//-----------------------------------------------------------------
        //  名稱: K1～K4按鍵狀態顯示
        //-----------------------------------------------------------------
        //  說明: K1,K2按下時LED點亮,松開時熄滅,
        //       K3,K4按下并釋放時LED點亮,再次按下并釋放時熄滅。
        //-----------------------------------------------------------------
        #include <reg51.h>
        #define uchar unsigned char
        #define uint  unsigned int
        sbit  LED1 = P0^0;
        sbit  LED2 = P0^1;
        sbit  LED3 = P0^2;
        sbit  LED4 = P0^3;
        sbit  K1 = P1^0;
        sbit  K2 = P1^1;
        sbit  K3 = P1^2;
        sbit  K4 = P1^3;
        //-----------------------------------------------------------------
        // 延時
        //-----------------------------------------------------------------
        void DelayMS(uint x)
        {
          uchar i;
          while(x--) for(i = 0; i < 120; i++);
        }
        //-----------------------------------------------------------------
        // 主程序
        //-----------------------------------------------------------------
        void main()
        {
          P1 = 0xFF;
          P0 = 0xFF;
          while(1)
          {
            LED1 = K1;
            LED2 = K2;
            if( K3 == 0)
            {
                while (K3==0);           //等待釋放按鍵
                LED3 = ～LED3;
            }
            if( K4 == 0)
            {
                while (K4==0);           //等待釋放按鍵
                LED4 = ～LED4;
            }
            DelayMS(10);
          }
        }