Keypad interfacing
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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.
![Attachments
[ App wireframe, Screenshots, ]](https://image.slidesharecdn.com/keypadinterfacing-220218065933/85/Keypad-interfacing-13-320.jpg)
Keypad interfacing
- 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.
- 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.
- 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