KEYPAD scan driver implementing RB interrupt

///////////////////////////////////////////////////////////////////////////
////                             keypad.c                              ////
////                  Generic keypad scan driver                       ////
////                                                                   ////
////  kbd_init()   Must be called before any other function.           ////
////                                                                   ////
////  c = kbd_getc(c)  Will return a key value if pressed or /0 if not ////
////                   This function should be called frequently so as ////
////                   not to miss a key press.                        ////
////                                                                   ////
///////////////////////////////////////////////////////////////////////////
#define __delay_cycles _delay
//Now using RB change interrupt

#ifndef BYTE
#define BYTE unsigned char
#endif

#define KBD_DEBOUNCE_FACTOR 33    // Set this number to apx n/333 where
                                  // n is the number of times you expect
                                  // to call kbd_getc each second

// Keypad layout:
char const KEYS[4][4] = {{'7','8','9','/'},
                         {'4','5','6','x'},
                         {'1','2','3','-'},
                         {'C','0','=','+'}};


BYTE kchar;
BYTE col;
BYTE row;
BYTE kbd_down=0;
BYTE last_key=0;

//--Interrupt Service function--
interrupt ISR (void) // Interrupt function definition
{
    if (RBIF) // If the RB change flag is 1, do .....
    {
        RBIE=0; //Disable RB change interrupt
        {
            kbd_down=1;
            col = (PORTB >> 4) & 0x0F;
            if (col == 0b1110)
            col = 0;
            else if (col == 0b1101)
            col = 1;
            else if (col == 0b1011)
            col = 2;
            else
            col = 3;

            last_key = KEYS[row][col];
            kchar = last_key;
        }
        RBIF=0; // Reset the external interrupt flag
        RBIE=1; // RE-Enable RB change interrupt
    }
    GIE=1; //Enable Global Interrupt
}

void kbd_init( )
{
    TRISB = 0xF0; //Port B in/output
    nRBPU = 0; // Enable PORTB pullups
    RBIE=1; //Enable the RB port change interrupt
    GIE=1; // Global interrupt enable
    RBIF=0; //Clear RB port change interrupt flag
}

char kbd_getc( )
{
   static BYTE kbd_call_count;

   kchar=last_key;
    for (row=0;row<4; row++)
    {
        switch (row)
        {
            case 0 : PORTB = PORTB | 0b11111110;
            RB0 = 0;
            break;

            case 1 : PORTB = PORTB | 0b11111101;
            RB1 = 0;
            break;

            case 2 : PORTB = PORTB | 0b11111011;
            RB2 = 0;
            break;

            case 3 : PORTB = PORTB | 0b11110111;
            RB3 = 0;
        }
        __delay_cycles(KBD_DEBOUNCE_FACTOR );

        if (kbd_down==1)
        {
            kbd_down=0;
            break;
        }
        else kchar = 0;
    }

    PORTB = PORTB | 0xFF;    //Reset Port B
    return(kchar);
}  //End of keypad