APPENDIX I PIC 16F877 - MICROCONTROLLERshodhganga.inflibnet.ac.in/bitstream/10603/24134/14/14_appendix.pdf · APPENDIX I PIC 16F877 - MICROCONTROLLER A1.1 CONCEPTS OF MICROCONTROLLER

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APPENDIX I

PIC 16F877 - MICROCONTROLLER

A1.1 CONCEPTS OF MICROCONTROLLER

Microcontroller is a general purpose device, which integrates a

number of the components of a microprocessor system on to single chip. It

has inbuilt CPU, memory and peripherals to make it as a mini computer. A

microcontroller combines on to the same microchip:

1) The CPU core

2) Memory (both ROM and RAM)

3) Some parallel digital I/O

Microcontrollers will combine other devices such as:

1) A timer module to allow the microcontroller to perform tasks

for certain time periods.

2) A serial I/O port to allow data to flow between the controller

and other devices such as a PIC or another microcontroller.

3) An ADC to allow the microcontroller to accept analogue input

data for processing.

119

Microcontrollers are :

1) Smaller in size

2) Consumes less power

3) Inexpensive

Micro controller is a stand alone unit ,which can perform functions

on its own without any requirement for additional hardware like I/O ports

and external memory. The heart of the microcontroller is the CPU core.

In the past, this has traditionally been based on a 8-bit

microprocessor unit. For example Motorola uses a basic 6800 microprocessor

core in their 6805/6808 microcontroller devices. In the recent years,

microcontrollers have been developed around specifically designed CPU

cores, for example the microchip PIC range of microcontrollers.

A1.2 INTRODUCTION TO PIC

The microcontroller that has been used for this project is from PIC

series. PIC microcontroller is the first RISC based microcontroller fabricated

in Complimentary Metal Oxide Semiconductor (CMOS) that uses separate

bus for instruction and data allowing simultaneous access of program and data

memory.

The main advantage of CMOS and RISC combination is low power

consumption resulting in a very small chip size with a small pin count. The

main advantage of CMOS is that it has immunity to noise than other

fabrication techniques.

120

A1.2.1 Memory

Various microcontrollers offer different kinds of memories.

EEPROM, EPROM, FLASH etc. are some of the memories of which FLASH

is the most recently developed.

Technology that is used in PIC 16F87x is flash technology, so that

data is retained even when the power is switched off. Easy Programming

and Erasing are other features of PIC 16F87x.

A1.2.2 PIC Start Plus Programmer

The PIC start plus development system from microchip technology

provides the product development engineer with a highly flexible low cost

microcontroller design tool set for all microchip PIC micro devices. The PIC

start plus development system includes PIC start plus development

programmer.

The PIC start plus programmer gives the product developer ability

to program user software in to any of the supported microcontrollers. The PIC

start plus software running under MP Lab provides for full interactive control

over the programmer.

A1.2.3 Special Features of PIC Microcontroller

High-performance RISC CPU

Only 35 single word instructions to learn

All single cycle instructions except for program branches

which are two cycle

Operating speed: DC - 20 MHz clock input

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DC - 200 ns instruction cycle

Up to 8K x 14 words of Flash Program Memory

Up to 368 x 8 bytes of Data Memory (RAM)

Up to 256 x 8 bytes of EEPROM data memory

Pin out compatible to the PIC16C73/74/76/77

Interrupt capability (up to 14 internal/external

Eight level deep hardware stack

Direct, indirect, and relative addressing modes

Power-on Reset (POR)

Power-up Timer (PWRT) and Oscillator Start-up Timer

(OST)

Watchdog Timer (WDT) with its own on-chip RC Oscillator

for reliable operation

Programmable code-protection

Power saving SLEEP mode

Selectable oscillator options

Low-power, high-speed CMOS EPROM/EEPROM

technology

Fully static design

In-Circuit Serial Programming (ICSP) via two pins

Only single 5V source needed for programming capability

In-Circuit Debugging via two pins

Processor read/write access to program memory

Wide operating voltage range: 2.5V to 5.5V

122

High Sink/Source Current: 25 mA

Commercial and Industrial temperature ranges

Low-power consumption:

< 2mA typical @ 5V, 4 MHz

20mA typical @ 3V, 32 kHz

< 1mA typical standby current

A1.2.4 Peripheral Features

Timer0: 8-bit timer/counter with 8-bit prescaler

Timer1: 16-bit timer/counter with prescaler, can be incremented

during sleep via external crystal/clock

Timer2: 8-bit timer/counter with 8-bit period register, prescaler

and postscaler

Two Capture, Compare, PWM modules

Capture is 16-bit, max resolution is 12.5 ns,

Compare is 16-bit, max resolution is 200 ns,

PWM max. resolution is 10-bit

10-bit multi-channel Analog-to-Digital converter

Synchronous Serial Port (SSP) with SPI. (Master Mode) and

I2C. (Master/Slave)

Universal Synchronous Asynchronous Receiver Transmitter

(USART/SCI) with

9- bit address detection.

Brown-out detection circuitry for Brown-out Reset (BOR)

123

Note

1. This buffer is a Schmitt Trigger input when configured as an

external interrupt

2. This buffer is a Schmitt Trigger input when used in serial

programming mode.

3. This buffer is a Schmitt Trigger input when configured as general

purpose I/O and a TTL input when used in the Parallel Slave Port

mode (for interfacing to a microprocessor bus).

4. This buffer is a Schmitt Trigger input when configured in RC

oscillator mode and a CMOS input otherwise.

Legend: I = input O = output I/O = input/output P = power

— = Not used TTL = TTL input ST = Schmitt Trigger input

A1.2.5 I/O Ports

Some pins for these I/O ports are multiplexed with an alternate

function for the peripheral features on the device. In general, when a

peripheral is enabled, that pin may not be used as a general purpose I/O pin.

A1.2.6 Porta and the Trisa Register

PORTA is a 6-bit wide bi-directional port. The corresponding data

direction register is TRISA. Setting a TRISA bit (=1) will make the

corresponding PORTA pin an input, i.e., put the corresponding output driver

in a Hi-impedance mode. Clearing a TRISA bit (=0) will make the

corresponding PORTA pin an output, i.e., put the contents of the output latch

on the selected pin. Reading the PORTA register reads the status of the pins

whereas writing to it will write to the port latch. All write operations are read-

modify-write operations. Therefore a write to a port implies that the port pins

124

are read; this value is modified, and then written to the port data latch. Pin

RA4 is multiplexed with the Timer0 module clock input to become the

RA4/T0CKI pin. The RA4/T0CKI pin is a Schmitt Trigger input and an open

drain output. All other RA port pins have TTL input levels and full CMOS

output drivers. Other PORTA pins are multiplexed with analog inputs and

analog VREF input.

The operation of each pin is selected by clearing/setting the control

bits in the ADCON1 register (A/D Control Register1).

The TRISA register controls the direction of the RA pins, even

when they are being used as analog inputs. The user must ensure the bits in

the TRISA register are maintained set when using them as analog inputs.

Table A1.1 PORTA Function

Name Bit# Buffer Function

RA0/AN0 bit0 TTL Input/output or analog input



RA3/AN3/VREF bit3 TTL Input/output or analog input or

VREF

RA4/T0CKI bit4 SL Input/output or external clock

input for Timer0 Output is open

drain type

RA5/SS/AN4 Bit5 TTL Input/output or slave select input

for synchronous serial port or

analog input

Legend: TTL = TTL input, ST = Schmitt Trigger input

125

Table A1.2 Summary of Registers Associated with PORTA

Address Name Bit7

Bit6

Bit5

Bit4

Bit3

Bit2

Bit1

Bit0

Valueon:POR,BOR

Value onall otherresets

05h PORTA - - RA5 RA4 RA3 RA2 RA1 RA0 --0x 0000 -0u 000085h TRISA - - PORTA Data Direction Register -11 1111 --1111119fh AdCON1 AD

FM- PCF

G3PCFG2

PCFG1

PCFG0

-0- 0000 --0- 0000

Legend: x = unknown, u = unchanged, - = unimplemented locations

read as '0'. Shaded cells are not used by PORTA.

