Keypad (PIC16F690)

How to decode a signal from a keypad

In many applications we want to give to user the possibility to use a keypad. We are going to use 4x3 keypad (1, 2, 3, 4, 5, 6, 7, 8, 9, 0, *, #). In order to be able to decode the user input we have to understand how it works. So please take a look on the schematic below:

keypad 4x3

This is nothing more than 3 vertical lines (columns), and 4 horizontal lines (rows) which are connected with 12 push buttons. So if i apply voltage to C1 and the user press any button between 1, 4, 7 or * the corresponding row will be connected to the supplying voltage to. The only thing i have to do is to search which row is "high" and i can immediately understand which button  is pressed given that i already know which column is "high". The same logic can be used for the columns 2 and 3. Simple!!! But really smart.

In today's article we are going to use a microcontroller in order to decode the user inputs. The software will follow the flowchart below (is big but is really simple):

Keypad reading flowchart

As you can see on the flow chart, we are in an infinite loop with three stages. On the first stage we "set" the first column (C1), and we check every row if its high, and accordingly to that we "export" the user input to a  seven segment display. For example if the button 4 is pressed by the user, this will be recognized on the second decision of the first stage as you can see on the flow chart. (a better solution will have to use debounce time, but this requires a longer flow chart and more code).

Please take a look on the following schematic:

proteus simulation of keypad

There is nothing more than the micro controller, the keypad, the seven segment display and off course some resistors (pull up resistors, safety resistors for the leds). 

The wiring of the seven segment display is easy if you keep in mind the picture below:

Image from Wikipedia

The first wire is A, the second wire is B etc. The last wire is the cathode of leds.

Now let me show you the software i wrote for this project:

1. start
2. ; remaining code goes here
3. call Initialization
4. setC1
5. bsf PORTA, 0
6. bcf PORTA, 1
7. bcf PORTA, 2
8. setC1_A
9. btfss PORTB, 4
10. goto setC1_B
11. movlw b'00000110'
12. movwf PORTC
13. goto setC1
14. setC1_B
15. btfss PORTB, 5
16. goto setC1_C
17. movlw b'01100110'
18. movwf PORTC
19. goto setC1
20. setC1_C
21. btfss PORTB, 6
22. goto setC1_D
23. movlw b'00000111'
24. movwf PORTC
25. goto setC1
26. setC1_D
27. btfss PORTB, 7
28. goto setC2
29. movlw b'01000000'
30. movwf PORTC
31. goto setC1
33. setC2
34. bcf PORTA, 0
35. bsf PORTA, 1
36. bcf PORTA, 2
37. setC2_A
38. btfss PORTB, 4
39. goto setC2_B
40. movlw b'01011011'
41. movwf PORTC
42. goto setC1
43. setC2_B
44. btfss PORTB, 5
45. goto setC2_C
46. movlw b'01101101'
47. movwf PORTC
48. goto setC1
49. setC2_C
50. btfss PORTB, 6
51. goto setC2_D
52. movlw b'01111111'
53. movwf PORTC
54. goto setC1
55. setC2_D
56. btfss PORTB, 7
57. goto setC3
58. movlw b'00111111'
59. movwf PORTC
60. goto setC1
61. 
    
62. setC3
63. bcf PORTA, 0
64. bcf PORTA, 1
65. bsf PORTA, 2
66. setC3_A
67. btfss PORTB, 4
68. goto setC3_B
69. movlw b'01001111'
70. movwf PORTC
71. goto setC1
72. setC3_B
73. btfss PORTB, 5
74. goto setC3_C
75. movlw b'01111100'
76. movwf PORTC
77. goto setC1
78. setC3_C
79. btfss PORTB, 6
80. goto setC3_D
81. movlw b'01100111'
82. movwf PORTC
83. goto setC1
84. setC3_D
85. btfss PORTB, 7
86. goto setC1
87. movlw b'00110110'
88. movwf PORTC
89. goto setC1
90. goto    start             ; loop forever
91. 
    
92. Initialization
93. bsf STATUS, RP0
94. bcf STATUS, RP1 ;bank 1
95. 
    
96. movlw 0xff
97. movwf TRISB ;PORTB is input
98. clrf TRISA ;PORTA is output
99. clrf TRISC ;PORTC is output
101. bcf STATUS, RP0
102. bsf STATUS, RP1 ;bank 2
104. clrf ANSEL
105. clrf ANSELH ;set all I/Os as digitals
106. 
     
107. bcf STATUS, RP1 ;bank0
108. 
     
109. clrf PORTA
110. clrf PORTC
111. return

        

For this project you can download all the files from here