/*********************************/
/* program: keypad.c             */
/*  by Jack Changfan             */
/*     Jan 8 '97                 */
/*********************************/

#progma SMALL db CODE
#include<reg51.h>

sfr keypad = 0x90;  /* Port 1: Data in/out */

void LCD_DISPLAY(unsigned char ch) 
{
  /* you can write this part by yourself */
}

unsigned char scan()
{
  int i;
  unsigned char scandrive = 0xfe;  /* scanner drive */
               /* higher 4-bit write 1's to setup input mode */
               /* lower 4-bit output exactly one 0 to the column
                    to be scanned */                    
  unsigned char key, row, col;
  unsigned char keyTab[16] = {'1','2','3','A','4','5','6','B',
                              '7','8','9','C','*','0','#','.'};

  key=0xf0;  /* preset key */

  while ((scandrive != 0xef) && ((key & 0xf0)==0xf0))
     /* when scandrive=0xef, all columns has been scanned */
     /* when (key & 0xf0)!=0xf0, some 0 has been read */
  {
     Keypad = scandrive;  
     key = Keypad;    
     scandrive = (scandrive*2)+1;  /* shift left */ 
  } 

  if (key == 0xf7) return 0xff;  /* scan all columns, no key pressed */
  else
  {
     for (i=0; i<4; i++)
     {
         if ( (key & 0x01) == 0 ) col=i;  /* locate the column with 0 */
         key = (key >> 1);
     }
   
    for (i=0; i<4; i++)
    {
        if ( (key & 0x01) == 0 ) row=i;   /* locate the row with 0 */
        key = (key >> 1);
    }

    return keyTab[row*4+col];
  }
}

void main()
{
  unsigned char keypress;    /* variable for keypress */

  while (1)  
  {
     keypress = scan();
     if (keypress != 0xff) LCD_DISPLAY();  /* some key is pressed */ 
      /* note that scan return 0xff when no key is pressed */
     Keypad = 0xf0;  /* scan all columns */
     while (Keypad != 0xf0);  /* wait until all keys are released */   
  }
}