A.1.2.7 PORTB and TRISB Register

PORTB is an 8-bit wide bi-directional port. The corresponding data

direction register is TRISB. Setting a TRISB bit (=1) will make the

corresponding PORTB pin an input, i.e., put the corresponding output driver

in a hi-impedance mode. Clearing a TRISB bit (=0) will make the

corresponding PORTB pin an output, i.e., put the contents of the output latch

on the selected pin. Three pins of PORTB are multiplexed with the Low

Voltage Programming function; RB3/PGM, RB6/PGC and RB7/PGD.

The alternate functions of these pins are described in the Special

Features Section. Each of the PORTB pins has a weak internal pull-up. A

single control bit can turn on all the pull-ups.

This is performed by clearing bit RBPU (OPTION_REG<7>). The

weak pull-up is automatically turned off when the port pin is configured as an

output. The pull-ups are disabled on a Power-on Reset.

Four of PORT B’s pins, RB7:RB4, have an interrupt on change

feature. Only pins configured as inputs can cause this interrupt to occur

126

(i.e. any RB7:RB4 pin configured as an output is excluded from the interrupt

on change comparison).

1) The input pins (of RB7:RB4) are compared with the old value

latched on the last read of PORTB. The “mismatch” outputs of

RB7:RB4 are OR’ed together to generate the RB Port Change

Interrupt with flag bit RBIF (INTCON<0>).

2) This interrupt can wake the device from SLEEP. The user, in

the interrupt service routine, can clear the interrupt in the

following manner:

Any read or write of PORTB. This will end the mismatch

condition.

Clear flag bit RBIF. A mismatch condition will continue to

set flag bit RBIF.

Reading PORTB will end the mismatch condition, and

allow flag bit RBIF to be cleared.

The interrupt on change feature is recommended for wake-

up on key depression operation and operations where

PORTB is only used for the interrupt on change feature.

Polling of PORTB is not recommended while using the interrupt on

change feature.

This interrupt on mismatch feature, together with software

configurable pull-ups on these four pins, allow easy interface to a keypad and

make it possible for wake-up on key depression.

127

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128

A1.2.8 PORTC and the TRISC Register

PORTC is an 8-bit wide bi-directional port. The corresponding datadirection register is TRISC. Setting a TRISC bit (=1) will make thecorresponding PORTC pin an input, i.e., put the corresponding output driverin a hi-impedance mode. Clearing a TRISC bit (=0) will make thecorresponding PORTC pin an output, i.e., put the contents of the output latchon the selected pin. PORTC is multiplexed with several peripheral functions.PORTC pins have Schmitt Trigger input buffers.

Table A.1.5 PORTC Functions

Name Bit# BufferType Function

RC0/T1OSO/T1CKI MO ST Input/output port pin or Timerl oscillatoroutput/Timer 1 clock input

RC1/T10SI/CCP2 Ml ST Input/output port pin or Timerloscillator input or Capture2 input'Compare2 output/PWM2 output

RC2/CCP1 m ST Input/output port pin or Capture 1input/Compare! output/PWMI output

RC3/SCK/SCL m ST RC3 can also be the synchronous serialclock for both SPI and l2C modes.

RC4/SDI/SDA m ST RC4 can also be the SPI Data In (SPImode) or data I/O (l2C mode).

RC5/SD0 bit5 ST Input/output port pin or SynchronousSerial Port data output

RC6TOCK bite ST Input/output port pin or USARTAsynchronous Transmit or SynchronousClock

RC7/RX/DT ST Input/output port pin or USARTAsynchronous Receive or SynchronousData

Legend : ST = Schmitt Trigger input

When the I2C module is enabled, the PORTC (3:4) pins can beconfigured with normal I2C levels or with SMBUS levels by using the CKEbit (SSPSTAT <6>). When enabling peripheral functions, care should betaken in defining TRIS bits for each PORTC pin.

