the normal range of human hearing. While technically a

dog whistle is an ultrasonic transducer that converts mechanical energy in the

form of air pressure into ultrasonic sound waves, the term is more apt to be used

to refer to piezoelectric transducers that convert electrical energy into sound.

Piezoelectric crystals have the property of changing size when a voltage is

applied, thus applying an alternating current ( AC ) across them causes them to

oscillate at very high frequencies, thus producing very high frequency sound

waves.

Detectors

Since piezoelectric crystal generate a voltage when force is applied to them, the same

crystal can be used as an ultrasonic detector. Some systems use separate transmitter

and receiver components while others combine both in a single piezoelectric

transceiver.

Alternative methods for creating and detecting ultrasound include magnetostriction

and capacitive actuation.

4.2 Applications of Ultrasonic sensors:

Use in medicine:

Medical ultrasonic transducers (probes) come in a variety of different shapes

and sizes for use in making pictures of different parts of the body. The transducer may

be passed over the surface of the body or inserted into an body opening such as the

rectum or vagina. Clinicians who perform ultrasound-guided procedures often use a

probe positioning system to hold the ultrasonic transducer.

Use in industry;

Ultrasonic sensors are used to detect the presence of targets and to measure the

distance to targets in many automated factories and process plants. Sensors with an on


http://en.wikipedia.org/wiki/Ultrasound

http://en.wikipedia.org/wiki/Dog_whistle

http://en.wikipedia.org/wiki/Piezoelectricity


http://en.wikipedia.org/wiki/Voltage


http://en.wikipedia.org/wiki/AC

http://en.wikipedia.org/wiki/Magnetostriction

http://en.wikipedia.org/wiki/Capacitor

http://en.wikipedia.org/wiki/Actuator

http://en.wikipedia.org/wiki/Body_opening

http://en.wikipedia.org/wiki/Rectum

http://en.wikipedia.org/wiki/Vagina

http://en.wikipedia.org/wiki/Probe_positioning_system

http://en.wikipedia.org/wiki/Ultrasound

http://en.wikipedia.org/wiki/Dog_whistle



http://en.wikipedia.org/wiki/AC

http://en.wikipedia.org/wiki/Magnetostriction

http://en.wikipedia.org/wiki/Capacitor

http://en.wikipedia.org/wiki/Actuator

http://en.wikipedia.org/wiki/Body_opening

http://en.wikipedia.org/wiki/Rectum

http://en.wikipedia.org/wiki/Vagina

http://en.wikipedia.org/wiki/Probe_positioning_system




or off digital output are available for detecting the presence of objects, and sensors

with an analog output which varies proportionally to the sensor to target separation

distance are commercially available.

Ultrasonic sensors are commonly used for a wide variety of noncontact presence,

proximity, or distance measuring applications. These devices typically transmit a

short burst of ultrasonic sound toward a target, which reflects the sound back to the

sensor. The system then measures the time for the echo to return to the sensor and

computes the distance to the target using the speed of sound in the medium.

Wavelength of Sound as a Function of Sound Speed and Frequency:

The wavelength of sound changes as a function of both the speed of sound and

the frequency, as shown by the expression:

λ = c/f

where:

λ = wavelength

C= speed of light

F = frequency

Figure 2 is a plot of the wavelength of sound as a function of frequency at room

temperature in air.

FIG (4.2) PLOT OF WAVE LENGTH VS FREQUENCY

BackgroundNoise





The level of background ultrasonic noise diminishes as the frequency

increases. The reason is that less noise at the higher frequencies is produced in the

environment, and the noise that is produced is greatly attenuated as it travels through

the air.






4.3 Ultrasonic sensor drive circuit:

The inverter is used for the drive of the ultrasonic sensor. The two inverters are

connected in parallel because of the transmission electric power increase.

The phase with the voltage to apply to the positive terminal and the negative terminal of the

sensor has been 180 degrees shifted. Because it is cutting the direct current with the capacitor,

about twice of Voltage of the inverter output are applied to the sensor.

Signalamplificationcircuit:

Generally, the positive and the negative power supply are used for the operational amplifier.

The circuit this time works with the single power supply of +9 V. Therefore, for the positive

input of the operational amplifiers, the half of the power supply voltage is appied as the bias

voltage and it is made 4.5 V in the central voltage of the amplified alternating current signal.

When using the operational amplifier with the negative feedback, the voltage of the positive

input terminal and the voltage of the negative input terminal become equal approximately. So,

by this bias voltage, the side of the positive and the side of the negative of the alternating

current signal can be equally amplified. When not using this bias voltage, the distortion causes

the alternating current signal.

Signal detector:

This circuit is the circuit which detects the ultrasonic which returned from the

measurement object. The output of the detection circuit is detected using the comparator. At the

circuit this time, the operational amplifier of the single power supply is used instead of the

comparator. The operational amplifier amplifies and outputs the difference between the

positive.

