由定時器產生PWM信號，可手動按鍵調節PWM的占空比，按鍵由外部中斷實現

#include <reg51.h>
#include <intrins.h>

#define CYCLE 6000               //周期6000 * 1us = 6ms
#define SCALE (CYCLE / 100)      //比例 1%

#define ONE        0x10
#define TWO        0x01
#define THREE    0x40
#define FOUR    0x04

typedef unsigned char uchar;
typedef unsigned 
 
int  uint;

sbit pwm_out = P2^6;               //PWM信號輸出
uchar key_value = 0;            //按鍵值

uchar const cycle = 100;              
uchar scale = 50;
uchar num = 0;

/*************************************************************
*ms延時函數
*************************************************************/
void delay_ms(uchar n)
{
    uchar j 
 
= 0;
    for(; n>0; n--)
        for(; j<125; j++);
}


/*************************************************************
*定時器初始化
*************************************************************/
void Init_Timer()
{
    TMOD = 0x01;
    TH0 = (65536 - SCALE)/256;            //定時器0初值
    TL0 = (65536 - SCALE)%256
 
;    
}


void main()
{
    Init_Timer();

    ET0 = 1;                              //開定時器中斷
//    IE0 = 1;
    IT0 = 0;
    EX0 = 1;
    EA = 1;                                //開總中斷

    pwm_out = 1;
    TR0 = 1;                              //定時器0開始計數
    
    while(1) 
    {
        switch(~key_value & 0x55)
        {
            case ONE :
                scale = 25;
            break;

            case TWO :
                scale = 50;
            break;

            case THREE :
                scale = 75;
            break;

            case FOUR :
                scale = 99;
            break;

            default :
            break;    
        }
       key_value = 0;
    }

}


/*************************************************************
*外部中斷0（按鍵）
*************************************************************/
void Int0_isr() interrupt 0
{
    key_value = P1;

    delay_ms(1);

    if(key_value != P1)
        key_value = 0;
}


/*************************************************************
*定時器中斷0（波形發生）
*************************************************************/
void Timer0_isr() interrupt 1                 
{     
    TH0 = (65536 - SCALE)/256;        
    TL0 = (65536 - SCALE)%256;
    num++;
    if(num == scale)
        pwm_out = 0;
    if(num == cycle)
    {
        num = 0;
        pwm_out = 1;
    }
}

51單片機PWM