LCD AND KEYBOARD

INTERFACING

The 8051 Microcontroller and Embedded

Systems: Using Assembly and C
Mazidi, Mazidi and McKinlay

Chung-Ping Young
楊中平

Home Automation, Networking, and Entertainment Lab

Dept. of Computer Science and Information Engineering
National Cheng Kung University, TAIWAN
 ‰ LCD is finding widespread use
LCD
INTERFACING
replacing LEDs
¾ The declining prices of LCD
LCD Operation ¾ The ability to display numbers, characters,
and graphics
¾ Incorporation of a refreshing controller
into the LCD, thereby relieving the CPU of
the task of refreshing the LCD
¾ Ease of programming for characters and
graphics

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 Pin Descriptions for LCD
Pin Symbol I/O Descriptions
LCD
1 VSS -- Ground
INTERFACING 2 VCC -- +5V power supply
3 VEE -- Power supply to control contrast
LCD Pin 4 RS I RS=0 to select command register,
Descriptions RS=1 to select data register
5 R/W I R/W=0 for write,
R/W=1 for read
used by the
6 E I/O Enable LCD to latch
7 DB0 I/O The 8-bit data bus information
- Send displayed 8 DB1 I/O The 8-bit data bus presented to
information or its data bus
9 DB2 I/O The 8-bit data bus
instruction
command codes to 10 DB3 I/O The 8-bit data bus
the LCD 11 DB4 I/O The 8-bit data bus
- Read the contents 12 DB5 I/O The 8-bit data bus
of the LCD’s 13 DB6 I/O The 8-bit data bus
internal registers 14 DB7 I/O The 8-bit data bus

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 LCD Command Codes
Code (Hex) Command to LCD Instruction Register

LCD 1 Clear display screen

2 Return home
INTERFACING
4 Decrement cursor (shift cursor to left)
6 Increment cursor (shift cursor to right)
LCD Command 5 Shift display right
Codes 7 Shift display left
8 Display off, cursor off
A Display off, cursor on
C Display on, cursor off
E Display on, cursor blinking
F Display on, cursor blinking
10 Shift cursor position to left
14 Shift cursor position to right
18 Shift the entire display to the left
1C Shift the entire display to the right
80 Force cursor to beginning to 1st line
C0 Force cursor to beginning to 2nd line
38 2 lines and 5x7 matrix

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 To send any of the commands to the LCD, make pin RS=0. For data,
make RS=1. Then send a high-to-low pulse to the E pin to enable the
LCD internal latch of the LCD. This is shown in the code below.
;calls a time delay before sending next data/command
INTERFACING ;P1.0-P1.7 are connected to LCD data pins D0-D7
;P2.0 is connected to RS pin of LCD
Sending Data/ ;P2.1 is connected to R/W pin of LCD
;P2.2 is connected to E pin of LCD
Commands to ORG 0H
LCDs w/ Time MOV A,#38H ;INIT. LCD 2 LINES, 5X7 MATRIX
ACALL COMNWRT ;call command subroutine
Delay ACALL DELAY ;give LCD some time
MOV A,#0EH ;display on, cursor on
+5V
8051 VCC
ACALL COMNWRT ;call command subroutine
P1.0 D0 ACALL DELAY ;give LCD some time
VEE 10k MOV A,#01 ;clear LCD
POT
LCD ACALL COMNWRT ;call command subroutine
P1.7 D7 VSS ACALL DELAY ;give LCD some time
RS R/W E
MOV A,#06H ;shift cursor right
ACALL COMNWRT ;call command subroutine
ACALL DELAY ;give LCD some time
P2.0
MOV A,#84H ;cursor at line 1, pos. 4
P2.1 ACALL COMNWRT ;call command subroutine
ACALL DELAY ;give LCD some time
P2.2 .....
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 .....
MOV A,#’N’ ;display letter N
ACALL DATAWRT ;call display subroutine
LCD
ACALL DELAY ;give LCD some time
MOV A,#’O’ ;display letter O
INTERFACING ACALL
AGAIN: SJMP
DATAWRT
AGAIN
;call display subroutine
;stay here
COMNWRT: ;send command to LCD
Sending Data/ MOV
CLR
P1,A
P2.0
;copy reg A to port 1
;RS=0 for command
Commands to CLR P2.1 ;R/W=0 for write
LCDs w/ Time
SETB P2.2 ;E=1 for high pulse
ACALL DELAY ;give LCD some time
Delay CLR
RET
P2.2 ;E=0 for H-to-L pulse

(cont’) +5V DATAWRT: ;write data to LCD

8051 VCC MOV P1,A ;copy reg A to port 1
P1.0 D0 SETB P2.0 ;RS=1 for data
VEE 10k CLR P2.1 ;R/W=0 for write
POT
LCD SETB P2.2 ;E=1 for high pulse
ACALL DELAY ;give LCD some time
P1.7 D7 VSS
CLR P2.2 ;E=0 for H-to-L pulse
RS R/W E RET
DELAY: MOV R3,#50 ;50 or higher for fast CPUs
P2.0
HERE2: MOV R4,#255 ;R4 = 255
HERE: DJNZ R4,HERE ;stay until R4 becomes 0
P2.1 DJNZ R3,HERE2
RET
P2.2 END
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 ;Check busy flag before sending data, command to LCD
;p1=data pin
LCD ;P2.0 connected to RS pin
INTERFACING ;P2.1 connected to R/W pin
;P2.2 connected to E pin
ORG 0H
Sending Data/ MOV A,#38H ;init. LCD 2 lines ,5x7 matrix
Commands to
ACALL COMMAND ;issue command
MOV A,#0EH ;LCD on, cursor on
LCDs w/ Time ACALL COMMAND ;issue command
MOV A,#01H ;clear LCD command
Delay ACALL COMMAND ;issue command
(cont’) +5V
MOV A,#06H ;shift cursor right
8051 VCC ACALL COMMAND ;issue command
P1.0 D0 MOV A,#86H ;cursor: line 1, pos. 6
10k
VEE
POT
ACALL COMMAND ;command subroutine
LCD MOV A,#’N’ ;display letter N
P1.7 D7 VSS ACALL DATA_DISPLAY
RS R/W E MOV A,#’O’ ;display letter O
ACALL DATA_DISPLAY
HERE:SJMP HERE ;STAY HERE
P2.0 .....
P2.1

P2.2

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 .....
COMMAND:
ACALL READY ;is LCD ready?
LCD MOV
CLR
P1,A
P2.0
;issue command code
;RS=0 for command
INTERFACING CLR P2.1 ;R/W=0 to write to LCD
SETB P2.2 ;E=1 for H-to-L pulse
Sending Codes
CLR P2.2 ;E=0,latch in
RET
and Data to DATA_DISPLAY:
ACALL READY ;is LCD ready?
LCDs w/ Busy MOV P1,A ;issue data
Flag
SETB P2.0 ;RS=1 for data
CLR P2.1 ;R/W =0 to write to LCD
(cont’) +5V
SETB P2.2 ;E=1 for H-to-L pulse
8051 VCC CLR P2.2 ;E=0,latch in
P1.0 D0 RET To read the command register, we make R/W=1,
VEE 10k READY:
POT RS=0, and a H-to-L pulse for the E pin.
LCD SETB P1.7 ;make P1.7 input port
P1.7 D7 VSS CLR P2.0 ;RS=0 access command reg
SETB P2.1 ;R/W=1 read command reg
RS R/W E
;read command reg and check busy flag
BACK:SETB P2.2 ;E=1 for H-to-L pulse
P2.0 CLR P2.2 ;E=0 H-to-L pulse
JB P1.7,BACK ;stay until busy flag=0
P2.1
RET If bit 7 (busy flag) is high, the LCD is busy
P2.2 END
and no information should be issued to it.
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 LCD Timing for Read

LCD tD = Data output delay time

INTERFACING
D0 – D7 Data
Sending Codes tD

and Data to
LCDs w/ Busy E

Flag R/W tAS tAH

(cont’)
RS
tAH = Hold time after E has
come down for both RS and
R/W = 10 ns (minimum)

tAS = Setup time prior to E

(going high) for both RS and
R/W = 140 ns (minimum)

Note : Read requires an L-to-H pulse for the E pin

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 LCD Timing for Write

LCD tDSW = Data set up time

INTERFACING = 195 ns (minimum)

Data
Sending Codes tH = Data hold time
and Data to tH = 10 ns (minimum)
LCDs w/ Busy E tDSW

Flag R/W tAS tPWH tAH

(cont’)
RS
tAH = Hold time after E has
come down for both RS and
R/W = 10 ns (minimum)

tPWH = Enable pulse width

= 450 ns (minimum)

tAS = Setup time prior to E

(going high) for both RS and
R/W = 140 ns (minimum)

