The human and physical interfaces Chapter Eight 8.1 – 8.9 Dr. Gheith Abandah1.

The human and physical interfaces

Chapter Eight8.1 – 8.9

Dr. Gheith Abandah 1

Outline

• Introduction• Keypads• Seven-segment displays• LCDs• Sensors• Actuators• Summary


Introduction

• A human interface is an important part of 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


Human Interface - Examples


Human Interface - Examples


Human interface types

• Input:– Switch– Push button– Keypad

• Output:– light-emitting diode

(LED)– Seven-segment LED– Liquid crystal display

(LCD)


The LED version of the Derbot AGV


The LCD version of the Derbot AGV


The Keypad


Flow diagramReading a keypad with a microcontroller port


Outputs for the keypad

Port Bit Function7 Row 16 Row 25 Row 34 Row 43 Column 12 Column 21 Column 30 Unused


Flow diagram of program example


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

Keypad Example – ISRkpad_to_lcd

call kpad_rd

call kp_code_conv

bsf portc,lcd_RS ;set for character op

movwf lcd_op

call lcd_write

rel_test ;test now for keypad release

call kpad_rd

movf kpad_pat,0

andlw 0fe ;suppress lsb, not used

sublw 0fe ;test if inactive

btfss status,z

goto rel_test

bcf intcon,rbif ;clear interrupt flag

retfieDr. Gheith Abandah 14

Keypad Example – Read keypadkpad_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 pattern


Keypad Example – Read keypad 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


Seven-segment displays


Common Anode

Common Cathode

Connecting multiple digits


Need 1.2 kΩ line resistors

Timing diagram


7-seg. display example – page 1


7-seg. display example – page 2;Initialise

bcf status,rp1

bsf status,rp0;bank 1

movlw B’00000000’ ;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


Liquid crystal displays (LCDs)

• Liquid crystal responds to an applied electric field by changing the alignment of its molecules, and in so doing changing the direction of the light polarization that it introduces.

• Liquid crystal can be trapped between two parallel sheets of glass, with a matching pattern of transparent electrode on each sheet.

• When a voltage is applied to the electrodes, the optical character of the crystal changes and the electrode pattern appears in the crystal.


Interfacing with LCDs

• Hitachi developed a special microcontroller (HD44780) for interfacing LCDs.

• This microcontroller is usually integrated with LCDs.

• Features:– 8- or 4-bit data transfer– Simple instruction set to initialize, clear, display,

and position cursor– Has instruction register and data register


HD44780 timing diagram


Derbot’s LCD


Each digit is a liquid crystal dot matrix

LCD Drive Example – Page 1

lcd_write

call busy_check

bcf portc,lcd_rw

bcf status,c

rrf 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


LCD Drive Example – Page 2

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


LCD Drive Example – Page 3busy_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


Sensors

• Convert physical variables to electrical.• Examples:

– The microswitch– Light-dependent resistor– Ultrasonic object sensor


The Microswitch


Light-dependent resistors

• A light-dependent resistor (LDR) is made from a piece of exposed semiconductor material. When light falls on it, it creates hole–electron pairs in the material, which improve the conductivity.

• 20M Ω to a few hundred ohms


Optical object sensingInfrared LED and phototransistor


The opto-sensor applied as a shaft encoder


Ultrasonic object sensor


Digital inputIf a microcontroller is to receive logic signals, then it is essential that those signals are at voltage levels which are recognized by it as being either Logic 0 or Logic 1.


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.

Input protection

• For Rprot = 1KΩ and max. diode current =20 mA

• What is the maximum voltage spike?

Vmax =

[(20mA × 1 k Ω) + 5.3]= 25V


Ensuring legal logic levels

• Can use Schmitt trigger for speeding up slow logic edges.

• Schmitt trigger with RC filter can be used to filter voltage spikes.

• Digital filtering: sample the input three times and use a majority vote.


Isolation or level shifting with the opto-isolator


Switch bouncing


Hardware switch debouncing


Software switch debouncing


Typically 10 ms

Actuators: motors and servos

• Often need to cause physical movement

• For linear movement use solenoids

• For angular movement, use ‘servos’

• For angular or rotary, use DC or stepper motors


Comparison

DC Motors• Range from the extremely

powerful to the very small• Wide speed range• Controllable speed• Good efficiency• Can provide accurate

angular positioning with angular shafts

• Only the armature winding needs to be driven

Stepper Motors• Simple interface with digital

systems• Can control speed and

position• Awkward start-up

characteristics• Lose torque at high speed• Limited top speed• Less efficient• More complex to drive


Derbot DC Motor


Servo input and output characteristics


Interfacing to actuators

• Simple DC switching– Bipolar transistors– MOSFET transistors

• Reversible switching– The H-bridge


Bipolar transistor switching of DC resistive loads


MOSFET transistor switching of DC resistive loads


MOSFET transistor switching of DC inductive loads


Characteristics of two popular logic-compatible MOSFETs


Driving piezo sounder and opto-sensors


I = (5 − 3.4)/91I = 17.6 mA

Reversible switching: the H-bridge


The L293D dual H-bridge


The L293D applied in the Derbot motor drive circuit


Summary• An embedded microcontroller must be able to interface with

the physical world and possibly the human world as well.• Much human interfacing can be done with switches, keypads

and displays.• To interface with the physical world, the microcontroller

must be able to interface with a range of transducers. The designer needs an understanding of the main sensors and actuators available.

• Interfacing with sensors requires a reasonable knowledge of signal conditioning techniques.

• Interfacing with actuators requires a reasonable knowledge of power switching techniques.


The human and physical interfaces Chapter Eight 8.1 – 8.9 Dr. Gheith Abandah1.

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