PIC Microcontroler based Keyboards

4x4 Keypad Standard Scanning with 8 io pins

Ken Webster says:

I just happened to have an example [of a keyboard to PIC interface]. It is for a 4x4 keypad connected to a 17C756 on PORTB. It doesn't do the fast scan ... just a plain old one-row-at-a-time scan. I had PORTB (output latch) set to 0. Weak pullups were enabled.

Pinout is kinda funky (to make PCB layout easier):

6---(1)-(2)-(3)-(A)
     |   |   |   |
7---(4)-(5)-(6)-(B)
     |   |   |   |
5---(7)-(8)-(9)-(C)
     |   |   |   |
2---(*)-(0)-(#)-(D)
     |   |   |   |
     3   4   0   1



;------------------------------------------------------------
;Called by interrupt handler to scan the keypad
;Returns with W=ASCII if a key is pressed, W=0 otherwise
;Interrupt handler performs debounce
SCAN1   MOVLW   ~0X40         ;Enable ROW0 output
        MOVWF   DDRB
        NOP
        NOP
        BTFSS   PORTB,3
        RETLW   '1'
        BTFSS   PORTB,4
        RETLW   '2'
        BTFSS   PORTB,0
        RETLW   '3'
        BTFSS   PORTB,1
        RETLW   'A'
        MOVLW   ~0X80         ;ROW1
        MOVWF   DDRB
        NOP
        NOP
        BTFSS   PORTB,3
        RETLW   '4'
        BTFSS   PORTB,4
        RETLW   '5'
        BTFSS   PORTB,0
        RETLW   '6'
        BTFSS   PORTB,1
        RETLW   'B'
        MOVLW   ~0X20         ;ROW2
        MOVWF   DDRB
        NOP
        NOP
        BTFSS   PORTB,3
        RETLW   '7'
        BTFSS   PORTB,4
        RETLW   '8'
        BTFSS   PORTB,0
        RETLW   '9'
        BTFSS   PORTB,1
        RETLW   'C'
        MOVLW   ~0X04         ;ROW3
        MOVWF   DDRB
        NOP
        NOP
        BTFSS   PORTB,3
        RETLW   '*'
        BTFSS   PORTB,4
        RETLW   '0'
        BTFSS   PORTB,0
        RETLW   '#'
        BTFSS   PORTB,1
        RETLW   'D'
        RETLW   0

Questions: