Chapter 6 - Hardware Interfacing

8/10/2019 Chapter 6 - Hardware Interfacing

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Chapter

HARDWARE INTERFACING


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Chapter Objectives

Serial Port Programming

Interrupt Programming

LCD interfacing

Keypad interfacing ADC, DAC and sensor interfacing

Relay interface and Motor Control


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The two most basic types of communication are serialandparallel.

They are so common that even the cabling use the name serial

cable andparallel cable.

Since electricity behaves according to the laws of physics, it is

impossible to get the electrical signal to go any faster.

There are two ways to get the data from one place to the other

faster. The first is to squish the data bits tighter together (leave less

distance between them when they travel down the wire). The

second way is to transmit more bits simultaneously.

Serial port programming


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Two ways of computer data transfer:

1. Parallel

- 8 or more wire line used to transfer data. E.g.

Printers & hard disk

2. Serial- data sent one bit at a time. E.g. data communication

between 2 computer.


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Serial Vs. Parallel

Parallel Serial

More wire use Less wire used

8 bit data transfer 1 bit data transfer

Fast data transmission Slow data transmissionExpensive Cheaper


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Serial versus Parallel Data Transfer


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Serial Port Basics (Synchronous vs.

asynchronous communication)

2 methods, asynchronous and synchronous synchronous method transfers a block of data

(characters) at a time

asynchronous method transfers a single byte at a time

Uses special IC chips called UART (universalasynchronous receiver-transmitter) and USART(universal synchronousasynchronous receiver-transmitter)

PIC chips has a built-in UART


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Fundamental of serial communication


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Simplex transmission

- Data is transmitted from the sender to receiver only

- The communication can only take place in one direction

and it is not possible for the receiver to send data back.


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Half-Duplex

&Full-Duplex


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Half- and full-duplex transmission

if the data can be transmitted and received, it is aduplex transmission

simplex transmissions the computer only sends data duplex transmissions can be half or full duplex

depends on whether or not the data transfer can besimultaneous

If one way at a time, it is half duplex If can go both ways at the same time, it is full duplex

full duplex requires two wire conductors for the datalines (in addition to the signal ground)


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Asynchronous serial communication and

data frame

data coming in 0s and 1s

to make sense of the data sender and

receiver agree on a set of rules

Protocol how the data is packed

how many bits/character

when the data begins and ends


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Start and stop bits

asynchronous method, each character isplaced between start and stop bits

called framing

start bit is always one bit

stop bit can be one or two bits

start bit is always a 0 (low) stop bit(s) is 1 (high)

LSB is sent out first Framing ASCII A (41H)


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in modern PCs one stop bit is standard

when transferring a text file of ASCII characters

using 1 stop bit there is total of 10 bits for eachcharacter

8 bits for the ASCII code (1 parity bit), 1 bit each forthe start and stop bits

for each 8-bit character there are an extra 2 bits,which gives 20% overhead


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Data transfer rate

rate of data transfer bps (bits per second) widely used terminology for bps is baud

rate

baud and bps rates are not necessarily

equal

baud rate is defined as the number of

signal changes per second


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RS232 standard using DB9 connector


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RS232 standards

most widely used serial I/O interfacing standard

input and output voltage levels are not TTL compatible 1 bit is represented by -3 to -25 V

0 bit is +3 to +25 V

-3 to +3 is undefined

to connect RS232 to a microcontroller system 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


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Data communication classification

DTE (data terminal equipment)

DCE (data communication equipment)

DTE - terminals and computers that send and receive data DCE - communication equipment responsible for

transferring the data

simplest connection between a PC and microcontroller

requires a minimum of three pins, TxD, RxD, and ground

Null Modem

Connection


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The use of MAX232 chips


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MAX232

converts from RS232 voltage levels to TTL voltage

levels uses a +5 V power source

MAX232 has two sets of line drivers for transferringand receiving data

line drivers used for TxD are called T1 and T2

line drivers for RxD are designated as R1 and R2 T1 and R1 are used together for TxD and RxD of the

PIC

second set is left unused


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Baud Rate in PIC18

The USART (Universal Synchronous Asynchronous Receiver) in

PIC18 has both synchronous and asynchronous features.

