L14 kb-lcd-interfacing-with-atmega328 p

Interfacing Keyboard and LCD Displays
(Lecture-14)
R S Ananda Murthy
Associate Professor
Department of Electrical & Electronics Engineering,
Sri Jayachamarajendra College of Engineering,
Mysore 570 006
R S Ananda Murthy Interfacing Keyboard and LCD Displays

Linear Arrangement of Keys
MCU
PB0
PB1
PB2
PB3
Keyboard is a collection of push-buttons.
Each key has an associated internal pull-up resistor
enabled.
In linear arrangement, number of input lines required is
equal to the number of keys.

Matrix Arrangement of Keys
When large number of keys are to be interfaced, linear
arrangement is not preferred since the number of input
lines required will be more.
In matrix arrangement, a key is connected between a row
and a column.
If there are R rows and C columns, then, the maximum
number of keys in the matrix could be N = R ×C and the
number of lines required for interfacing would be R +C and
not N.

Matrix Arrangement of Keys
0123
4567
89AB
CDEF
10k 10k 10k 10k
10k
10k
10k
10k
PD4PD5PD6PD7
Input Lines
PD3
PD2
PD1
PD0
OutputLines
Vcc
Only one 8-bit port is needed to interface 4×4 keyboard
matrix instead of two 8-bit ports as in linear arrangement.

Steps to Identity Depressed Key
1 Poll the keyboard for any previously pressed key to be
released.
2 Poll the keyboard for any key to be pressed.
3 When any key is pressed, wait for about 20 ms to
debounce the key.
4 Scan the keyboard to ﬁnd which key is pressed to generate
the keycode.
5 Perform the operation assigned to a key.
6 Go back to Step-1.

Keyboard Interface Logic
Start
Ground all rows
Read columns
All keys
open
?
No
Yes
A
A
Read columns
Any key
down
?
No
Yes
Wait for debounce
B
B
Read columns
Any key
down
?
No
Yes
A
C
Identify Key
C
Do operation
assigned to key
D
D
In case of encoded keyboard, the keycode is generated by
hardware.
In case of un-encoded keyboard, the keycode has to be
generated by software.

Identifying the Depressed Key
Ground next row
Read columns
key on
in this row
?
No Yes Scan columns
to identify key

16×2 LCD Panel

Advantages of LCD Panels
LCD panels are becoming cheaper.
LCD panels can display numbers, characters, and graphics
unlike LEDs which are limited to numbers and few
characters.
Since LCDs have an in-built refreshing controller the MCU
need not refresh the display as in the case of LEDs.
LCDs are easy to program since ASCII codes are used for
display by LCDs instead of 7-segment codes used for
LEDs.
LCDs consume less power.

LCD Pin Descriptions
Pin Signal I/O Description
1 VSS – Ground
2 VCC – +5 V Supply
3 VEE – Power supply to control contrast
4 RS I
RS = 0 selects Command Register
RS = 1 selects Data Register
5 R/W I
R/W = 0 for write operation
R/W = 1 for read operation
6 E I/O High-to-low pulse latches data to LCD
7-14 D0-D7 I/O Lines for sending or reading data
LCD panel sizes are speciﬁed as C ×L where
C =Characters per line and L = No. of lines.
ASCII codes are used for display.
8-bits or 4bits data lines can be used for interfacing to the
MCU.

LCD Connections using 8-bit Data Bus
10 k
Pot
PD0
PD7
.
.
.
.
.
.
.
Atmega328P LCD
D0
.
.
.
.
.
.
.
D7
PB0
PB1
PB2
RS R/W E
+ 5 V
Commands to initialize the LCD panel: 0x38, 0x0E, 0x01.

LCD Connections using 4-bit Data Bus
10 k
Pot
PD4
PD7
.
.
.
.
Atmega328P LCD
D4
.
.
.
.
D7
PB0
PB1
PB2
RS R/W E
+5 V
Requires PB0, PB1, PB2 and higher nibble of Port D on
which data is sent, higher nibble ﬁrst.
Initialization commands: 0x33, 0x32, 0x28, 0x0E, 0x01,
0x06

LCD Connections using Only One Port
10 k
Pot
PD4
PD7
.
.
.
.
Atmega328P LCD
D4
.
.
.
.
D7
PD0
PD1
PD2
RS R/W E
+ 5V
Requires only Port D.
Initialization commands: 0x33, 0x32, 0x28, 0x0E, 0x01,
0x06

LCD Command Codes
Hex Code Command to LCD Command Register
01 Clear display screen
02 Return home
04 Shift cursor to left by one position
06 Shift cursor to right by one position
05 Shift display right
07 Shift display left
08 Display off, cursor off
0A Display off, cursor on
0C Display on, cursor off

LCD Command Codes
Hex Code Command to LCD Command Register
0E Display on, cursor blinking
0F 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 the beginning of 1st line
C0 Force cursor to the beginning of 2nd line
28 2-lines and 5×7 matrix D4-D7, 4 bits
33 Go into 4-bit operating mode
32 Go into 4-bit operating mode
38 2-lines and 5×7 matrix D0-D7, 8 bits

Steps for Sending Commands and Data to LCD
1 Initialize LCD.
2 Send any of the commands to the LCD for the required
operation.
3 Send the character to be shown to the LCD.

Initializing the LCD Panel
1 After power-on, wait for about 15 ms before sending the
initializing commands to the LCD panel.
2 For eg. to initialize the LCD for 5×7 matrix and 8-bit
operation, send the following commands to the command
register of the LCD panel: 0x38, 0x0E, and 0x01.

Sending Commands to the LCD Panel
Make pins RS = 0 and R/W = 0.
Place the command bits on the data pins.
Send a high-to-low pulse to the E pin of the panel.
Wait for about 100µs before sending the next command.
But in case of clear LCD (0x01) and Return Home (0x02)
commands wait for about 2 ms before issuing the next
command.

Sending Data to the LCD Panel
Make pins RS = 1 and R/W = 0.
Place the data on the data lines in ASCII format. For
example, to display A, the ASCII code to be placed on the
data lines would be 0x41.
Send a high-to-low pulse to the E pin of the panel.
Wait for about 100µs before sending the next data.

License
This work is licensed under a
Creative Commons Attribution 4.0 International License.

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