The Human and Physical Interfaces

8/2/2019 The Human and Physical Interfaces

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The human and

physical interfaces

Chapter Eight

8.1 8.9

Dr. Gheith Abandah 1


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Outline

Introduction

Keypads

Seven-segment displays LCDs

Sensors

Actuators Summary



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Introduction

A human interface is an important partof most embedded systems.

Users need to conveniently get

information from the embedded system. They also need to conveniently control

the operation of this system.

Examples: Domestic fridge

Photocopier

Car dashboard



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Human Interface - Examples



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Human interface types

Input:

Switch

Push button

Keypad

Output:

light-emittingdiode (LED)

Seven-segmentLED

Liquid crystal

display (LCD)



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The LED version of theDerbot AGV



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The LCD version of theDerbot AGV



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The Keypad



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Flowdiagram

Reading akeypad witha

microcontroller port



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Outputs for the keypad

Port Bit Function

7 Row 1

6 Row 2

5 Row 34 Row 4

3 Column 1

2 Column 2

1 Column 3

0 Unused



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Flow diagram of programexample



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Keypad Example Initialization

;Initialize

bsf status,rp0 ;select memory bank 1

movlw B'11110000' ;Port B initially Row bits

;are input, column output

movwf trisb

bcf status,rp0 ;select bank 0

...

clrf portb ;initialize keypad value

bcf intcon,rbif ;enable interrupt bsf intcon,rbie

bsf intcon,gie

loop

goto loop ;await keypad entriesDr. Gheith Abandah 13


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Keypad Example Readkeypad

kpad_rd

movf portb,w ;read portb value, row pattern

andlw B'11110000' ;suppress unwanted bits

movwf kpad_pat

bsf status,rp0 ;set row to op, column to ip

movlw B'00001110'

movwf trisb

bcf status,rp0

movlw 00 movwf portb ;ensure output values still 0

movf portb,w ;read portb value, col. pattern

andlw B'00001110' ;suppress unwanted bits

iorwf kpad_pat,1 ;OR results into the patternDr. Gheith Abandah 15


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Keypad Example Readkeypad 2

;reset keypad interface

bsf status,rp0 ;set row to ip, column to op

movlw B'11110000'

movwf trisb

bcf status,rp0

clrf portb ;ensure output values still 0

return



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Seven-segment displays


Common

Anode

CommonCathode


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Connecting multiple digits


Need 1.2 k

line resistors


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Timing diagram



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7-seg. display example page 1



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7-seg. display example page 2

;Initialise

bcf status,rp1

bsf status,rp0;bank 1

movlw B00000000 ;out

movwf trisa

movwf trisb

movwf trisc

bcf status,rp0;bank 0

;

loop

;set digit 1

movlw B'00011101' ;H

movwf porta

bcf portc,6 ;seg a

bsf portc,7 ;seg b

bsf portc,1 ;dig 1

call delay5

bcf portc,1;set digit 2

goto loop



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Liquid crystal displays(LCDs)

Liquid crystal responds to an appliedelectric field by changing the alignment ofits molecules, and in so doing changing

the direction of the light polarization thatit introduces.

Liquid crystal can be trapped between twoparallel sheets of glass, with a matching

pattern of transparent electrode on eachsheet.

When a voltage is applied to the

electrodes, the optical character of theDr. Gheith Abandah 22


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Interfacing with LCDs

Hitachi developed a specialmicrocontroller (HD44780) forinterfacing LCDs.

This microcontroller is usuallyintegrated with LCDs.

Features:

