The microcontroller generates a square wave with adjustable duty cycle (PWM)

            Program name: Brightness control of light-emitting diode D1
            Version: VER1.0
            Applicable board: 51hei learning board      
Use timer control to generate PWM wave with variable duty cycle to control the brightness of light-emitting diode D1
Press S2, the PWM value increases, the duty cycle decreases, and the D1 light becomes brighter.
Press S3, the PWM value decreases, the duty cycle increases, and the D1 light becomes darker.
When the PWM value increases to the maximum value or decreases to the minimum value, the buzzer will alarm.
******************************************************************/

#include
#include
sbit S2 =P3^4 ; //PWM value decrease key
sbit S3 =P3^5 ; //PWM value increase key
sbit BEEP =P2^3 ; //Buzzer
unsigned char PWM=0x03 ; //Assign initial value

/////////////////////////////////////////
void Beep();
void delayms(unsigned char ms );
void delay(unsigned char t);

/************************************************************/
void main()
{   
    P1=0xff;
    TMOD=0x00 ;
  TH0=0x00 ; //Delay constant
    TL0=0x00 ; //Frequency adjustment
    TH1=PWM ; //Pulse width adjustment
    TL1=0xff ;
EA=1;
ET0=1;
ET1=1;
    TR0=1 ;

   while(1)
   {
 do{
     if(PWM!=0xff)
    {PWM++ ;delayms(10);}
        else Beep() ; 
   }
    while(S3==0);

 do{
      if(PWM!=0x02)
     {PWM-- ;delayms(10);}
      else Beep() ; 
   }
    while(S2==0);
  }
}

/************************************************************/
// Timer 0 interrupt service routine (frequency)
/************************************************************/
void timer0() interrupt 1 
{  
    TR1=0 ;
    TH0=0x00;
    TL0=0x00 ;
    TH1=PWM ;
    TR1=1 ;
    P1=0xfe ; // Start output
}

/************************************************************/
// Timer 1 interrupt service routine (pulse width)
/************************************************************/
void timer1() interrupt 3 
{ 
    TR1=0 ;
    P1=0xff ; //End output
}

/************************************************************/
//Buzzer subroutine
/****************************************************************/

void Beep()     
  {
    unsigned char i ;
    for (i=0 ;i<100 ;i++)
      {
        delay(100) ;
        BEEP=!BEEP ; //Invert Beep
      } 
    BEEP=1 ; //Turn off the buzzer
 delayms(100);
  }

/************************************************************/
// Delay subroutine
/************************************************************/  
void delay(unsigned char t)
 { 
   while(t--) ;
 }

/************************************************************/
// Delay subroutine
/************************************************************/
void delayms(unsigned char ms)

{
   unsigned char i ;
   while(ms--)
    {
      for(i = 0 ; i < 120 ; i++) ;
    }
}

/****************************************************** **********/