The document describes interfacing a 3x4 keypad with an 8051 microcontroller to display the digit corresponding to the pressed key on a 7-segment display. It explains that the keypad uses row and column scanning to detect key presses. The 8051 will be configured with port 2 connected to the keypad and port 1 connected to the 7-segment display. An algorithm is provided to scan the keypad rows and columns to determine the pressed key and display the corresponding digit on the 7-segment display.

1. INTERFACING KEYPAD AND DISPLAY THE KEY
PRESSED ON SSD USING 8051

2. Objectives of the Lab
 Learning how a keypad works
 Learning how to take input using 8051
 Understanding the interfacing of keypad with 8051
 Displaying SSD according to the key pressed on keypad

3. Deciding Pins or Ports to use
Use with
caution
If EA high
*As keypad is
being used as
input, we can
use any port
except port0.
Let’s use port2
for keypad and
port1 for SSD

4. Keypad
 Keypads are usually used
for user input.
 The numbers on the top
make them very easy for
a user to be used as an
input for a program.
 It is widely used in
applications that use
MENU to work on.
 There are 7 pins for 3x4
and are 8 pins for 4x4
keypad.

5. Keypad Internal Circuit
 Whenever a key is
pressed, a certain row
and a column gets short.
 With keypad, we only
need to find out which
row and column is short.
 Then, we only need to
know which key will
make that particular row
and column short.
 We will use this property
of the device to our
advantage.

6. Keypad with 8051
 We will use a 3x4 keypad, so we will have 7 pins.
 These pin represents 3 columns and 4 rows.
 We need to detect when a key is pressed and which key
is pressed.
 The process of finding the key is done by scanning.
 Scanning ROWs is done by first making rows as input
and giving columns a ground.
 When the key is pressed, we know the row from which
the button is pressed.
 Then, we scan COLs by making cols as input and
giving rows a ground.
 This process happens so fast that still the same key is
pressed and we know the column from which the
button is pressed. Thus we have the corresponding key.

7. Instructions used in this Lab
 JB Px.y,label
 JNB Px.y,label
 CJNE REG,DATA,LABEL (See instruction set)
 See instruction set for the details of above commands.

8. Taking input using 8051
 To interface a keypad, we need to learn how to take input.
 There are two formats to take input, one is 1-bit and other
is 8-bit commonly known as single bit input and port
input.
 In both processes, we simply set the pin or port (all pins)
to ‘one’ and then make decisions later in the code.
 e.g. setb p1.0
jb p1.0,$
jmp myfunc
 Also, mov p1,#0FFh
cjne p1,#55,newfunc
jmp exit1

9. Keypad connections with
8051
 First, we need to define the pins that can
be sorted into the columns and rows.
 We can do that by using DMM; if the
same pin is short when all the keys from
the same column are pressed then this key
is corresponding to column 1.
 After that, we will know which pin is for
which column or row.
 Then, we can assign pins to rows and
columns and write our code.
 Normally the order followed is
C3
P1.0
C2
P1.1
C1
P1.2
R1
P1.6
R2
P1.5
R1
P1.4
R1
P1.3
R1 R2 R3 R4 C1 C2 C3
P1.6,5,4,3,2,1,0
R1 R2 R3 R4 C1 C2 C3
P1.6 P1.5 P1.4 P1.3 P1.2 P1.1 P1.0

10. Algorithm for Keypad and SSD
Display
Start
Scan Rows
main:
mov p2,#0FFh
mov p2,#11111000b
row1:
mov p2,#0FFh
mov p2,#00000111b
jnb p1.2,one
jnb p1.1,two
jnb p1.3,three
jmp main
one: mov p1,#0F9h
jmp main
. . .
jnb p1.6,row1
jnb p1.5,row2
jnb p1.4,row3
jnb p1.3,row4
jmp main
Key from
which
col?
Scan
Cols
Scan
Cols
Scan
Cols
Scan
Cols
row1 row4row2 row3
Key from
which
row?
Digit
Display
0 1 2 3 4
0C0h 0F9h 0A4h 0B0h 099h
5 6 7 8 9
092h 082h 0F8h 080h 098h

11. Configuration of Keypad available
in Lab
7 1 2 4 3 5 6
R1 R2 R3 R4 C1 C2 C3
P1.6 P1.5 P1.4 P1.3 P1.2 P1.1 P1.0

12. Proteus Devices needed in this Lab
1. AT89c51
2. Keypad
3. 7-seg-CA

13. Lab Tasks
 Show the digit on SSD corresponding to the key
pressed on Keypad.