A 4x4 matrix keypad can be interfaced with an 8051 microcontroller to detect key presses. The rows of the keypad are connected to output port pins on the microcontroller, which are set high or low to scan each row. The columns are connected to input port pins, which the microcontroller reads to detect a low value, indicating a key press. By scanning each row and detecting the low column, the microcontroller can identify the specific key pressed. The document provides a circuit diagram and pin assignments for interfacing a 4x4 keypad with an 8051 development board to allow scanning and detecting keys.

1. Key board interfacing with 8051

2. 8051 Development Board
• The 8051 Development board is specifically designed
to help students to master the required skills in the
area of embedded systems. The kit is designed in such
way that all the possible features of the
microcontroller will be easily used by the students.
The kit supports in system programming (ISP) which is
done through serial port.
• NXP’s 8051 (AT89V51RD2), 8051 Development Kit is
proposed to smooth the progress of developing and
debugging of various designs encompassing of speed
8-bit Microcontrollers.

3. Keypad:
A keypad is a set of buttons arranged in a block or "pad" which
usually bear digits, symbols and usually a complete set of
alphabetical letters. If it mostly contains numbers then it can also
be called a numeric keypad. Here we are using 4 X 4 matrix
keypad.
Interfacing keypad:
• Figure shows how to interface the 4 X 4 matrix keypad to two
ports in microcontroller. The rows are connected to an output
port and the columns are connected to an input port.

4. • To detect a pressed key, the microcontroller grounds all rows by
providing 0 to the output latch, and then it reads the columns. If
the data read from the columns is D3-D0=1111, no key has
been pressed and the process continues until a key press is
detected. However, if one of the column bits has a zero, this
means that a key press has occurred. For example, if D3-
D0=1101, this means that a key in the D1 column has been
pressed.
• After a key press is detected, the microcontroller will go through
the process of identifying the key. Starting with the top row, the
microcontroller grounds it by providing a low to row D0 only;
then it reads the columns.

5. Circuit Diagram to Interface keypad with
8051

6. • If the data read is all 1s, no key in that row is activated and the
process is moved to the next row. It grounds the next row,
reads the columns, and checks for any zero. This process
continues until the row is identified. After identification of the
row in which the key has been pressed, the next task is to find
out which column the pressed key belongs to.

7. Pin Assignment with 8051

8. Interfacing keypad with 8051
• We now want to scan a keypad in 8051 Development board. In
case of 4X4 matrix Keypad both the ends of switches are
connected to the port pin i.e. four rows and four columns. So in
all sixteen switches have been interfaced using just eight lines.
• 1Keypads arranged by matrix format, each row and column
section pulled by high or low by selection J5, all row lines(P2.4
– P2.7) and column lines(P2.0 to P2.3) connected directly by
the port pins.