#include <AT89X51.H>    //Include file for 8051
#define keyport P2      //keypad connected to P2
#define col1 P2_0       //column 1
#define col2 P2_1       //column 2
#define col3 P2_2       //column 3
#define col4 P2_3       //column 4
#define TRUE 1          //some defines
#define FALSE 0

/*
+---------------------------------------+
| Prototype: void key_init(void);       |
| Return Type: void                     |
| Arguments: None                       |
| Description: Initialize ports and     |
|              Keypad.                  |
+---------------------------------------+
*/
void key_init(){
        keyport &=0x0F; //make Rows as o/p and cols are i/p
}

/*
+-----------------------------------------------+
| Prototype: unsigned char get_key(void);       |
| Return Type: unsigned char                    |
| Arguments: None                               |
| Description: To read key from the keypad      |
+-----------------------------------------------+
*/
unsigned char get_key(){
        unsigned char i,k,key=0;
        k=1;
        for(i=0;i<4;i++){               //loop for 4 rows
                keyport &=~(0x80>>i);   //to make rows low 1 by 1
                        if(!col1){      //check if key1 is pressed
                                key = k+0;      //set key number
                                while(!col1);   //wait for release
                                return key;     //return key number
                        }
                        if(!col2){      //check if key2 is pressed
                                key = k+1;      //set key number
                                while(!col2);   //wait for release
                                return key;     //return key number
                        }
                        if(!col3){      //check if key3 is pressed
                                key = k+2;      //set key number
                                while(!col3);   //wait for release
                                return key;     //return key number
                        }
                        if(!col4){      //check if key4 is pressed
                                key = k+3;      //set key number
                                while(!col4);   //wait for release
                                return key;     //return key number
                        }
                k+=4;                   //next row key number
                keyport |= 0x80>>i;     //make the row high again
        }
        return FALSE;                   //return false if no key pressed
}