问个c51的程序 是按键扫描的

#include
#define N    5

sbit SEG_En = P1^0;
sbit DIG_En = P1^1;
sbit LED_En = P1^2;
sbit Key_En = P1^3;

unsigned char Key_Value;


/***********************************************/
/*延时函数。                                   */
/***********************************************/
void delay_ms(unsigned int TData)
{
   unsigned int i,j;
   for(j=0; j<TDATA; J++)
      for(i=0; i<120; i++);
}


/***********************************************/
/*按键扫描函数                                 */
/***********************************************/
void key_scan()
{
   unsigned char TD;
   P0       = 0x00;
   SEG_En   = 1;
   DIG_En   = 1;
   SEG_En   = 0;
   DIG_En   = 0;

   P0       = 0xFE;                //扫描第一行
   DIG_En   = 1;
   DIG_En   = 0;
   Key_En   = 0;
   TD       = P0;
   switch(TD)  
   {
      case 0xFE : Key_Value = 0x00; break;
      case 0xFD : Key_Value = 0x01; break;
      case 0xFB : Key_Value = 0x02; break;
      case 0xF7 : Key_Value = 0x03; break;
      default   : break;
   }
   Key_En = 1;

   P0       = 0xFD;                //扫描第二行
   DIG_En   = 1;
   DIG_En   = 0;
   Key_En   = 0;
   TD       = P0;
   switch(TD)  
   {
      case 0xFE : Key_Value = 0x04; break;
      case 0xFD : Key_Value = 0x05; break;
      case 0xFB : Key_Value = 0x06; break;
      case 0xF7 : Key_Value = 0x07; break;
      default   : break;
   }
   Key_En = 1;

   P0       = 0xFB;                //扫描第三行
   DIG_En   = 1;
   DIG_En   = 0;
   Key_En   = 0;
   TD       = P0;
   switch(TD)  
   {
      case 0xFE : Key_Value = 0x08; break;
      case 0xFD : Key_Value = 0x09; break;
      case 0xFB : Key_Value = 0x0A; break;
      case 0xF7 : Key_Value = 0x0B; break;
      default   : break;
   }
   Key_En = 1;

   P0       = 0xF7;                //扫描第四行
   DIG_En   = 1;
   DIG_En   = 0;
   Key_En   = 0;
   TD       = P0;
   switch(TD)  
   {
      case 0xFE : Key_Value = 0x0C; break;
      case 0xFD : Key_Value = 0x0D; break;
      case 0xFB : Key_Value = 0x0E; break;
      case 0xF7 : Key_Value = 0x0F; break;
      default   : break;
   }
   Key_En = 1;
}

void main()
{                           
   unsigned char temp,i,j;
   unsigned char a[5];
  
   SEG_En    = 0;
   DIG_En    = 0;
   Key_En    = 1;
   LED_En    = 1;
   Key_Value = 0xFF;
   temp      = 0xFF;
   i         = 0x00;
   j         = N-1;

   while(1)
   {
      key_scan();
      if(i == N)
      {        
         P0     = a[j] << 4;
         LED_En = 0;
         delay_ms(500);
         LED_En = 1;
         delay_ms(500);
         j++;
         if(j == N)
            j = 0x00;                 
      }
      else
      {
         if(Key_Value != 0xFF)
         {        
            P0     = Key_Value << 4;
            LED_En = 0;
            delay_ms(20);
            LED_En = 1;

            if(temp != Key_Value)
            {              
                temp = Key_Value;
                a[i] = Key_Value;
                i++;
            }
         }
      }
   }
}







这里面while(1)
   {
      key_scan();
      if(i == N)
      {        
         P0     = a[j] << 4;
         LED_En = 0;
         delay_ms(500);
         LED_En = 1;
         delay_ms(500);
         j++;
         if(j == N)
            j = 0x00;                 
      }
      else
      {
         if(Key_Value != 0xFF)
         {        
            P0     = Key_Value << 4;
            LED_En = 0;
            delay_ms(20);
            LED_En = 1;

            if(temp != Key_Value)
            {              
                temp = Key_Value;
                a[i] = Key_Value;
                i++;
            }
         }
      }
   }
}尤其的不理解是什么意思啊 谁能帮我解释下