The asynchronous mode can be used to connect the PIC18-based

system to serial port for the purpose of full-duplex serial data

transfer.

6 major registers are associated with the UART are:

- SPBGR ( serial port baud rate generator)

- TXREG (transfer register)

- RCREG (receiver register)

- TXSTA (transmit status and control register)

- RCSTA (receive status and control register)

-PIR1 (peripheral interrupt request register 1)


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a. To transfer data serially


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b. To receive data serially


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Interrupt Programming

Interrupts are mechanisms which enable instantresponse to events such as counter overflow, pin

change, data received, etc.

In normal mode, microcontroller executes the main

program as long as there are no occurrences that

would cause an interrupt.

Upon interrupt, microcontroller stops the execution of

main program and commences the special part of the

program(ISR) which will analyze and handle the

interrupt.


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PIC can serve multiple devices using

mechanisms of

Polling PIC continuously monitors the status of each

device

Each device get the attention of the CPU as the

same level of priority Wastes u-Controllers time by polling devices that

do not need service.

Interrupt

Devices get the attention of the CPU only when it

needs a service

Can service many devices with different level of

priorities


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Why interrupts over polling? Because polling

Ties up the CPU in one activity

Uses cycles that could be used more effectively

Code cant be any faster than the tightest pollingloop

Bottom line: an interrupt is an asynchronoussubroutine call (triggered by a hardware event) thatsaves both the return address and the system status


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Interrupt service routine (ISR)

When an interrupt is

invoked the uC runs the

Interrupt Service

Routine(ISR)

Interrupt vector tableholds the address of ISRs

Power-on Reset 0000h

High priority interrupt 0008h

Low priority interrupt 0018h


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When an Interrupt Occurs

Finish the current instruction

Save minimal state information on stack

Transfer to the interrupt handler, also known as the interruptservice routine (ISR)


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Steps in executing Interrupts


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Upon activation of interrupt the microcontroller

Finishes executing the current instruction

Pushes the PC of next instruction in the stack

Jumps to the interrupt vector table to get the address of ISR

and jumps to it Begin executing the ISR instructions to the last instruction of

ISR (RETFIE)

Executes RETFIE

Pops the PC from the stack

Starts to execute from the address of that PC

Steps in executing Interrupts (simplified)


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PIC18 Interrupt Sources

External sources

Three pins of PORTB RB0/INTO, RB1/INT1,and RB2/INT2

Can be used to connect external interrupting sources

Keypads or switches

PORTB Interrupt (RBI) Change in logic levels of pins RB4-RB7

Internal peripheral sources Examples

Timers

A/D Converter Serial I/O

CCP (compare capture pulse-width-modulation)


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Special Function Registers (SFRs)

RCON

Priority Enable

INTCON

External interrupt sources

IPR, PIE, and PIR

Internal peripheral interrupts

Valid interrupt

Interrupt request bit (flag)

Interrupt enable bit

Priority bit


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INTERUPT

LABEL

REGISTER

CODE

INTERRUPT

SOURCE

INT_EXT 0x0B10 External interrupt has been detected

INT_RB 0x0B08 Change on Port B has been detected

INT_RTCC 0x0B20 Timer 0 has overflowed (same as TIMER0

INT_TIMER0 0x0B20 Timer 0 has overflowed (same as RTCC)

INT_TIMER1 0x8C10 Timer 1 has overflowed

INT_CCP1 0x8C04 Timer 1 matches preset value?

INT_TIMER2 0x8C02 Timer 2 has overflowed

INT_CCP2 0x8D01 Timer 2 matches preset value?

