This blog post explains how to interface matrix keypad with cortex m4

Procedure:

1. Power: In the PCONP register, set the PCGPIO bit
2. Configure all the Keypad pins as GPIO using IOCON3_0 to IOCON3_7
3. Configure all the row pins as output and make it HIGH
4. Configure all the column pins as input and make it HIGH
5. Configure all the LCD pins as output and Initialize LCD
6. Make a row pin Zero and check all the four column lines one by one for zero
7. If any key is pressed, the corresponding column will get zero
8. Read the key value and display it in LCD

Interface Matrix key pad with cortex m4 -C Source code

#include <LPC407x_8x_177x_8x.h>
#include "LCD.h"

#define keyportc  LPC_GPIO3->CLR
#define keyports  LPC_GPIO3->SET

#define COL1			(LPC_GPIO3->PIN & 0x10) 
#define COL2			(LPC_GPIO3->PIN & 0x20)
#define COL3			(LPC_GPIO3->PIN & 0x40)
#define COL4			(LPC_GPIO3->PIN & 0x80)

unsigned char k=0,key=0,i=0;
unsigned char Lcd_LINE1[]={"KEYPAD DEMO:    "};
unsigned char get_key(void);

int main(void)
{
		
		//1. Set the PCGPIO bit
		LPC_SC->PCONP |= (1<<15);
	
		//2. Configure all Keypad pins as GPIO
		LPC_IOCON->P3_0 = 0x00;
		LPC_IOCON->P3_1 = 0x10;
		LPC_IOCON->P3_2 = 0x10;
		LPC_IOCON->P3_3 = 0x10;
		LPC_IOCON->P3_4 = 0x10;
		LPC_IOCON->P3_5 = 0x10;
		LPC_IOCON->P3_6 = 0x10;
		LPC_IOCON->P3_7 = 0x10;
	
		//3. Configure all the row pins as output and make it HIGH
		//   Configure all the column pins as input and make it HIGH
		LPC_GPIO3->DIR = 0x0F;
		LPC_GPIO3->PIN = 0x0F;	 
	
		//5. Configure all the LCD pins as GPIO outputs and Initialize LCD
		lcd_init();
		
		//Send First line command
		lcd_command(0x80);
		//Send 16 characters from the line1 array
		for(i=0;Lcd_LINE1[i]!='\0';i++)
		{
			lcd_data(Lcd_LINE1[i]);
			delay_ms(50);
		}
		delay_ms(1000);
		
		while (1)
		{
				if(get_key()!=0){
					 lcd_command(0xc0);
					 if(key<=10)lcd_data(key-1+'0');
					 else lcd_data(key-11+'A');
				}
			  delay_ms(100);
		}
}

unsigned char get_key(void)
{
				LPC_GPIO3->PIN=0x0F;
        k=1;
        for(i=0;i<4;i++){               					
                keyportc |=(0x01<<i);   										//Scan for a Key by sending '0' on ROWS
								lcd_command(0xc0);
                        if(COL1==0){      									//when a key pressed numbers 1--16 will be returned
                                key = k+0;    
                                while(COL1==0);  
                                return key;     
                        }
                        if(COL2==0){      
                                key = k+1;      
                                while(COL2==0); 
                                return key;     
                        }
                        if(COL3==0){      
                                key = k+2;      
                                while(COL3==0);  
                                return key;     
                        }
                        if(COL4==0){      
                                key = k+3;      
                                while(COL4==0);   
                                return key;     
                        }
               k+=4;        
               keyports |=(0x01<<i);    		
        }
        return 0;                   
}