1. I/O Devices Switches, LED, LCD
Week 9

2. I/O devices (Peripherals)
Examples: switches, LED, LCD, printers, keyboard, keypad
Interface chips
are needed to resolve the speed problem
synchronizes data transfer between CPU and I/O device
Connection of Interface and CPU
Data pins are connected to CPU data bus
I/O port pins are connected to I/O device
CPU may be connected to multiple interface
IO ports are simplest interface

3. I/O Interfacing
Dedicated instructions for IO operations (Isolated I/O)
same instruction for memory and IO (memory-mapped I/O)
MCS-51 (8051) is memory mapped

4. Synchronization of CPU and interface chip
To make sure that there are valid data in the interface
two ways
Polling method: Read status bit - Simple method
Interrupt driven method: interface interrupts the CPU when it has new data - CPU executes the ISR

5. Synchronization of CPU and interface chip
Output synchronization: two ways of doing this
Polling method
interface chip uses a status bit to indicate that the data register is empty
CPU keeps checking status bit until it is set, and then writes data into interface chip
Interrupt driven method: interface chip interrupts the CPU when it data register is empty. CPU executes the ISR

6. Synchronization of CPU and interface chip
Methods used to synchronize data transfer between interface chip and I/O devices:
Brute force method: interface chip returns voltage levels in its input ports to CPU and makes data written by CPU directly available on its output ports
All 8051 port can perform brute force I/O
Strobe method:
During input, the I/O device activates a strobe signal when data are stable. Interface chip latches the data
For output, interface chip places output data on output port. when data is stable, it activates a strobe signal. I/O device latches the data
Handshake method: two handshake signals are needed
One is asserted by interface chip and the other by I/O device

7. 8051 - Switch On I/O Ports Case-1: Case-2: Case-3:
Gives a logic 0 on switch close
Current is 0.5ma on switch close
Case-2:
Gives a logic 1 on switch close
High current on switch close
Case-3:
Can damage port if 0 is output

8. Simple input devices DIP switches usually have 8 switches
Use the case-1 from previous page
Sequence of instructions to read is:
MOV P1,#FFH
MOV A,P1,

9. Bouncing contacts Contact:
Push-button switches
Toggle switches
Electromechanical relays
Make and break Contact normally open switch
The effect is called "contact bounce" or, in a switch, "switch bounce”.
If used as edge-triggered input (as INT0), several interrupt is accorded

10. Hardware Solution An RC time constant to suppress the bounce
The time constant has to be larger than the switch bounce
Vcc
OUT

11. Hardware Solution

12. Software Solution Read the new state of switch N time
Wait-and-see technique
When the input drops
an “appropriate” delay is executed (10 ms)
then the value of the line is checked again to make sure the line has stopped bouncing

13. Interfacing a Keypad Place a 0 on R0 port Read C port
16 keys arranged as a 4X4 matrix
Place a 0 on R0 port
Read C port
If there is a 0 bit then the button at the column/row intersection has been pressed.
Otherwise, try next row
Repeat constantly 1 2 3 5 6 7 D E F 9 A B C 8 4

14. Interfacing a Keypad 8051 scan: mov P1,#EFH jnb P1.0,db_0
scan1: jnb P1.1,db_1
scan2: jnb P1.2,db_2
scan3: jnb P1.3,db_3
scan4: mov P1,#DFH
jnb P1.0,db_4
…..
8051 F E D C B A 9 8 7 6 5 4 3 2 1

15. Interfacing a Keypad get_code: mov DPTR, #key_tab movc A, @A+DPTR
db_0: lcall wt_10ms
jb P1.0, scan1
mov A, #0
ljmp get_code
db_1: lcall wt_10ms
jb P1.1, scan2
mov A, #1
….. …
get_code: mov DPTR, #key_tab
movc
ljmp scan
key_tab: db ‘ ABCDEF’
END

16. Simple output devices Case-1 Case-2 Case-3 LED is ON if output=zero
Most LEDs drop 1.7 volts and need about 10ma
Current is (5-1.7)/470
Case-2
Too much current
Failure of Port or LED
Case-3
Not enough drive (1ma)
LED is too dim

17. The 7-Segment Display 7 LEDs arranged to form the number 8.
By turning on and off (LEDs), different combinations can be produced.
useful for displaying the digits 0 through 9, and some characters. a b c f e g d

18. The 7-segment Display (Cont.)
7-segment displays come in 2 configurations:
Common Anode Common Cathode
Connect cathode to the output Connect cathode to the output
Therefore, the common anode variety would be better for our interfacing needs.

