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

Outline Introduction Keypads Seven-segment displays LCDs Sensors Actuators Summary Dr. Gheith Abandah2

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 Dr. Gheith Abandah3

Human Interface - Examples Dr. Gheith Abandah4

Human Interface - Examples Dr. Gheith Abandah5

Human interface types Input: – Switch – Push button – Keypad Output: – light-emitting diode (LED) – Seven-segment LED – Liquid crystal display (LCD) Dr. Gheith Abandah6

The LED version of the Derbot AGV Dr. Gheith Abandah7

The LCD version of the Derbot AGV Dr. Gheith Abandah8

The Keypad Dr. Gheith Abandah9

Flow diagram Reading a keypad with a microcontroller port Dr. Gheith Abandah10

Outputs for the keypad Port BitFunction 7Row 1 6Row 2 5Row 3 4Row 4 3Column 1 2Column 2 1Column 3 0Unused Dr. Gheith Abandah11

Flow diagram of program example Dr. Gheith Abandah12

Keypad Example – Initialization ;Initialize bsf status,rp0 ;select memory bank 1 movlw B' ' ;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 entries Dr. Gheith Abandah13

Keypad Example – ISR kpad_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 retfie Dr. Gheith Abandah14

Keypad Example – Read keypad kpad_rd movf portb,w ;read portb value, row pattern andlw B' ' ;suppress unwanted bits movwf kpad_pat bsf status,rp0 ;set row to op, column to ip movlw B' ' movwf trisb bcf status,rp0 movlw 00 movwf portb ;ensure output values still 0 movf portb,w ;read portb value, col. pattern andlw B' ' ;suppress unwanted bits iorwf kpad_pat,1 ;OR results into the pattern Dr. Gheith Abandah15

Keypad Example – Read keypad 2 ;reset keypad interface bsf status,rp0 ;set row to ip, column to op movlw B' ' movwf trisb bcf status,rp0 clrf portb ;ensure output values still 0 return Dr. Gheith Abandah16

Seven-segment displays Dr. Gheith Abandah17 Common Anode Common Cathode

Connecting multiple digits Dr. Gheith Abandah18 Need 1.2 kΩ line resistors

Timing diagram Dr. Gheith Abandah19

7-seg. display example – page 1 Dr. Gheith Abandah20

7-seg. display example – page 2 ;Initialise bcf status,rp1 bsf status,rp0;bank 1 movlw B’ ’ ;out movwf trisa movwf trisb movwf trisc bcf status,rp0;bank 0 ; loop ;set digit 1 movlw B' ' ;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 Dr. Gheith Abandah21

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. Dr. Gheith Abandah22

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 Dr. Gheith Abandah23

HD44780 timing diagram Dr. Gheith Abandah24

Derbot’s LCD Dr. Gheith Abandah25 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 Dr. Gheith Abandah26

LCD Drive Example – Page 2 busy_check bsf status,rp0 ;bank 1 movlw B' ' ;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 Dr. Gheith Abandah27

LCD Drive Example – Page 3 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' ‘ ;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 Abandah28

Sensors Convert physical variables to electrical. Examples: – The microswitch – Light-dependent resistor – Ultrasonic object sensor Dr. Gheith Abandah29

The Microswitch Dr. Gheith Abandah30

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 Dr. Gheith Abandah31

Optical object sensing Infrared LED and phototransistor Dr. Gheith Abandah32

The opto-sensor applied as a shaft encoder Dr. Gheith Abandah33

Ultrasonic object sensor Dr. Gheith Abandah34

Digital input If 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. Dr. Gheith Abandah35

Forms of signal corruption Dr. Gheith Abandah36 (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 R prot = 1KΩ and max. diode current =20 mA What is the maximum voltage spike? V max = [(20mA × 1 k Ω) + 5.3] = 25V Dr. Gheith Abandah37

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. Dr. Gheith Abandah38

Isolation or level shifting with the opto-isolator Dr. Gheith Abandah39

Switch bouncing Dr. Gheith Abandah40

Hardware switch debouncing Dr. Gheith Abandah41

Software switch debouncing Dr. Gheith Abandah42 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 Dr. Gheith Abandah43

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 Dr. Gheith Abandah44

Derbot DC Motor Dr. Gheith Abandah45

Servo input and output characteristics Dr. Gheith Abandah46

Interfacing to actuators Simple DC switching – Bipolar transistors – MOSFET transistors Reversible switching – The H-bridge Dr. Gheith Abandah47

Bipolar transistor switching of DC resistive loads Dr. Gheith Abandah48

MOSFET transistor switching of DC resistive loads Dr. Gheith Abandah49

MOSFET transistor switching of DC inductive loads Dr. Gheith Abandah50

Characteristics of two popular logic-compatible MOSFETs Dr. Gheith Abandah51

Driving piezo sounder and opto- sensors Dr. Gheith Abandah52 I = (5 − 3.4)/91 I = 17.6 mA

Reversible switching: the H-bridge Dr. Gheith Abandah53

The L293D dual H-bridge Dr. Gheith Abandah54

The L293D applied in the Derbot motor drive circuit Dr. Gheith Abandah55

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. Dr. Gheith Abandah56