Scanning algorithm for 3 x 4 Keypad Matrix

When a keypad is connected with a microprocessor, e.g 8051, a scanning algorithm is used to detect the pressed keys. See shown in [1]

Scanning algorithm

1.       Start.

2.       Make All Pins High.

3.       Make Column 1 pin low.

4.       Check if Row 1 is low, if yes then Switch 1 has been pressed.

5.       Check if Row 2 is low, if yes then Switch 4 has been pressed.

6.       Check if row 3 is low if yes then Switch 7 has been pressed.

7.       Check if row 4 is low if yes then Switch 10 has been pressed.

8.       Make Column 1 Pin high & Column 2 Pin Low.

9.       Check if Row 1 is low, if yes then Switch 2 has been pressed.

10.   Check if Row 2 is low, if yes then Switch 5 has been pressed.

11.   Check if row 3 is low if yes then Switch 8 has been pressed.

12.   Check if row 4 is low if yes then Switch 11 has been pressed.

13.   Make Column 2 Pin high & Column 3 Pin Low.

14.   Check if Row 1 is low, if yes then Switch 3 has been pressed.

15.   Check if Row 2 is low, if yes then Switch 6 has been pressed.

16.   Check if row 3 is low if yes then Switch 9 has been pressed.

17.   Check if row 4 is low if yes then Switch 12 has been pressed.

18.   Make column 3 pin high

19.   Stop

Reference: 
[1]http://www.dnatechindia.com/index.php/Tutorials/8051-Tutorial/Interfacing-Matrix-Keypad-to-8051.html