INT_AD 0x8C40 Analogue to digital converter has finished

INT_SSP 0x8C08 Serial data has been received

INT_PSP 0x8C80 Data ready at parallel serial port

INT_EEPROM 0x8D10 Data EEPROM write completed


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Enabling and disabling interrupt INTCON ( interrupt

control register)

Set the GIE bit fromINTCON REG

Set the IE bit for

that interrupt

If the interrupt is

one of theperipheral (timers

1,2 , serial,etc ) set

PEIE bit from

INTCON reg


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In general, interrupt sources have three bits to control

their operation. They are:

Flag bit to indicate that an interrupt event occurred

Enable bit

that allows program execution to branch to the

interrupt vector address when the flag bit is set Priority bit

to select high priority or low priority


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Interrupt priority in PIC18

What happens when 2 interrupt are

activated at the same time?

Which of these 2 interrupt activated first?


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2 level of interrupt priority:

1. low level

2. high level

Interrupt vector level


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Interrupt priorities

High-priority interrupt vector 000008H

Low-priority interrupt vector 000018H

A high-priority interrupt can interrupt a low-priority

interrupt in progress.

Interrupt priority enable

Bit7 (IPEN) in RCON register

RCON


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Multiple Interrupt Sources

All interrupt requests are directed to one of twomemory locations (interrupt vectors)

000008H (high-priority)

000018H (low-priority) When there are multiple requests

The interrupt source must be identified by checking

the interrupt flags


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Determine Interrupts in Timer programming

Interrupt Flag Bit Register Enable Bit RegisterTimer0 TMR0IF INTCON TMR0IE INTCON

Timer1 TMR1IF PIR1 TMR1IE PIE1



Timer Interrupt Flag Bits and Associated Registers

INTCON Register with Timer0 Interrupt Enable and Interrupt Flag


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ORG 0000H ORG 00100H T0 ISR


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ORG 0000H

GOTO MAIN

ORG 0008H

BTFSS INTCON,TMR0IF

RETFIEGOTO T0_ISR

ORG 00100H

MAIN BCF TRISB,5

CLRF TRISD

SETF TRISC

MOVLW 0x08

MOVWF T0CONMOVLW 0xFF

MOVWF TMR0H

MOVLW 0xF2

MOVWF TMR0L

BCF INTCON,TMR0IF

BSF T0CON,TMR0ON

BSF INTCON,TMR0IE

BSF INTCON,GIE

OVER MOVFF PORTC,PORTD

BRA OVER

T0_ISR

ORG 200H

MOVLW 0xFF

MOVWF TMR0H

MOVLW 0xF2

MOVWF TMR0L

BTG PORTB,5

BCF

INTCON,TMR0IF

RETFIE

END

Timer0 Interrupt


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External Hardware Interrupt programming

INTCON Register

INTCON2 Register

INTCON3 Register

ORG 0000H


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ORG 0000H

GOTO MAIN

ORG 0008H

BTFSS INTCON,INT0IF

RETFIEGOTO INT0_ISR

ORG 00100H

MAIN

BCF TRISB,7

BSF TRISB,INT0

CLRF TRISD

SETF TRISC

BSF INTCON,INT0IE

BSF INTCON,GIE

OVER MOVFF PORTC,PORTD

BRA OVER

INT0_ISRORG 200H

BTG PORTB,7

BCF INTCON,INT0IF

RETFIE

END

S


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Interrupt in Serial communication programming

Interrupt Flag Bit Register Enable Bit Register

TXIF(Transmit)

TXIF PIR1 TXIE PIE1

RCIF

(Receive)

RCIF PIR1 RCIE PIE1

Serial Port Interrupt Flag Bits and Associated Registers

PIE1 Register Bits Holding TXIE and RCIE


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Serial Interrupt Enable Flags

8 bit switch is connected to port.D. the PIC18 reads data from PORTD and


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ORG 0000H

GOTO MAIN

ORG 0008H

BTFSC PIR1,TXIF

BRA TX_ISR

RETFIEORG 0040H

TX_ISR

MOVWFF

PORTD,TXREG

RETFIE

ORG 00100H

MAIN SETF TRISD

MOVLW 0x20

MOVWF TXSTA

MOVLW D'15'

MOVWF SPBRG

BCF TRISC, TX

BSF RCSTA, SPENBSF PIE1,TXIE

BSF INTCON,PEIE

BSF INTCON,GIE

OVER BRA OVER

END

Serial Port Interrupt

Enable peripheral Interrupt

8 bit switch is connected to port.D. the PIC18 reads data from PORTD and

writes it to TXREG.