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 ‰ One can put data at any location in the
LCD
INTERFACING
LCD and the following shows address
locations and how they are accessed
LCD Data Sheet RS R/W DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0
0 0 1 A A A A A A A

¾ AAAAAAA=000_0000 to 010_0111 for line1

¾ AAAAAAA=100_0000 to 110_0111 for line2
The upper address
range can go as LCD Addressing for the LCDs of 40×2 size
high as 0100111
DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0
for the 40-
character-wide Line1 (min) 1 0 0 0 0 0 0 0
LCD, which Line1 (max) 1 0 1 0 0 1 1 1
corresponds to Line2 (min) 1 1 0 0 0 0 0 0
locations 0 to 39 Line2 (max) 1 1 1 0 0 1 1 1

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 ;Call a time delay before sending next data/command
LCD ; P1.0-P1.7=D0-D7, P2.0=RS, P2.1=R/W, P2.2=E
INTERFACING ORG 0
MOV DPTR,#MYCOM
Sending C1: CLR A
MOVC A,@A+DPTR
Information to ACALL COMNWRT ;call command subroutine
LCD Using ACALL
INC
DELAY
DPTR
;give LCD some time

MOVC JZ SEND_DAT
Instruction
SJMP C1
SEND_DAT:
MOV DPTR,#MYDATA
D1: CLR A
MOVC A,@A+DPTR
ACALL DATAWRT ;call command subroutine
ACALL DELAY ;give LCD some time
INC DPTR
JZ AGAIN
SJMP D1
AGAIN: SJMP AGAIN ;stay here
.....

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 .....
COMNWRT: ;send command to LCD
LCD MOV
CLR
P1,A
P2.0
;copy reg A to P1
;RS=0 for command
INTERFACING CLR P2.1 ;R/W=0 for write
SETB P2.2 ;E=1 for high pulse
Sending ACALL
CLR
DELAY
P2.2
;give LCD some time
;E=0 for H-to-L pulse
Information to RET
DATAWRT: ;write data to LCD
LCD Using MOV P1,A ;copy reg A to port 1
MOVC SETB
CLR
P2.0
P2.1
;RS=1 for data
;R/W=0 for write
Instruction SETB P2.2 ;E=1 for high pulse
ACALL DELAY ;give LCD some time
(cont’) CLR P2.2 ;E=0 for H-to-L pulse
RET
DELAY: MOV R3,#250 ;50 or higher for fast CPUs
HERE2: MOV R4,#255 ;R4 = 255
HERE: DJNZ R4,HERE ;stay until R4 becomes 0
DJNZ R3,HERE2
RET
ORG 300H
MYCOM: DB 38H,0EH,01,06,84H,0 ; commands and null
MYDATA: DB “HELLO”,0
END

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 Example 12-2
LCD Write an 8051 C program to send letters ‘M’, ‘D’, and ‘E’ to the LCD
INTERFACING using the busy flag method.

Sending Solution:

Information to #include <reg51.h>

sfr ldata = 0x90; //P1=LCD data pins
LCD Using sbit rs = P2^0;
sbit rw = P2^1;
MOVC sbit en = P2^2;
Instruction sbit busy = P1^7;
void main(){
(cont’) lcdcmd(0x38);
lcdcmd(0x0E);
lcdcmd(0x01);
lcdcmd(0x06);
lcdcmd(0x86); //line 1, position 6
lcdcmd(‘M’);
lcdcmd(‘D’);
lcdcmd(‘E’);
}
.....

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 .....
LCD void lcdcmd(unsigned char value){
lcdready(); //check the LCD busy flag
INTERFACING ldata = value; //put the value on the pins
rs = 0;
Sending rw = 0;
en = 1; //strobe the enable pin
Information to MSDelay(1);
LCD Using en = 0;
return;
MOVC }
Instruction void lcddata(unsigned char value){
(cont’) lcdready(); //check the LCD busy flag
ldata = value; //put the value on the pins
rs = 1;
rw = 0;
en = 1; //strobe the enable pin
MSDelay(1);
en = 0;
return;
}
.....