8- or 4-bit data transfer

Simple instruction set to initialize, clear,display, and position cursor

Has instruction register and dataDr. Gheith Abandah 23


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HD44780 timing diagram



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Derbots LCD


Each digit

is a liquidcrystal dot

matrix


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LCD Drive Example Page 1

lcd_write

call busy_check

bcf portc,lcd_rw

bcf status,crrf lcd_op,1

bcf portc,6

btfsc status,c

bsf portc,6

bcf status,c

rrf lcd_op,1

bcf portc,7

btfsc status,c bsf portc,7

movf lcd_op,0

movwf porta

bsf portc,lcd_E

bcf portc,lcd_E

return



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busy_check

bsf status,rp0 ;bank 1

movlw B'00111111' ;set port A all ip

movwf trisa bcf status,rp0

bcf flags,0

btfsc portc,lcd_RS ;save RS in flags, 0

bsf flags,0

bcf portc,lcd_RS ;access instr register

bsf portc,lcd_RW ;set to read



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busy_loop

bcf portc,lcd_E

bsf portc,lcd_E

btfsc porta,lcd_busy ;test the busy flag

goto busy_loop

bcf portc,lcd_E

bsf status,rp0 ;select memory bank 1

movlw B'00000000 ;set port A all op

movwf trisa bcf status,rp0

bcf portc,lcd_RS

btfsc flags,0 ;reinstate RS bit

bsf portc,lcd_RS

return Dr. Gheith Abandah 28


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Sensors

Convert physical variables toelectrical.

Examples:

The microswitch

Light-dependent resistor

Ultrasonic object sensor



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The Microswitch



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Light-dependent resistors

A light-dependentresistor (LDR) ismade from a pieceof exposedsemiconductormaterial. When lightfalls on it, it createsholeelectron pairsin the material,which improve theconductivity.

20M to a fewhundred ohms



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Optical object sensingInfrared LED and phototransistor



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The opto-sensor applied asa shaft encoder



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Ultrasonic object sensor



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Digitalinput

If amicrocontrolleris to receivelogic signals,

then it isessential thatthose signals

are at voltagelevels whichare recognizedby it as being

either Logic 0 Dr. Gheith Abandah 35


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Forms of signal corruption


(a) Spikes in signal, potentially harmful to device input. (b) Spikes in

signal.

(c) Excessively slow edges. (d) DC offset in signal.


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Input protection

For Rprot = 1K and

max. diode current=20 mA

What is themaximum voltagespike?

Vmax =

[(20mA 1 k ) +5.3]

= 25VDr. Gheith Abandah 37


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Ensuring legal logic levels

Can use Schmitttrigger forspeeding up slow

logic edges. Schmitt trigger

with RC filter canbe used to filter

voltage spikes. Digital filtering:

sample the input

three times and Dr. Gheith Abandah 38


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Switch bouncing


H d i h


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Hardware switchdebouncing



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Software switch debouncing


Typically

10 ms

A t t t d


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Actuators: motors andservos

Often need tocause physicalmovement

For linearmovement usesolenoids

For angular

movement, useservos

For angular or

rotary, use DC orDr. Gheith Abandah 43


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Comparison

DC Motors

Range from theextremely powerful to

the very small Wide speed range

Controllable speed

Good efficiency

Can provide accurateangular positioningwith angular shafts

Only the armature

winding needs to be

Stepper Motors

Simple interface withdigital systems

Can control speed andposition

Awkward start-upcharacteristics

Lose torque at highspeed

Limited top speed

Less efficient

More complex to driveDr. Gheith Abandah 44


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Derbot DC Motor


S i t d t t


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Servo input and outputcharacteristics



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Interfacing to actuators

Simple DC switching

Bipolar transistors

MOSFET transistors

Reversible switching

The H-bridge


Bi l t i t it hi


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Bipolar transistor switchingof DC resistive loads


MOSFET t i t it hi


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MOSFET transistor switchingof DC resistive loads


MOSFET t i t it hi


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MOSFET transistor switchingof DC inductive loads


aracter st cs o two


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aracter st cs o twopopular logic-compatible

MOSFETs


Dri ing pie o so nder and


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Driving piezo sounder andopto-sensors


I = (5 3.4)/91I = 17.6 mA

Reversible switching the H


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Reversible switching: the H-bridge



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The L293D dual H-bridge


The L293D applied in the


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The L293D applied in theDerbot motor drive circuit



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Summary An embedded microcontroller must be able to

interface with the physical world and possiblythe human world as well.

Much human interfacing can be done withswitches, keypads and displays.

To interface with the physical world, themicrocontroller must be able to interface with arange of transducers. The designer needs anunderstanding of the main sensors andactuators available.

Interfacing with sensors requires a reasonableknowledge of signal conditioning techniques.

Documents

The Human and Physical Interfaces