129

130

Some peripherals override the TRIS bit to make a pin an output, while otherperipherals override the TRIS bit to make a pin an input. Since the TRIS bitoverride is in effect while the peripheral is enabled, read-modify writeinstructions (BSF, BCF, XORWF) with TRISC as destination should beavoided. The user should refer to the corresponding peripheral section for thecorrect TRIS bit settings.

A1.2.9 PORTD and TRISD Registers

This section is not applicable to the 28-pin devices. PORTD is an 8-

bit port with Schmitt Trigger input buffers. Each pin is individually

configurable as an input or output. PORTD can be configured as an 8-bit wide

microprocessor Port (parallel slave port) by setting control bit PSPMODE

(TRISE<4>). In this mode, the input buffers are TTL.

Table A1.7 PORTD Functions

Name Bit# BufferType

Function

RDO/PSPO bitO 5T/TTL11' Input-output port pin or parallel slave port bit0RD1/PSP1 bill ST>TTL|1> Input-output port p n or parallel slave port bit1RD2/PSP2 bit2 Input/output port pin or parallel slave port bit2RD3/PSP3 W STfTTL'1' Input/output port pin or parallel slave port bit3RD4.PSP4 m ST;n|1) Input-output port pin or parallel slave port bit4RD5/PSP5 biti Input-output port pin or parallel slave port bitsRD6/PSP6 bite ST/TTL*1' Input-output port pin or parallel slave port bitsRD7/PSP7 bit7 STTL111 Input-output port p:n or parallel slave port bit7Legend : St = Schmitt Trigger input TTL = TTL input

Note : 1: Input buffers are Schmitt Triggers when in I/O mode and TTL buffer

when in parallel Slave Port Mode

131

132

A1.2.10 PORTE and TRISE Register

PORTE has three pins RE0/RD/AN5, RE1/WR/AN6 and

RE2/CS/AN7, which are individually configurable as inputs or outputs. These

pins have Schmitt Trigger input buffers.

The PORTE pins become control inputs for the microprocessor port

when bit PSPMODE (TRISE<4>) is set. In this mode, the user must make sure

that the TRISE<2:0> bits are set (pins are configured as digital inputs). Ensure

ADCON1 is configured for digital I/O. In this mode the input buffers are TTL.

PORTE pins are multiplexed with analog inputs. When selected as

an analog input, these pins will read as '0's. TRISE controls the direction of

the RE pins, even when they are being used as analog inputs. The user must

make sure to keep the pins configured as inputs when using them as analog

inputs.

Table A1.9 PORTE Functions

Name Bitff Buffer

Type

Function

RE0/RD/AN5 bis ST..'TTL(1) Input/output port pin or read control input

in parallel slave port mode or

analog input:

RD

1 = Not a read operation

0 = Read operation. Reads PORTD

register (if chip selected)

133

Table A1.9 (Continued)

Name Bitff Buffer

Type

Function

RE1/VVR/AN6 bid ST/TTL'1' Input-output port pin or write control input

in parallel slave port mode or

analog input:

WR

1 =Not a write operation

0 ='vVrite operation. Writes PORTD

register (if chip selected)

RE2/CS/AN7 bit2 ST/TTL<1> Input/output port pin or chip select control

input in parallel slave port

mode or analog input:

OS

1 = Device is not selected 0 = Device is

selected

Legend : St = Schmitt Trigger input TTL = TTL input

Note :1: Input buffers are Schmit Triggers when in I/O mode and TTL

buffer when in parallel Slave Port Mode

134

135

A1.2.11 Memory Organisation

There are three memory blocks in each of the PIC16F87x MUC’s.

The program memory and Data Memory have separate buses so that

concurrent access can occur.

All instructions are executed within one single instruction cycle,

unless a conditional test is true or the program counter is changed as a result

of an instruction.

1) In this case, the execution takes two instruction cycles with

the second cycle executed as a NOP.

2) One instruction cycle consists of four oscillator periods.

3) Thus, for an oscillator frequency of 4 MHz, the normal

instruction execution time is 1 ms.

