This document discusses interfacing a 4x4 matrix keypad with a microcontroller. It describes the advantages of a matrix keypad, how the microcontroller can detect which key is pressed by scanning the rows and columns, and two methods (interrupt and scanning) for key press detection. It then provides code for implementing the interrupt method, including initializing ports, enabling interrupts, scanning rows/columns to identify the pressed key, debouncing, and displaying the key on another port. Lookup tables are used to map rows/columns to ASCII codes for each key.

1. Keypad
Interfacing
Done by
Akhash Kumar
Sriram Prasad MI
Kamlesh Kumar R

2. Keypad Interfacing
4×4 matrix keypads are
commonly used keypads in
embedded applications. Such
keypads are seen in telephones
and other commonly seen
applications.

3. Advantages of Matrix Keypad
The advantage of a matrix
keypad is that the use of it
will allow the programmer
to reduce the number of
pins to be used. In a 4×4
matrix keypad

4. ● THE CPUI ACCESSES BOTH
ROWS AND COLUMNS
THROUGH PARTS
● WHEN A KEY IS PRESSED A
ROW AND COLUMN MAKE A
CONTACT. OTHERWISE THERE
IS NO CONTACT BETWEEN
THEM.

5. Matrix keypad

7. Debouncing
These are spurious spikes generated during keypress events. Normally
several spikes get generated during a key press because of the
mechanical contacts in the switch and it causes multiple spikes

8. Key-press Detection
FOR KEYPAD INTERFACING TWO PROCSSES SHOULD HAPPEN:
• KEY-PRESS DETECTON
• KEY-PRESS IDENTIFICATION
THERE ARE TWO WAYS BY WHICH THE PIC18 CAN PERFORM THE KEY
DETECTION.
• INTERRUPT METHOD
• SCANNING METHOD

9. INTERRUPT METHOD OF KEY PRESS
DETECTION
• Keypad connected to Port B
• Rows are connected to Port B high
• Columns are connected to Port B low.

11. Scanning

12. Introduction
● In this method, To detect a
key , the microcontroller
grounds all the rows by
providing zero

13. Attachments
[ App wireframe, Screenshots, ]

14. Step 1
Assume we pressed a button

16. Step 2
If it detects a logic 0 in
any one of the columns,
then a key press was
made in that column.

17. Step 3
The logic is that if a
button in that row
was pressed, then the
value written that row
will be reflected in the
corresponding column
(C2) as they are short
circuited.

18. Step 4
The procedure is followed
till C2 goes high when logic
high is written to a row. In
this case, a logic high to
the second row will be
reflected in the second
column.

19. CODE for
Interrupt
Method

20. D15H EQU D'100’; delay high byte of value
D15L EQU D'255’; low byte of value
COL EQU 0x08; holds the column found
DR15mH EQU 0x09; registers for 15 ma delay
DR15mL EQU 0x0A;
ORG 0x000000
Initialization

21. High priority Interrupt
ORG 0x000008
HI_ISR BTFSC INTCON,RBIP; Was it a PORTB change?
BRA RBIF_ISR; yes then go to ISR
RETFIE; else return

22. Main Function
ORG 0X00
CLRF TRISD; make PORTD output port
BCF INTCON2, RBPU; enable PORTB pull-up resistors.
MOVLW 0XF0; (Make PORTB high input ports
MOVWF TRISB; Take PORTB low output ports )
MOVWF PORTB; ground all rows
CPFSEQ PORTB; are all keys open
GOTO KEYOPEN; wait until keypad ready
KEYOPEN
0XFE =
1111 0000

23. MOVLW upper(KCODEO)
MOVWF TBLPTRU ; load upper byte of TBLPTR
MOVLW high(KCODEO)
MOVWF TBLPTRH ; load high byte of TBLPTR
BSF INTCON, RBIE; enable PORTB change interrupt
BSF INTCON, GIE; enable all interrupts globally
LOOP GOTO LOOP; wait for key press.

24. --key identification ISR
RBIF_ISR CALL DELAY; wait for debounce
MOVFF PORTB,COL ; get the column of key press
MOVLW 0XFE
MOVWF PORTB; ground row 0
CPFSEQ PORTB; Did PORTB change?
BRA ROW0; yes then row 0
MOVLW 0XFD;
MOVWF PORTB; ground row 1
CPFSEQ PORTA; Did PORTB change?
BRA ROW1; yes then row 1
0XFE =
1111 1110
0XFD =
1111 1101

25. MOVLW 0XFB;
MOVWF PORTB; ground row 2
CPFSEQ PORTB; Did PORTB change?
BRA ROW2; yes then row 2
MOVLW 0XF7;
MOVWF PORTB; ground row 3
CPFSEQ PORTB; Did PORTB change?
BRA ROW3; yes then row 3
GOTO BAD RBIF; no then key press too short
0XFB =
1111 1011
0XF7 =
1111 0111

26. ROW0 MOVLW Low (KCODEO); set TBLPTR start of row 0
BRA FIND; find the column
ROW1 MOVLW low (KCODE1); set TBLPTR= start of row I
BRA FIND; find the column
ROW2 MOVLW low (KCODE2); set TBLPTR = start of row 2
BRA FIND; find the column
ROW3 MOVLW low (KCODE3); set TBLPTR start of row 3
BRA FIND; find the column
BAD RBIF MOVLW 0x00; return null
GOTO WAIT; wait for key release

27. FIND MOVWF TBLPTRL; load low byte of TBLPTR
MOVLW 0XF0
XORWF COL ; invert high nibble
SWAPF COL, F; bring to low nibble
AGAIN RRCF COL; rotate to find column
BC MATCH ; column found, get the ASCII code
INCF TBLPTRL; else point to next col. address
BRA AGAIN; keep searching
0XF0 =
1111 0000

28. MATCH TBLRD*+; get ASCII code from table.
MOVFF TABLAT, PORTD; display pressed key on PORTD
WAIT MOVLM 0XF0;
MOVWF PORTB; reset PORTB
CPPSEQ PORTB; Did PORTB change?
BRA WAIT; yes then wait for key release.
BCF INTCON, RBIF; clear PORTB, change flag
RETFIE; return and wait for key press.

29. -------------delay
DELAY MOVLW D15mH; high byte of delay.
MOVWF DR15mH; store in register
D2: MOVLW D15mL; low byte of delay
MOVWF DR15mL; store in register
D1: DECF DR15mL, F; Stay until DR15ml becomes 0
BNZ D1
DECP DR15mH, F; loop until all DR15m= 0x0000
BNZ D2
RETURN

30. ----key scan code look-up table
ORG 300H
KCODE0: DB '0','1', '2', '3'; ROW 0
KCODE1: DB '4','5', '6', '7'; ROW 1
KCODE2: DB '8','9', 'A', 'B'; ROW 2
KCODE3: DB 'C' ,'D', 'E', 'F'; ROW 3

31. Doubts

33. Thank You