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 .....
LCD void lcdready(){
INTERFACING busy = 1;
rs = 0;
//make the busy pin at input

rw = 1;
Sending while(busy==1){ //wait here for busy flag
en = 0; //strobe the enable pin
Information to MSDelay(1);
LCD Using en = 1;
}
MOVC
Instruction void lcddata(unsigned int itime){
unsigned int i, j;
(cont’) for(i=0;i<itime;i++)
for(j=0;j<1275;j++);
}

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 ‰ Keyboards are organized in a matrix of
KEYBOARD
INTERFACING
rows and columns
¾ The CPU accesses both rows and columns
through ports
ƒ Therefore, with two 8-bit ports, an 8 x 8 matrix
of keys can be connected to a microprocessor
¾ When a key is pressed, a row and a
column make a contact
ƒ Otherwise, there is no connection between
rows and columns
‰ In IBM PC keyboards, a single
microcontroller takes care of hardware
and software interfacing

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 ‰ A 4x4 matrix connected to two ports
KEYBOARD
INTERFACING ¾ The rows are connected to an output port
and the columns are connected to an
Scanning and input port
Matrix Keyboard Connection to ports
Identifying the
Key Vcc

3 2 1 0
D0
7 6 5 4
D1 If no key has
B A 9 8 been pressed,
If all the rows are D2 reading the
grounded and a key F E D C input port will
is pressed, one of D3 yield 1s for all
the columns will columns since
have 0 since the key Port 1 they are all
pressed provides the connected to
(Out) D3 D2 D1 D0 Port 2
path to ground high (Vcc)
(In)
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 ‰ It is the function of the microcontroller
KEYBOARD to scan the keyboard continuously to
INTERFACING detect and identify the key pressed
‰ To detect a pressed key, the
Grounding microcontroller grounds all rows by
Rows and providing 0 to the output latch, then it
Reading reads the columns
Columns ¾ If the data read from columns is D3 – D0 =
1111, no key has been pressed and the
process continues till key press is detected
¾ If one of the column bits has a zero, this
means that a key press has occurred
ƒ For example, if D3 – D0 = 1101, this means that
a key in the D1 column has been pressed
ƒ After detecting a key press, microcontroller will
go through the process of identifying the key

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 ‰ Starting with the top row, the
KEYBOARD microcontroller grounds it by providing
INTERFACING a low to row D0 only
¾ It reads the columns, if the data read is all
Grounding
1s, no key in that row is activated and the
Rows and process is moved to the next row
Reading
Columns ‰ It grounds the next row, reads the
(cont’) columns, and checks for any zero
¾ This process continues until the row is
identified
‰ After identification of the row in which
the key has been pressed
¾ Find out which column the pressed key
belongs to

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 Example 12-3
KEYBOARD From Figure 12-6, identify the row and column of the pressed key for
each of the following.
INTERFACING (a) D3 – D0 = 1110 for the row, D3 – D0 = 1011 for the column
(b) D3 – D0 = 1101 for the row, D3 – D0 = 0111 for the column
Grounding Solution :
Rows and From Figure 13-5 the row and column can be used to identify the key.
Reading (a) The row belongs to D0 and the column belongs to D2; therefore,
key number 2 was pressed.
Columns (b) The row belongs to D1 and the column belongs to D3; therefore,
(cont’) key number 7 was pressed.

3 2 1 0
D0
7 6 5 4
D1
B A 9 8
D2
F E D C
D3
Vcc
Port 1
(Out) D3 D2 D1 D0 Port 2
(In)

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 ‰ Program 12-4 for detection and
KEYBOARD
INTERFACING
identification of key activation goes
through the following stages:
Grounding 1. To make sure that the preceding key has
Rows and been released, 0s are output to all rows
Reading at once, and the columns are read and
Columns checked repeatedly until all the columns
(cont’) are high
ƒ When all columns are found to be high, the
program waits for a short amount of time
before it goes to the next stage of waiting for
a key to be pressed

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 2. To see if any key is pressed, the columns
KEYBOARD are scanned over and over in an infinite
INTERFACING loop until one of them has a 0 on it
ƒ Remember that the output latches connected
Grounding to rows still have their initial zeros (provided
Rows and in stage 1), making them grounded
Reading ƒ After the key press detection, it waits 20 ms
Columns for the bounce and then scans the columns
(cont’) again
(a) it ensures that the first key press
detection was not an erroneous one due a
spike noise
(b) the key press. If after the 20-ms delay the
key is still pressed, it goes back into the
loop to detect a real key press