LCD i t f i & K d


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LCD interfacing & Keypad

interfacing


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Driver HD44780 8-bit data bus (RD7-RD0)

Three control signals

RS Register Select(RA3)

R/W Read/Write(RA2)

E Enable (RA1)

Three power connections

Power, ground, andvariable resistor tocontrol brightness

LCD Interfacing

I t f i LCD


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Interfacing LCD

Can be interfaced either in 8-bit mode or 4-bit mode

In 8-bit mode, all eight data lines are connected In 4-bit mode, only four data lines are connected

Two transfers per character (or instruction) are needed

Driver has two 8-bit internal registers

Instruction Register (IR) to write instructions to set upLCD

Data Register (DR) to write data (ASCII characters)


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LCD Operation

When the MPU writes an instruction to IR or data toDR, the controller:

Sets DB7 high indicating that the controller is busy

Sets DB7 low after the completion of the operation

The MPU should always check whether DB7 is low

before sending an instruction or a data byte Writing to or Reading from LCD (Table 9-4)

The MPU: Asserts RS low to select IR

Asserts RS high to select DR Reads from LCD by asserting the R/W signal high

Writes into LCD by asserting the R/W signal low

Asserts the E signal high and then low (toggles) to latch adata byte or an instruction


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Timing diagram: writing to LCD


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Software

To write into the LCD

Send the initial instructions to set up the LCD 4-bit or 8-bit mode

Continue to check DB7 until it goes low

Write instructions to IR to set up LCD

parameters Number of display lines and cursor status

Write data to display a message

Sending commands and data to LCD with time delay


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g y

Sending commands and data to LCD busy flag


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g y g


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Software

To recognize and encode the key pressed,

the program should: Ground all the columns by sending zeros.

Check each key in a row for logic zero.

Ground one column at a time and check all the

rows in that column. Once a key is identified, it is encoded based on itsposition in the column.


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ADC and sensor interfacing


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Introduction

Analog to digital converters widely used for dataacquisition

Digital computers use binary values (discrete) but

physical world everything is analog (continuous).

Temperature, pressure, humidity and velocity are all inanalog.

A physical quantity is converted to electrical signals

using transducer or called as sensor

ADC is use to translate the analog signals to digitalnumbers so that the microcontroller can read and

process them

ADC and sensor interfacing

ADC (analog to digital converter)


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ADC (analog to digital converter) Most signals we want to process are analog

i.e.: they are continuous and can take an

inifinity of values

x(t)

t


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Digital systems require discrete digital data

ADC converts an analog information into a

digital information

Digital System?Analog Digital

Resolution & Conversion time


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Resolution & Conversion time

Resolution

ADC has n-bit resolution (n = 8,10,12,16 or even 24 bits)

Higher ADC resolution, a smaller step size (step size smallest step

change can be discerned by ADC).

Step size can be change with the help of Vref.

Conversion time


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Conversion time

Define as the time it takes the ADC to convert the analog input to

digital (binary) number.

Dictated by: Clock source connected to the ADC

Technology used in the fabrication of the ADC chips (CMOS or

TTL)

Digital data output

Dout digital data output in decimal

Vin analog input voltage

Step size smallest change, Vref/256 for

8 bit ADC

ADC program using assembly language


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ADC program using C language


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ADC program using C language

ADC and temperat re sensor interfacing


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ADC and temperature sensor interfacing



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Relay an electrically operated switch.

Many relays use an electromagnet to operate a switching

mechanism mechanically, but other operating principles are also

used.

Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and

controlled circuits), or where several circuits must be controlled by

one signal.