If a conditional test is true or the program counter is changed as a

result of an instruction, then the instruction execution time is 2 ms.

136

APPENDIX 2

PROGRAMMING

The Assembly program is written using Keil software for

displaying triggering pulse frequency measurement given to the MOSFET for

producing variable voltage:

sbit LCD_RS at RB4_bit;

sbit LCD_EN at RB5_bit;

sbit LCD_D4 at RB0_bit;




sbit LCD_RS_Direction at TRISB4_bit;

sbit LCD_EN_Direction at TRISB5_bit;

sbit LCD_D4_Direction at TRISB0_bit;




char txt1[] = "T=";

char txt2[] = "Sec";

char i;

void Move_Delay() {

Delay_ms(500);

137

}

void main() {

unsigned char value,value1,g,h,TE,TE1;

TRISC=0X01;

TRISD=0x00;

TRISB=0x00;

T1CON=0X00;

Lcd_Init(); // Initialize LCD

Lcd_Cmd(_LCD_CLEAR); // Clear display

Lcd_Cmd(_LCD_BLINK_CURSOR_ON); // Cursor off

Lcd_Out(1,1,txt1); // Write text in first row

Lcd_Out(1,6,txt2);

Delay_ms(200);

while(1)

{

if(PIR1.TMR1IF==1)

{

TMR1H =0x00;

TMR1L=0X00;

T1CON.TMR1ON=0;

}

if(PIR1.TMR1IF==0)

{

/*value=TMR1L ;

h=value & 0x0f;

g=value& 0xf0;

g=g>>4;*/

/*PORTB=h|0x30;

delay_ms(10);

138

PORTB=g|0x30;*/

value1=TMR1l;

h=value1 & 0x0f;

g=value1 & 0xf0;

g=g>>4;

TE=g|0x30;

TE1=h|0x30;

Lcd_Out(1,3,TE);

Delay_ms(200);

Lcd_Out(1,4,TE1);

Delay_ms(200);

/*PORTD=h|0x30;

delay_ms(10);

PORTD=g|0x30;*/

}

T1CON.TMR1ON=1;

PIR1.TMR1IF=0;

}

}

In ASM

Just to count the pulse alone

LIST p=16F877 ; PIC16F844 is the target processor

#include "P16F877.INC" ; Include header file

BSF STATUS,RP0

MOVLW 0X01

MOVWF TRISC

MOVLW 0X00

MOVWF TRISD

MOVWF TRISB

139

BCF STATUS,RP0

MOVLW 0X00

MOVWF T1CON

HERE:MOVLW 0X00

MOVWF TMR1H

MOVLW 0X00

MOVWF TMR1L

BCF PIR1,TMR1IF

BSF T1CON,TMR1ON

AGAIN: MOVF TMR1H,W

MOVWF PORTD

MOVF TMR1L,W

MOVWF PORTB

BTFSS PIR1,TMR1IF

GOTO AGAIN

BCF T1CON,TMR1ON

GOTO HERE

END

140

APPENDIX 3

LM35 TEMPERATURE SENSOR

Basic centigrade temperature sensor(+2oC to +150oC)

Full-Range centigrade temperature sensor

141

142

143

144

145

146

147

APPENDIX 4

LM555 TIMER

148

149

150

Pulse Position Modulator: This application uses the timer connected for

astable operation as shown in figure, with a modulating signal again applied to the

control voltage terminal. The pulse position varies with the modulating signal, since

the threshold voltage and hence the time delay is varied. Figure shows the

waveforms generated for a triangle wave modulation signal.

151

APPENDIX 5

DESIGN AND SPECIFICATIONS OF HIGH VOLTAGE

FERRITE CORE TRANSFORMER

(Reference Book: ”A Course in Electrical MachineDesign”,by A.K.Sawhney,

6th edition 2006,Publisher :DhanpatRai&Co.,Delhi,India.)