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 3. To detect which row key press belongs to,
KEYBOARD it grounds one row at a time, reading the
INTERFACING columns each time
ƒ If it finds that all columns are high, this means
that the key press cannot belong to that row
Grounding – Therefore, it grounds the next row and
Rows and continues until it finds the row the key
press belongs to
Reading ƒ Upon finding the row that the key press
Columns belongs to, it sets up the starting address for
(cont’) the look-up table holding the scan codes (or
ASCII) for that row
4. To identify the key press, it rotates the
column bits, one bit at a time, into the
carry flag and checks to see if it is low
ƒ Upon finding the zero, it pulls out the ASCII
code for that key from the look-up table
ƒ otherwise, it increments the pointer to point to
the next element of the look-up table
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 Flowchart for Program 12-4
1
KEYBOARD
Read all columns
INTERFACING Start

Grounding Ground all rows

no All keys
down?
Rows and
yes
Reading Read all columns
Columns Wait for debounce
(cont’)
All keys
open? Read all columns

yes
no no All keys
1 down?
yes

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 2
KEYBOARD
INTERFACING
Ground next row

Grounding
Rows and no All keys
down?
Reading
yes
Columns
(cont’) Find which key
is pressed

Get scan code

from table

Return

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 Program 12-4: Keyboard Program
KEYBOARD ;keyboard subroutine. This program sends the ASCII
INTERFACING ;code for pressed key to P0.1
;P1.0-P1.3 connected to rows, P2.0-P2.3 to column

Grounding MOV P2,#0FFH ;make P2 an input port

K1: MOV P1,#0 ;ground all rows at once
Rows and MOV A,P2 ;read all col
Reading ;(ensure keys open)
ANL A,00001111B ;masked unused bits
Columns CJNE A,#00001111B,K1 ;till all keys release
(cont’) K2: ACALL DELAY ;call 20 msec delay
MOV A,P2 ;see if any key is pressed
ANL A,00001111B ;mask unused bits
CJNE A,#00001111B,OVER;key pressed, find row
SJMP K2 ;check till key pressed
OVER: ACALL DELAY ;wait 20 msec debounce time
MOV A,P2 ;check key closure
ANL A,00001111B ;mask unused bits
CJNE A,#00001111B,OVER1;key pressed, find row
SJMP K2 ;if none, keep polling
....

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 ....
KEYBOARD OVER1: MOV P1, #11111110B ;ground row 0
INTERFACING MOV A,P2 ;read all columns
ANL A,#00001111B ;mask unused bits
CJNE A,#00001111B,ROW_0 ;key row 0, find col.
Grounding MOV P1,#11111101B ;ground row 1
Rows and MOV A,P2 ;read all columns
Reading ANL A,#00001111B ;mask unused bits
CJNE A,#00001111B,ROW_1 ;key row 1, find col.
Columns MOV P1,#11111011B ;ground row 2
(cont’) MOV A,P2 ;read all columns
ANL A,#00001111B ;mask unused bits
CJNE A,#00001111B,ROW_2 ;key row 2, find col.
MOV P1,#11110111B ;ground row 3
MOV A,P2 ;read all columns
ANL A,#00001111B ;mask unused bits
CJNE A,#00001111B,ROW_3 ;key row 3, find col.
LJMP K2 ;if none, false input,
;repeat
....

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 ....
ROW_0: MOV DPTR,#KCODE0 ;set DPTR=start of row 0
KEYBOARD SJMP FIND ;find col. Key belongs to
INTERFACING ROW_1: MOV DPTR,#KCODE1 ;set DPTR=start of row
SJMP FIND ;find col. Key belongs to
ROW_2: MOV DPTR,#KCODE2 ;set DPTR=start of row 2
Grounding SJMP FIND ;find col. Key belongs to
ROW_3: MOV DPTR,#KCODE3 ;set DPTR=start of row 3
Rows and FIND: RRC A ;see if any CY bit low
Reading JNC MATCH ;if zero, get ASCII code

Columns
INC DPTR ;point to next col. addr
SJMP FIND ;keep searching
(cont’) MATCH: CLR A ;set A=0 (match is found)
MOVC A,@A+DPTR ;get ASCII from table
MOV P0,A ;display pressed key
LJMP K1
;ASCII LOOK-UP TABLE FOR EACH ROW
ORG 300H
KCODE0: DB ‘0’,’1’,’2’,’3’ ;ROW 0
KCODE1: DB ‘4’,’5’,’6’,’7’ ;ROW 1
KCODE2: DB ‘8’,’9’,’A’,’B’ ;ROW 2
KCODE3: DB ‘C’,’D’,’E’,’F’ ;ROW 3
END

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