1. Simple I/O Interfacing
EEE3410 Microcontroller Applications Department of Electrical Engineering
│ Lecture 9 │
Simple I/O Interfacing

2. In this Lecture ….
Interface 8051 with the following Input/Output Devices
Switches
Solenoid and relays
LEDs
Seven Segment Display
Dot matrix display

3. Introduction
The 8051 microcontroller is commonly used for real-world applications, e.g. display control, lighting control, machine control, etc. Various input and output devices are connected to the I/O ports of the microcontroller to deal with different applications. We will introduce some common I/O devices for simple application in this lecture. They are:
Mechanical switches
Electromagnetic relays
Solid-state relays
LEDs
7-segment Display
Dot Matrix Display

4. Mechanical Switches Mechanical switches are common input devices
One or more pairs of contacts that can be open or close.
Typical switch designations are:
SPST (single-pole-single-throw)單刀單擲
SPDT (single-pole-double-throw)單刀雙擲
DPDT (double-pole-double-throw)
Normally open (N.O.) contacts close when the switch is activated and normally close (N.C.) contacts OPEN when the switch is activated.
N.O. SPST
N.O. SPDT
N.O. DPDT
雙刀雙擲

6. Connect mechanical switches to 8051
+5V
8051
EA VCC
Reset
XTAL1
XTAL2
VSS
P3.7
P3.6
P3.5
P3.4
SW1
SW2
SW3
SW4
ORG H :
JNB P3.7, CASE1
JNB P3.6, CASE2
JNB P3.5, CASE3
JNB P3.4, CASE4
CASE1: :
CASE2: :
CASE3: :
CASE4: :
Figure 9.1
Check the status of SW1
SW opens, input to 8051 is HIGH (1)
SW closes, input to 8051 is LOW (0)

7. Electromagnetic Relays
Electromagnetic relay consists of two parts - solenoid and relay contacts
Solenoid is a coil of wire used to produce a magnetic field to move a steel actuator where points of contacts are located.
The actuator is used to close/open the contact points, such construction is called a relay.
NC: NORMAL CLOSE
NO: NORMAL OPEN
Figure 9.2 construction of a Electromagnetic relay

8. Driving an Electromagnetic Relays
Figure 9.3 show a typical driving circuit of an electromagnetic relay.
The transistor will act as a switch to allow current passing through the solenoid.
The diode placed across the coil terminal is to protect the transistor damaged from spike voltage during ON/OFF.
The external circuit connected to relay terminals will be turned ON/OFF by the TTL.
Contact closed if TTL = High
Contact opened if TTL = Low Vs
+5V
TTL
Figure 9.3 Driving circuit of an electromagnetic relay

9. Solid State Relays (SSR)/Switches
Solid-state relays has no mechanical parts and made of semi-conductor materials
It combines the isolation, driving, and contact closure functions into a single package
It is commonly use to control ac loads
Relay closed if input = High
Relay opened if input = Low
SSR +  T1 T2
a.c. power supply
Figure 9.4 Driving a solid state relay
Input Control

10. Light-Emitting Diode (LED)
Light-emitting diodes (LEDs) can be turned on when a current passes through it.
+5V
TTL
Figure 9.5
330
Figure 9.5 Shows a typical TTL circuit driving a LED
The 330 resistor is used to limit the amount of current to pass through the LED to prevent it burning off.
The TTL output is LOW, the LED will ON
The TTL output is HIGH, the LED will OFF

11. Control of LEDs 0 = on 1 = off Figure 9.6
P1.7
P1.6
P1.5
P1.4
P1.3
P1.2
P1.1
P1.0
+5V
8051
EA VCC
Reset
XTAL1
XTAL2
VSS
8 LEDs are connected to Port 1
and linked to 5V supply
The LEDs can directly be turned
ON/OFF by the 8051
Under this connection, the LEDs
will be
OFF when port bit is at logic 1
ON when port bit is at logic 0
Figure 9.6

12. Example 9.1 : Control the 8 LEDs ON/OFF at the same time
All LEDs ON
All LEDs OFF

13. Move the content of A to P1
Program Listing for Example 9.1
ORG 0000H
CLR A
LOOP: MOV P1, A
CPL A
ACALL DELAY
AJMP LOOP
DELAY: MOV R6, #250
DL1: MOV R7, #200
DL2: DJNZ R7, DL2
DJNZ R6, DL1
RET
END
Start
Set A = 00
Move the content of A to P1
Delay for 0.1s
Invert the content of A
Assume 12MHz clock, determine the delay time.