1. Core Design:The starting point in the design of small transformers is the choice ofturns per volt.Flux in the core

f = output frequency (Hz)The frequency of the transformer is specified and the value of turns pervolt ( ) is taken from taken from table.Net area of core

The value of maximum flux density ( ) is assumed to be 1 Wb/m2.Gross area of core (stacking factor is assumed to be 0.9)The core is made up of E pairs combination of stampings for thisdesign.Width of the central limbDepth of the core = width of the central limb.

2. Winding Design:2.1 Primary windingCurrent in the primary winding A

The efficiency of small transformers varies from 80 to 96 % can bechosen.Area of primary winding conductor mm2=

Current density ( ) =2.3 A/mm2.Diameter of bar conductor can be calculated from area of primarywinding.Using enameled conductors, from the table, the nearest standardconductor has

152

bare diameter (d) and the diameter of insulated conductor (d1) valuescan be taken.Space factor for primary winding

Area of primary conductor used mm2

Number of primary winding turns

Window space required by primary winding =

2.2 Secondary Winding: Secondary winding current ( ) is our estimated values in amps.Area of secondary winding conductor mm2=

Current density ( ) =2.3 A/mm2.Diameter of bar conductor can be calculated from area of primarywinding.Using enamelledconductors, from the table, the nearest standardconductor hasbare diameter (d) and the diameter of insulated conductor (d1) valuescan be taken.Space factor for secondary winding

Area of secondary conductor used mm2

Number of secondary winding turns

Window space required by secondary winding =

3. Stamping Size:Total window space required Space factor for primary&secondary winding) in mm2.Now to select a stamping which givesfrom stamping table,Width of the central limbUsing a combination of E pairs stampings. From the table we hadreferred the value of the window.Width of the windowHeight of the windowArea of the window providedInductance =

153

APPENDIX 6

SENSORS CHARACTERISTICS

Table A6.1 Sensors Characteristics

NTCThermistor Platinum RTD Thermocouple Semiconductor

SensorCeramic(metal-

oxide spinel)

Platinum wire-wound or metal

filmThermoelectric Semiconductor

Junction

TemperatureRange (typical)

-100 to+325 C -200 to +650 C -200 to +1750 C -70 to 150 C

Accuracy(typical) 0.05 to 1.5 C 0.1 to 1.0 C 0.5 to 5.0 C 0.5 to 5.0 C

Long-termStability @

100 C

0.2 C/year(epoxy)

0.02 C/year(glass)

0.05 C/year (film)0.002 C/year

(wire)

Variable, sometypes very prone

to aging>1 C/year

Output NTCResistance-4.4%/ C

typical

PTC resistance0.00385 /°C

Thermovoltage10 V to40 V/°C

Digital, variousOutputs

Linearity Exponential Fairly linear Most types non-linear Linear

PowerRequired

Constantvoltage

or current

Constant voltageor current Self-powered 4 to 30 VDC

Response TimeFast

0.12 to 10seconds

Generally slow1 to 50 seconds

Fast0.10 to 10seconds

Slow5 to 50 seconds

Susceptibilityto

ElectricalNoise

Rarelysusceptible

Highresistance

only

Rarely susceptibleSusceptible/Cold

junctioncompensation

Board layoutdependent

LeadResistance

Effects

Lowresistanceparts only

Very susceptible.3 or 4-wire

configurationsrequired

None over shortruns. TC

extension cablesrequired.

N/A

Cost Low tomoderate

Wire-wound –High

Film - LowLow Moderate

154

Table A6.2 Sensors Advantages and Disadvantages

NTCThermistor Platinum RTD Thermocouple Semiconductor

Sensor Ceramic (metal-oxide spinel)

Platinum wire-wound or metalfilm

Thermoelectric SemiconductorJunction

Advantages • Sensitivity• Accuracy• Cost• Rugged• Flexible• Packages• Hermetic Seal• Surface Mount

• Accuracy• Stability• Linearity

• Temperature• Range• Self-Powered• No Self-heat• Rugged

• Ease of Use• Board

Mounting• Rugged• Overall Cost

Disadvantages • Non-linearity• Self-heating• Moisture

failures (non-glass only)

• Leadresistanceerror

• Response time• Vibration

resistance• Size• Package• limitations

• Cold-junctioncompensation

• Accuracy• Stability• TC extension

leads

• Accuracy• Limited

applications• Stability• Response time

Each of the major types of sensors above differs in their basic theory

of operation. Temperature ranges vary for each sensor type. Accuracy

depends upon basic sensor characteristics. Generally the better the accuracy,

the higher the price.