19. Interfacing a 7-segment display
A resistor will be needed to control the current
This leaves two possibilities:
Case 2 would be more appropriate
Case 1 will produce different brightness depending on the number of LEDs turned on.
Case1
Case 2

20. ET2640 Microprocessors and Microcontrollers

21. ET2640 Microprocessors and Microcontrollers

22. ET2640 Microprocessors and Microcontrollers

23. Continued on next slide
ET2640 Microprocessors and Microcontrollers

24. Continued from previous
ET2640 Microprocessors and Microcontrollers

25. Use of current buffer
Interfacing to a DIP switch and 7-segment display
Output a ‘1’ to ON a segment
We can use to common cathode 7_seg

26. LCD Connections
ET2640 Microprocessors and Microcontrollers

27. BCD to 7_Seg lookup table
p g f e d c b a
7_seg
hex
0000 3f
0001 30
0010 5b
0011 4f
0100 66
0101 6d
0110 7d
0111 07
1000 7f
1001 6f
mov a,p3
anl a,0fh
get_code: mov DPTR, #7s_tab
movc
mov p1,a
7s_tab: db 3fh,30h,5bh,4fh,66h
db 6dh,7dh,07h,7fh,6fh
END a a a a a a a a f b f b b f b f f b f b f b g g g g g g g e c e e c c c e c c e c c d d d d d d d

28. LCD Interfacing Liquid Crystal Displays (LCDs)
cheap and easy way to display text
Various configurations (1 line by 20 X char up to 8 lines X 80)
Integrated controller
The display has two register
command register
data register
By RS you can select register
Data lines (DB7-DB0) used to transfer data and commands

29. Alphanumeric LCD Interfacing
R/W RS
DB7–DB0
LCD controller
communications bus
Microcontroller 8
LCD Module
Pinout
8 data pins D7:D0
RS: Data or Command Register Select
R/W: Read or Write
E: Enable (Latch data)
RS – Register Select
RS = 0  Command Register
RS = 1  Data Register
R/W = 0  Write , R/W = 1  Read
E – Enable
Used to latch the data present on the data pins.
D0 – D7
Bi-directional data/command pins.
Alphanumeric characters are sent in ASCII format.

30. LCD Commands
The LCD’s internal controller can accept several commands and modify the display accordingly Such as:
Clear screen
Return home
Decrement/Increment cursor
After writing to the LCD, it takes some time for it to complete its internal operations. During this time, it will not accept any new commands or data.
We need to insert time delay between any two commands or data sent to LCD

31. Command Codes

32. LCD Addressing

33. LCD Timing

35. Interfacing LCD with 8051 8051 LM015 RW E RS P1.7-P1.0 D7-D0 P3.4 P3.5

36. Interfacing LCD with 8051 mov A, command call cmd delay
mov A, another_cmd
mov A, #’A’
call data
mov A, #’B’ ….
Command and Data Write Routines
data:mov P1, A ;A is ascii data
setb P3.3 ;RS=1 data
clr P3.4 ;RW=0 for write
setb P3.5 ;H->L pulse on E
clr P3.5
ret
cmd:mov P1,A ;A has the cmd word
clr P3.3 ;RS=0 for cmd
setb P3.5 ;H->L pulse on E
Interfacing LCD with 8051

37. Example

38. 8255 Usage: Simple Example
8255 memory mapped to 8051 at address C000H base
A = C000H, B = C001H, C = C002H, CR = C003H
Control word for all ports as outputs in mode0
CR : b = 80H
test: mov A, #80H ; control word
mov DPTR, #C003H ; address of CR
movx @DPTR, A ; write control word
mov A, #55h ; will try to write 55 and AA
; alternatively
repeat:mov DPTR,#C000H ; address of PA
movx @DPTR, A ; write 55H to PA
inc DPTR ; now DPTR points to PB
movx @DPTR, A ; write 55H to PB
inc DPTR ; now DPTR points to PC
movx @DPTR, A ; write 55H to PC
cpl A ; toggle A (55AA, AA55)
acall MY_DELAY ; small delay subroutine
sjmp repeat ; for (1)

39. Next Week—Week10
Unit 10 Quiz
Interfacing with A/D and D/A Converters