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1 Microprocessors 6-1 I/O Devices Switches, LED, LCD Lec note 6

2 Microprocessors 6-2 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 Microprocessors 6-3 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 Microprocessors 6-4 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 Microprocessors 6-5 Synchronization of CPU and interface chip  Output synchronization: two ways of doing this 1.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 2.Interrupt driven method: interface chip interrupts the CPU when it data register is empty. CPU executes the ISR

6 Microprocessors 6-6 Synchronization of CPU and interface chip  Methods used to synchronize data transfer between interface chip and I/O devices: 1.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 2.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 3.Handshake method: two handshake signals are needed  One is asserted by interface chip and the other by I/O device

7 Microprocessors Switch On I/O Ports  Case-1:  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 Microprocessors 6-8 Simple input devices  DIP switches usually have 8 switches  Use the case-1 from previous page  Sequence of instructions to read is: MOVP1,#FFH MOVA,P1,

9 Microprocessors 6-9 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 Microprocessors 6-10 Hardware Solution  An RC time constant to suppress the bounce  The time constant has to be larger than the switch bounce Vcc OUT

11 Microprocessors 6-11 Hardware Solution

12 Microprocessors 6-12 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 Microprocessors 6-13 Interfacing a Keypad 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 DEF 9AB C 8 4

14 Microprocessors 6-14 Interfacing a Keypad scan:movP1,#EFH jnbP1.0,db_0 scan1:jnbP1.1,db_1 scan2:jnbP1.2,db_2 scan3:jnbP1.3,db_3 scan4:movP1,#DFH jnbP1.0,db_4 ….. P1.3 P1.2 P1.1 P1.0 P1.7 P1.6 P1.5 P DEF 9AB C

15 Microprocessors 6-15 Interfacing a Keypad db_0:lcallwt_10ms jbP1.0, scan1 movA, #0 ljmpget_code db_1:lcallwt_10ms jbP1.1, scan2 movA, #1 ljmpget_code ….. … ….. get_code:movDPTR, #key_tab ljmpscan key_tab:db‘ ABCDEF’ END

16 Microprocessors 6-16 Simple output devices  Case-1  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 Microprocessors 6-17 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 Microprocessors 6-18 The 7-segment Display (Cont.)  7-segment displays come in 2 configurations: Common AnodeCommon Cathode Connect cathode to the output Connect cathode to the output  Therefore, the common anode variety would be better for our interfacing needs.

19 Microprocessors 6-19 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.

20 Microprocessors 6-20 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

21 Microprocessors 6-21 BCD to 7_Seg lookup table mov a,p3 anl a,0fh get_code:movDPTR, #7s_tab mov p1,a 7s_tab:db3fh,30h,5bh,4fh,66h db 6dh,7dh,07h,7fh,6fh END a b c f e d f e a b e g d a b c g d b c f g a c f g d a c f e g d a b c a b c f e g d a b c f g d BCDp g f e d c b a 7_seg he x f b f d d f f

22 Microprocessors 6-22 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

23 Microprocessors 6-23 Alphanumeric LCD Interfacing  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. E R/W RS DB7–DB0 LCD controller communications bus Microcontrolle r 8 LCD Module

24 Microprocessors 6-24 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

25 Microprocessors 6-25 Pin Description

26 Microprocessors 6-26 Command Codes

27 Microprocessors 6-27 LCD Addressing

28 Microprocessors 6-28 LCD Timing

29 Microprocessors 6-29

30 Microprocessors 6-30 Interfacing LCD with 8051 LM P1.7-P1.0 D7-D0 RW RS E P3.4 P3.5 P3.3

31 Microprocessors 6-31 Interfacing LCD with 8051 mov A, command call cmd delay mov A, another_cmd call cmd delay mov A, #’A’ call data delay mov A, #’B’ call data delay …. 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 clr P3.4 ;RW=0 for write setb P3.5 ;H->L pulse on E clr P3.5 ret

32 Microprocessors 6-32 Example

33 Microprocessors 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 A ; write control word mov A, #55h ; will try to write 55 and AA ; alternatively repeat:mov DPTR,#C000H ; address of PA A ; write 55H to PA inc DPTR ; now DPTR points to PB A ; write 55H to PB inc DPTR ; now DPTR points to PC A ; write 55H to PC cpl A ; toggle A (55  AA, AA  55) acall MY_DELAY ; small delay subroutine sjmp repeat ; for (1)

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