Two types of relay:

1. Electromechanical relay2. Solid state relay

The following designations are commonly encountered:


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SPST Single Pole Single Throw.

SPDT Single Pole Double Throw.

DPST Double Pole Single Throw.

DPDT Double Pole Double Throw.

Electromchanical sensor
http://en.wikipedia.org/w/index.php?title=File:Relay_symbols.svg&page=1


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Electromchanical sensor A simple electromagnetic relay consists of a coil of wire wrapped

around a soft iron core, an iron yoke which provides a low

reluctance path for magnetic flux, a movable iron armature, and oneor more sets of contacts .

Other relays may have more or fewer sets of contacts depending on

their function.
http://1.bp.blogspot.com/-pZEyGn-KTXM/T4Mp0ca-eDI/AAAAAAAABJg/OgnorCSzq3A/s1600/1.JPG


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Solid state relay

A solid state relay (SSR) is a solid state electronic component that

provides a similar function to an electromechanical relay but doesnot have any moving components, increasing long-term reliability.

Every solid-state device has a small voltage drop across it. This

voltage drop limits the amount of current a given SSR can handle.

Solid-state relays rated to handle as much as 1,200 Amperes have

become commercially available. Compared to electromagnetic relays, they may be falsely triggered

by transients.

Driving a relay
http://en.wikipedia.org/wiki/File:Solid_state_relay.jpg


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g y

circuit to drive relay from I/O port using transistor and ULN2803

program code to drive relay


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program code to drive relay

Optoisolator


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Optoisolator also called an optocoupler, photocoupler, or optical isolator

"an electronic device designed to transfer electrical signals by

utilizing light waves to provide coupling with electrical isolationbetween its input and output".

The main purpose of an opto-isolator is "to prevent high voltages or

rapidly changing voltages on one side of the circuit from damaging

components or distorting transmissions on the other side.

Commercially available opto-isolators withstand input-to-outputvoltages up to 10 kV and voltage transients with speeds up to

10 kV/s.

Stepper Motor


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A stepper motor(or step motor) is a brushless DC electric motor

that divides a full rotation into a number of equal steps.

The motor's position can then be commanded to move and hold at

one of these steps without any feedback sensor (an open-loop

controller), as long as the motor is carefully sized to the application.

There are many kind of stepper motors. Unipolar type, Bipolar type,

Single-phase type, Multi-phase type...

* 2-phase unipolar PM type stepper motor

Driving a stepper motor


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DC Motor


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Motors come in many sizes and types, but their basic function is the

same. Motors of all types serve to convert electrical energy into

mechanical energy.

They can be found in VCR's, elevators, CD players, toys, robots,

watches, automobiles, subway trains, fans, space ships, air

conditioners, refrigerators, and many other places.

D.C. motors are motors that run on Direct Curr entfrom a battery or

D.C. power supply.

DC motor interface to control direction with H-Bridge motor
http://en.wikipedia.org/wiki/File:Ejs_Open_Source_Direct_Current_Electrical_Motor_Model_Java_Applet_(_DC_Motor_)_80_degree_split_ring.gif


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configuration

program code to control DC motor direction with H-Bridge

fi ti


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configuration

PWM ( Pulse Width Modulation )


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Pulse width modulation (PWM) is a powerful technique for

controlling analog circuits with a microprocessor's digital outputs.

PWM is employed in a wide variety of applications, ranging frommeasurement and communications to power control and conversion

PWM is a way of digitally encoding analog signal levels. Through

the use of high-resolution counters, the duty cycle of a square wave

is modulated to encode a specific analog signal level.

The PWM signal is still digital because, at any given instant of time,the full DC supply is either fully on or fully off.

The voltage or current source is supplied to the analog load by

means of a repeating series of on and off pulses.

The on-time is the time during which the DC supply is applied to the

load, and the off-time is the period during which that supply is

switched off. Given a sufficient bandwidth, any analog value can be

encoded with PWM.

( )


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PWM comparison

PWM motor control with CCP


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Program code


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Chapter 6 - Hardware Interfacing

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