I've created (yet another) 4x4 matrix keyboard decode-routine.
I've placed 4 NOP's in it, because in a similar routine, they also were
there (bad reason) , but I'm not sure if they are needed.
They are there to let the output-pins when the pattern on portb is
changed. But are they nescessary?

Scan_4x4        ****************        ;OK
        CLRF    Key                     ;Returns: key value in Key
        MOVLW   .4                      ;No keypress:   Key = b'00010000'
        MOVWF   Counter
Scan_loop
        MOVF    Mask,W                  ;Mask
        MOVWF   PORTB                   ;Put mask on port
        NOP                             ;Let pins settle
        NOP                             ;But is this nescessary??
        NOP
        NOP
        NOP
        NOP
        BTFSS   PORTB,0                 ;IS first bit clear?
        RETURN                          ;Yes, so that row was pressed
        INCF    Key
        BTFSS   PORTB,1
        RETURN
        INCF    Key
        BTFSS   PORTB,2
        RETURN
        INCF    Key
        BTFSS   PORTB,3
        RETURN
        INCF    Key
        RLF     Mask,F                  ;Check next column
        DECFSZ  Counter,F
        GOTO    Scan_loop
        RETURN                          ;No keypress: Bit 4 from Key = 1.

Hans