14. Example 9.2 : Glowing a LED in rotating sequence
ORG 0000H
START: MOV R1, #07H
MOV A, # B
LEFT: MOV P1, A
ACALL DELAY
RL A
DJNZ R1, LEFT ;
MOV R1, #07H
MOV A, # B
RIGHT: MOV P1, A
RR A
DJNZ R1, RIGHT
AJMP START
DELAY: ……….

15. Example 9.3: Turning LEDs ON/OFF in a preset sequence by using a Look-up Table

16. Program Listing of Example 9.3
ORG 0000H
START: MOV R0, #OKDATA+1
MOV DPTR, #DATA
MOV R1, #00H
LOOP: MOV A, R1
MOVC
MOV P1, A
ACALL DELAY
INC R1
DJNZ R0, LOOP
AJMP START ;
DELAY: MOV R5, #2
DL1: MOV R6, #250
DL2: MOV R7, #200
DL3: DJNZ R7, DL3
DJNZ R6, DL2
DJNZ R5, DL1
RET
DATA: DB B
DB B
DB B
DB B
DB B
DB B ;
OK: DB B
END

17. Exercise: Write a 8051 program, using a look-up table, to
light-up the LEDs in the sequence as shown below

18. Simple I/O applications
+5V
8051
EA VCC
Reset
XTAL1
XTAL2
VSS
P3.7
P3.6
P3.5
P3.4
SW1
SW2
SW3
SW4
Figure 9.7

19. Example 9. 4: Refer to the 8051 circuit in figure 9
Example 9.4: Refer to the 8051 circuit in figure 9.5, write a 8051 program to light-up the LEDs in the pattern as shown below when the respect switch is closed.
When SW1 Closed
When SW3 Closed
When SW4 Closed
When SW2 Closed
Priority: SW SW SW SW4
YELLOW=ON
RED =OFF

20. Flow Chart of Example 9.4 2 Start Y SW1 closed? SW1 Handler
Initialization
Set P3 as input port
Read SW1SW4 status
SW1 closed?
SW2 closed?
SW3closed?
SW4 closed?
SW1 Handler Y
SW2 Handler
SW3 Handler
SW4 Handler 2 1 N

21. Program Listing of Example 9.4
P1.x = 0, ON
P1.x = 1. OFF
Program Listing of Example 9.4
CASE2 MOV A, R2
MOV P1, A
ACALL DELAY
XRL A, # B
AJMP TEST ;
CASE3 MOV A, R3
XRL A, # B
MOV P1, A
CASE4 MOV A, R4
ACALL DELAY
XRL A, # B
DELAY: ………..
END
ORG 0000H
MOV R1, # B
MOV R2, # B
MOV R3, # B
MOV R4, # B ;
TEST: ORL P3, #0FFH
JNB P3.7, CASE1 JNB P3.6, CASE2
JNB P3.5, CASE3
JNB P3.4, CASE4
AJMP TEST
CASE1: MOV A, R1
MOV P1, A
ACALL DELAY
XRL A, # B

22. 7-Segment LED Numeric Display
A single-character display panel
Contains 7 LED segments arranged as an “8”
Two configurations: common-anode and common-cathode a b c d e f g
Segment Pattern Dp

23. 7-Segment LED Numeric Display
Common-anode configuration
Common-cathode configuration a b c d e f g Dp
Common
Pull-high
Pull-low

24. 7-Segment LED Numeric Display
Segment displays are driven by connecting each segment to a port bit, or they can be driven by decoder/driver IC
Output Port bit 7 6 5 4 3 2 1
Display segment Dp g f e d c b a
Normally the LED should be connected to the power via resistors to protect them from burning
When using common-cathode configuration, a segment will be lit only if the lead of the segment connected with a High voltage and the common cathode lead with Low voltage

25. 7-Segment LED Numeric Display
8051 b c d e f g a
XTAL1
XTAL2
P2.0
P2.1
P2.2
P2.3
P2.4
P2.5
P2.6
P2.7
Figure 9.8
Vcc
Dp.
Use R3 as counter, write a 8051 assembly language program using look-up table method, to display the value in R3 to a 7-segment display

26. Program Listing Display 0,1,2,,,,8,9,0,1,2,3…. BCD addition ORG 0000H
MOV R3, #00H
LOOP: MOV DPTR, #TABLE
MOV A, R3
MOVC ;
; Display numbers on 7-segment display
MOV P2, A
ACALL DELAY
; Increase R3 by 1 and loop back
ADD A, #1
DA A
ANL A, #0FH
MOV R3, A
AJMP LOOP
DELAY: …………..
TABLE: DB B ; 0
DB B ; 1
DB B ; 2
DB B ; 3
DB B ; 4
DB B ; 5
DB B ; 6
DB B ; 7
DB B ; 8
DB B ; 9
; =ON, 1=OFF
END
Output port , P2 7 6 5 4 3 2 1
Display segment Dp g f e d c b a
7-segment ‘0’
BCD addition

27. Dot-matrix LED Display
Consists of a number of LED arranged in the form of a matrix
e.g. 35 LED in a 5 columns x 7 rows matrix, or 64 LED in a 8 x 8 matrix
To display a digit/character, use the method of scanning a b c d e f g 1 2 3 4 5
i.e. scan a column at a time. If the scanning is fast enough, it “appears” that the digit/character is displayed (due to illusion of our eyes)

28. Dot-matrix LED Display
Internal circuitry of a 5 x 7 dot matrix display is shown on the right
Voltage should be supplied to terminals C and 1 to light up the LED indicated in this picture b c d e f g 1 2 3 4 5

29. Dot-matrix LED Display
8051
Reset
XTAL1
XTAL2
VSS EA
Vcc
P1.0
P1.1
P1.2
P1.3
P1.4 c d e f g a b 1 2 3 4 5
Repeat 100 times
Drive Column P1 Method
On column
Off
On column
On column
On column
On column

30. Dot-matrix LED Display
Example: Displaying the character “E” on the dot-matrix. b c d e f g a 1 2 3 4 5 b c d e f g a 1 2 3 4 5 b c d e f g a b c d e f g a 1 2 3 4 5 1 2 3 4 5
Step 1: signals on pins
pins – “10000”
pins gfedcba – “ ”
Step 2: signals on pins
pins – “01000”
pins gfedcba – “ ”
Step 3: signals on pins
pins – “00100”
pins gfedcba – “ ”
Displaying the character “E” on the 5x7dot-matrix. 1 2 3 4 5 b c d e f g a 1 2 3 4 5 b c d e f g a
Step 4: signals on pins
pins – “00010”
pins gfedcba – “ ”

31. Code Words and Displays of ‘0’ to ‘4’B B B B B B B B B B B B B B B B B B B B B B

32. Code Words and Displays of ‘5’ to ‘9’B B B B B B B B B B B B B B
B B B B B

33. Program Listing – character display on 5x7 Dot-matrix display (1/2)
ORG 0000H
START: MOV DPTR, #TABLE ; point to the starting add of 1st char
MOV R3, #10 ; display 10 characters: 0,1,2,3,,,,,9,0,1,….
LOOP: MOV R2, #100 ; scan 100 times for each character
SCAN: ACALL SCAN1 ; 10ms x 100 = 1000ms
DJNZ R2, SCAN
INC DPTR ; increase DPTR by 5 to
INC DPTR ; point to the starting address of
INC DPTR ; the next character
INC DPTR
DJNZ R3, LOOP ; loop back to display 10 char
AJMP START
; ======================
; == Scan Subroutine ==
SCAN1: MOV R1, #00H ; R1 points to the starting add of a char
MOV R5, # B ; start from the leftmost column
MOV R4, #05 ; there are 5 columns, R4 as a counter !
LOOP1: MOV A, R1 ; get the code at add R1+DPTR
MOVC
MOV P3, A ; send the code to the dot-matrix display (ROW)
MOV P1, R5 ; turn-on a particular transistor (COLUMN)

34. Program Listing – character display on 5x7 Dot-matrix display (2/2)
MOV R6, #5 ; delay for 2ms delay routine
DL1: MOV R7, # ; delay routine
DL2: DJNZ R7, DL ; delay routine
DJNZ R6, DL ; delay routine
ORL P1, # B ; turn-off display
ANL P1,# B ; turn-off display
MOV A, R5 ;
RL A ; move to next column
MOV R5, A ; totally there are 5 columns
INC R1 ; NEXT COLUMN DATA
DJNZ R4, LOOP1 ;
RET ; return the main program
; ============================
; == Character Table ==
TABLE: DB B ; codes for 0
DB B
DB B
DB B
DB B
DB B ; code for 1
: ; codes for 2 to 9
: ;
END

35. Simple I/O Interfacing
EEE3410 Microcontroller Applications Department of Electrical Engineering
│ END of Lecture 9 │
Simple I/O Interfacing