Using 0xFF in an array of pointers - c

Question

I don't understand the use of 0xFF in an array of pointers. I want to control a keypad. My code is:
// CONFIGURATION BITS
#pragma config FOSC = HS // Oscillator Selection bits (XT oscillator)
#pragma config WDTE = OFF // Watchdog Timer Enable bit (WDT disabled)
#pragma config PWRTE = OFF // Power-up Timer Enable bit (PWRT disabled)
#pragma config BOREN = OFF // Brown-out Reset Enable bit (BOR disabled)
#pragma config LVP = OFF // Low-Voltage In-Circuit Serial Programming Enable bit
#pragma config CPD = OFF // Data EEPROM Memory Code Protection bit
#pragma config WRT = OFF // Flash Program Memory Write Enable bits
#pragma config CP = OFF // Flash Program Memory Code Protection bit
#define _XTAL_FREQ 8000000
// DEFINING LCD PINS
#define RS RD2
#define EN RD3
#define D4 RD4
#define D5 RD5
#define D6 RD6
#define D7 RD7
// DEFINING KEYPAD PINS
#define KP_R1 PORTBbits.RB1
#define KP_R2 PORTBbits.RB2
#define KP_R3 PORTCbits.RC1
#define KP_R4 PORTCbits.RC2
#define KP_C1 PORTBbits.RB3
#define KP_C2 PORTBbits.RB4
#define KP_C3 PORTBbits.RB6
#define KP_C4 PORTBbits.RB7
#include <xc.h>
#include "lcd.h"
#include <stdio.h>
#include <stdlib.h>
const char *keyPadMatrix[] =
{
"1","2","3","A",
"4","5","6","B",
"7","8","9","C",
"*","0","#","D",
0xFF
};
void kbd_init();
void kbd_init(){
// SETTING THE DATA DIRECTION REGISTERS
PORTB = 0x00;
TRISB = 0xD9; // RB7-1 RB6-1 RB5-0 RB4-1 RB3-1 RB2-0 RB1-0 RB0-1 ( 1=Input, 0=Output)
PORTC = 0x00;
TRISC = 0x90; // RC7-1 RC6-0 RC5-0 RC4-1 RC3-0 RC2-0 RC1-0 RC0-0 ( 1=Input, 0=Output)
OPTION_REGbits.nRBPU = 1;
}
char old_key;
char *kbd_getc();
char *kbd_getc(){
char row;
char key = 0;
for( row = 0b00000001; row < 0b00010000; row <<= 1 )
{
{ // turn on row output
KP_R1 = (row & 0x0001)>>0;
KP_R2 = (row & 0x0002)>>1;
KP_R3 = (row & 0x0004)>>2;
KP_R4 = (row & 0x0008)>>3;
__delay_ms(1);
}
// read colums - break when key press detected
if( KP_C1 == 1 ){
break;
}
key++;
if( KP_C2 == 1 ){
break;
}
key++;
if( KP_C3 == 1 ){
break;
}
key++;
if( KP_C4 == 1 ){
break;
}
key++;
}
KP_R1 = 0;
KP_R2 = 0;
KP_R3 = 0;
KP_R4 = 0;
if (key!=old_key){
old_key=key;
return keyPadMatrix[key];
}
else{
return keyPadMatrix[0x10];
}
}
void main()
{
TRISD = 0x00; //PORTD as output for LCD
Lcd_Init();
Lcd_Clear();
while(1)
{
Lcd_Set_Cursor(1,1);
char *keypress;
keypress = kbd_getc();
Lcd_Write_String(keypress);
__delay_ms(50);
}
}
The idea is that when you press a key, an LCD show that key, and when you don't press anything, the LCD doesn't show anything. If you change 0xFF for something like "w" the keypad does the trick, but instead of showing anything, it shows "w" (as expected). But when you use 0xFF the keypad doesn't work. I don't understand the use of 0xFF in the array of pointers. I copied the code from a website, but I making some changes because the original doesn't work either.
Note:
void Lcd_Write_String(char *a)
{
int i;
for(i=0;a[i]!='\0';i++)
Lcd_Write_Char(a[i]);
}