In case of the operational amplifier which doesn't have the negative feedback, at a

little input voltage, the output becomes the saturation state. So, when the positive input

becomes higher a little than the negative input, the difference is tens of thousands of times

amplified and the output becomes the same as the power supply almost. (It is the saturation

state) Oppositely, when the positive input becomes lower a little than the negative input, the

difference is tens of thousands of times amplified and the output becomes 0 V almost.(It is in

the OFF condition) This operation is the same as the operation of the comparator. However,

because the inner circuit is different about the comparator and the operational amplifier, the

comparator can not be used .

CHAPTER 5

LCD DISPLAY CIRCUIT5.1 INTRODUCTION:

The alphanumeric 16 character X 2line LCD requires 8data lines and also 3 control

signals and they are interfaced to 3664.By using 2 ports, port 0&3 data pins are connected to

LCD as data bus. Port0 can be basically used as I/O port i n be programmed as an input




APPENDIX

THEORY OF MICROCONTROLLER

A micro-controller contains a CPU, clock circuitry, ROM, Ram and I/O

circuitry on a single integrated circuit package. The Micro-controller is therefore, a

self-contained device, which does not require a host of associated support chips for its

operation as conventional microprocessors do. It offers several advantages over

conventional multi-chip systems. There is a cost and space advantage as extra chip

costs and printed circuit board and connectors required to support multi-chip systems

are eliminated. The other advantages include cheaper maintenance, decreased

hardware design effort and decreased board density, which is relevant in portable

control equipment. The microcontroller that we use is AT89S52.

Memory Organization of Microcontroller:

The total memory of 89S52 system is logically divided in Program memoryand Data memory. Program memory stores the programs to be executed, while data

memory stores the data like intermediate results, variables and constants required for

the execution of the program. Program memory is invariably implemented using

EPROM, because it stores only program code which is to be executed and thus it need

not be written into. However, the data memory may be read from or written to and

thus it is implemented using RAM.

Further, the program memory and data memory both may be categorized as

on-chip (internal) and external memory, depending upon whether the memory

physically exists on the chip or it is externally interfaced. The 89S52 can address

8Kbytes on-chip memory whose map starts from 0000H and ends at 1FFFH. It can

address 64Kbytes of external program memory under the control of PSEN (low)

signal.

The AT89S52 implements 256 bytes of on-chip RAM. The upper 128 bytes

occupy a parallel address space to the Special Function Registers. That means the

upper 128bytes have the same addresses as the SFR space but are physically separate


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Gnd vcc preset rs rw en d0 d1 d2 d3 d4 d5 d6 d7 vcc gnd

LCD DISPLAY




from SFR space. When an instruction accesses an internal location above address

7FH, the address mode used in the instruction specifies whether the CPU accesses the

upper 128 bytes of RAM or the SFR space. Instructions that use direct addressing

access SFR space.

2.2 PIN CONFIGURATION:





FIG (2.2) PIN DIAGRAM OF 89S52 IC

2.3 PIN DESCRIPTION:

VCC

Pin 40 provides Supply voltage to the chip. The voltage source is +5v





GND.

Pin 20 is the grounded

Port 0

Port 0 is an 8-bit open drain bidirectional I/O port from pin 32 to 39. 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 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 bidirectional I/O port with internal pull-ups from pin 1 to 8.

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.In addition, P1.0 and P1.1 can be configured to be the timer/counter 2 external

count input (P1.0/T2) and the timer/counter 2 trigger input (P1.1/T2EX), respectively,

as shown in following table.

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

and program verification.

TABLE (2.3.1) PORT 1 FUNCTIONS

Port 2

Port 2 is an 8-bit bidirectional I/O port with internal pull-ups from pin 21 to

28. The Port 2 output buffers can sink / source four TTL inputs. When 1s are written

to Port 2 pins they are pulled high by the internal pull-ups and can be used as inputs.





As inputs, Port 2 pins that 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 memory that uses 16-bit addresses

(MOVX @ DPTR). In this application it uses strong internal pull-ups when emitting

1s. During accesses to external data memory that uses 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.

Port 3

Port 3 is an 8-bit bidirectional I/O port with internal pull-ups from pin 10 to

17. The Port 3 output buffers can sink / source four TTL inputs. When 1s are written

to Port 3 pins they are pulled high by the internal pull-ups and can be used as inputs.

As inputs, Port 3 pins that are externally being pulled low will source current (IIL)

because of the pull-ups.

TABLE (2.3.2) PORT 3 FUNCTIONS

RST

Pin 9 is the Reset input. It is active high. Upon applying a high pulse to this

pin, the microcontroller will reset and terminate all activities. A high on this pin for

two machine cycles while the oscillator is running resets the device.





ALE/PROG

Address Latch is an output pin and is active high. Address Latch Enable

output pulse for latching the low byte of the address during accesses to externalmemory. This pin is also the program pulse input (PROG) during Flash programming.

In normal operation ALE is emitted at a constant rate of 1/6 the 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. 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 AT89C52 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

when 12-volt programming is selected.

XTAL1

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

operating circuit.





XTAL2

Output 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 Figure

5.3. 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 .

FIGURE (2.3.1) CRYSTAL CONNECTIONS

FIGURE( 2.3.2) EXTERNAL CLOCK DRIVE CONFIGURATION

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.


http://www.howsstuffworks.com/

http://www.electronicsforu.com/

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IDLE MODE

In idle mode, the CPU puts itself to sleep while all the on-chip peripherals

remain active. The mode is invoked by software. The content of the on-chip RAM and

all the special functions registers remain unchanged during this mode. The idle mode

can be terminated by any enabled interrupt or by a hardware reset. It should be noted

that when idle is terminated by a hardware reset, the device normally resumes

program execution, from where it left off, up to two machine cycles before the

internal reset algorithm takes control.

On-chip hardware inhibits access to internal RAM in this event, but access to

the port pins is not inhibited. To eliminate the possibility of an unexpected write to a

port pin when Idle is terminated by reset, the instruction following the one that

invokes Idle should not be one that writes to a port pin or to external memory.

POWER DOWN MODE

In the power down mode the oscillator is stopped, and the instruction that

invokes power down is the last instruction executed. The on-chip RAM and Special

Function Registers retain their values until the power down mode is terminated. The

only exit from power down is a hardware reset. Reset redefines the SFRs but does not

change the on-chip RAM. The reset should not be activated before VCC is restored to

its normal operating level and must be held active long enough to allow the oscillator

to restart and stabilize.

Table (2.3.3) Status of External Pins During Idle and Power Down Mode

2.4 TIMERS:

• Timer 0 and 1





Timer 0 and Timer 1 in the AT89S52 operate the same way as Timer 0 and

Timer 1 in the AT89S52. Register pairs (TH0, TL1), (TH1, TL1) are the 16-bit

counter registers for timer/counters 0 and 1.

• Timer 2

Timer 2 is a 16-bit Timer/Counter that can operate as either a timer or an event

counter. The type of operation is selected by bit C/T2 in the SFR T2CON. Timer 2

has three operating modes: capture, auto-reload (up or down counting), and baud rate

generator. The modes are selected by bits in T2CON, as shown in Table 5.2. Timer 2

consists of two 8-bit registers, TH2 and TL2. In the Timer function, the TL2 register

is incremented every machine cycle. Since a machine cycle consists of 12 oscillator

periods, the count rate is 1/12 of the oscillator frequency.

Table (2.4) Timer 2 Operating Modes

In the Counter function, the register is incremented in response to a 1-to-0

transition at its corresponding external input pin, T2. In this function, the external

input is sampled during S5P2 of every machine cycle. When the samples show a high

in one cycle and a low in the next cycle, the count is incremented. The new count

value appears in the register during S3P1 of the cycle following the one in which the

transition was detected. Since two machine cycles (24 oscillator periods) are required

to recognize a 1-to-0 transition, the maximum count rate is 1/24 of the oscillator

frequency. To ensure that a given level is sampled at least once before it changes, the

level should be held for at least one full machine cycle.

There are no restrictions on the duty cycle of external input signal, but it

should for at least one full machine to ensure that a given level is sampled at least

once before it changes.





2.5 INTERRUPTS:

The AT89S52 has a total of six interrupt vectors: two external interrupts

(INT0 and INT1), three timer interrupts (Timers 0, 1, and 2), and the serial portinterrupt. These interrupts are all shown in Figure 2.5

FIGURE (2.5) INTERRUPTS SOURCE

Each of these interrupt sources can be individually enabled or disabled by

setting or clearing a bit in Special Function Register IE. IE also contains a global

disable bit, EA, which disables all interrupts at once.

Timer 2 interrupt is generated by the logical OR of bits TF2 and EXF2 in

register T2CON. Neither of these flags is cleared by hardware when the service

routine is vectored to. In fact, the service routine may have to determine whether it

was TF2 or EXF2 that generated the interrupt, and that bit will have to be cleared in

software.

The Timer 0 and Timer 1 flags, TF0 and TF1, are set at S5P2 of the cycle in

which the timers overflow.





Table (2.5) Interrupts Enable Register

2.6 MEMORY ORGANIZATION :

The total memory of 89S52 system is logically divided in Program memory

and Data memory. Program memory stores the programs to be executed, while data

memory stores the data like intermediate results, variables and constants required for

the execution of the program. Program memory is invariably implemented using

EPROM, because it stores only program code which is to be executed and thus it need

not be written into. However, the data memory may be read from or written to and

thus it is implemented using RAM.

Further, the program memory and data memory both may be categorized as

on-chip (internal) and external memory, depending upon whether the memory

physically exists on the chip or it is externally interfaced. The 89S52 can address

8Kbytes on-chip memory whose map starts from 0000H and ends at 1FFFH. It can

address 64Kbytes of external program memory under the control of PSEN (low)

signal.


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