Long-term stability is defined by how consistently a sensor

maintains its accuracy over time. Stability is dictated by basic physical

properties of the sensor. Stability is typically worsened by exposure to high

temperatures. Wire-wound platinum and glass encapsulated thermistors are

the most stable sensor types. Thermocouples and semiconductors are the least

stable.

Sensor outputs vary by type. Thermistors change resistance

inversely proportionally with temperature, thus the name negative

temperature coefficient (NTC). Base metals such as platinum have positive

155

temperature coefficients (PTC). Thermocouples have low milli-volt outputs

that change with temperature. Semiconductors are typically conditioned and

come in a variety of digital outputs.

Linearity defines how well over a range of temperature a sensor’s

output consistently changes. Thermistors are exponentially non-linear,

exhibiting a much higher sensitivity at low temperatures than at high

temperatures. Linearity of a sensor has become less of an issue over time, as

microprocessors are more widely used in sensor signal conditioning circuits.

When powering, both thermistors and platinum elements require

constant voltage or constant currents. Power regulation is important to limit

self-heat in either thermistors or platinum RTDs. Current regulation is not as

critical for semiconductors. Thermocouples generate a voltage output.

Response time, or how quickly a sensor indicates temperature, is

dependent on the size and mass of the sensor element (assuming no predictive

method is used). Semiconductors are the slowest responding. Platinum wire-

wound elements are next slowest. Platinum film, thermistors and

thermocouples are available in small packages, and thus have high-speed

options. Glass micro-beads are the fastest responding thermistor

configuration.

Electrical noise inducing errors in temperature indication is a

problem mostly with thermocouples. Thermistors with very high resistances

may present a problem in some cases.

Lead resistance may cause an error offset in resistive devices such as

thermistors or RTDs. This effect is more pronounced with low resistance

devices such as 100 platinum elements or low resistance thermistors. For

platinum, 3 or 4-wire lead configurations are used to eliminate the problem.

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For thermistors, typically choosing a higher resistance value eliminates the

effect. Thermocouples must use extension leads and connectors of the same

material as the leads themselves or an error may be introduced.

Although thermocouples are the least expensive and the most widely

used sensor, an NTC thermistor generally provides the greatest value for its

price. Each sensor type has advantages and disadvantages. For thermistors,

the major advantages are:

Sensitivity: This allows thermistors to sense very small changes in

temperature.

Accuracy: Thermistors offer both high absolute accuracy and

interchangeability.

Cost: For the high performance they offer, thermistors are very cost-

effective.

Ruggedness: Because of their construction, thermistors are very

rugged.

Flexibility: Thermistors can be configured into a wide variety of

physical forms, including very small packages.

Hermetic Seal: Glass encapsulation provides a hermetic package,

eliminating moisture induced sensor failure.

Surface Mount: A wide range of sizes and resistance tolerances are

available.

Of the thermistor disadvantages, typically only self-heating is a

design consideration. Proper care must be taken to limit the sensing current to

a low enough value that self-heat error is minimized to an acceptable value.

Non-linearity can be addressed by software or by circuitry, and

moisture induced failure by glass encapsulation.

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All sensors have specific advantages and disadvantages. For a

successful project, the key is to match the sensor capabilities with the

application. If you would like assistance in deciding if a thermistor is the best

design option, please contact MEAS application engineers.

Documents

APPENDIX I PIC 16F877 - MICROCONTROLLERshodhganga.inflibnet.ac.in/bitstream/10603/24134/14/14_appendix.pdf · APPENDIX I PIC 16F877 - MICROCONTROLLER A1.1 CONCEPTS OF MICROCONTROLLER