TheHuman _ andPhysical _ Interfaces .ppt

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

1. The human and physical
interfaces
Chapter Eight
8.1 – 8.9
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2. Outline
•Introduction
•Keypads
•Seven-segment displays
•LCDs
•Sensors
•Actuators
•Summary
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3. 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
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4. Human Interface - Examples
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5. Human Interface - Examples
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6. Human interface types
•Input:
–Switch
–Push button
–Keypad
•Output:
–light-emitting diode
(LED)
–Seven-segment LED
–Liquid crystal display
(LCD)
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7. The LED version of the Derbot AGV
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8. The LCD version of the Derbot AGV
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9. The Keypad
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10. Flow diagram
Reading a
keypad with a
microcontroller
port
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11. Outputs for the keypad
Port Bit Function
7 Row 1
6 Row 2
5 Row 3
4 Row 4
3 Column 1
2 Column 2
1 Column 3
0 Unused
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12. Flow diagram of program example
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13. 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 entries
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14. 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
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15. Keypad Example – Read keypad
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 pattern
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16. 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
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17. Seven-segment displays
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Common
Anode
Common
Cathode

18. Connecting multiple digits
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Need 1.2 kΩ
line resistors

19. Timing diagram
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20. 7-seg. display example – page 1
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21. 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
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22. 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.
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23. 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
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24. HD44780 timing diagram
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25. Derbot’s LCD
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Each digit
is a liquid
crystal dot
matrix

26. 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
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27. 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
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28. 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'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
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29. Sensors
•Convert physical variables to electrical.
•Examples:
–The microswitch
–Light-dependent resistor
–Ultrasonic object sensor
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30. The Microswitch
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31. 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
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32. Optical object sensing
Infrared LED and phototransistor
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33. The opto-sensor applied as a shaft
encoder
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34. Ultrasonic object sensor
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35. 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.
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36. Forms of signal corruption
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(a) Spikes in signal, potentially harmful to device input. (b) Spikes in
signal.
(c) Excessively slow edges. (d) DC offset in signal.

37. 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
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38. 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.
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39. Isolation or level shifting with the
opto-isolator
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40. Switch bouncing
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41. Hardware switch debouncing
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42. Software switch debouncing
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Typically
10 ms

43. 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
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44. 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
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45. Derbot DC Motor
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46. Servo input and output
characteristics
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47. Interfacing to actuators
•Simple DC switching
–Bipolar transistors
–MOSFET transistors
•Reversible switching
–The H-bridge
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48. Bipolar transistor switching of DC
resistive loads
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49. MOSFET transistor switching of DC
resistive loads
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50. MOSFET transistor switching of DC
inductive loads
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51. Characteristics of two popular
logic-compatible MOSFETs
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52. Driving piezo sounder and opto-
sensors
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I = (5 − 3.4)/91
I = 17.6 mA

53. Reversible switching: the H-bridge
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54. The L293D dual H-bridge
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55. The L293D applied in the Derbot
motor drive circuit
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56. 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.
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