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the beauty of the code shown above is that all the work is done in the interrupt so no reliance on the main() loop to generate the signal. if you don't change PWM_DC, the mcu will just continue to generate the output.

the pwm period in the above code is presumed to be 256 x PWM_STEPPING (because pwm_dc is an unsigned char type). I have been asked to allow a user-define pwm period, and to show an example of variable pwm output. so here it is the 2nd revision to the code above.

============code========
#include <regx51.h>
#include "gpio.h"

//hardware configuration
#define PWM_PORT        P2
#define PWM_DDR                 P2
#define PWM_OUT                 (1<<0)        //pwm output on p2.0
//end hardware configuration

#define PWM_ON(bits)        IO_CLR(PWM_PORT, bits)        //turn on bits
#define PWM_OFF(bits)        IO_SET(PWM_PORT, bits)        //turn off bits
#define PWM_STEPPING        150                 //pwm stepping, in  us - should leave enough time to fully execute the isr, 0x00 - 0xff
#define PWM_PERIOD                100                //pwm period, 0x00 - 0xff
unsigned char PWM_DC = 4;        //desired pwm duty cycle, from 0x00 - PWM_PERIOD

///////////////////???0????PWM////////////////////

void timer0(void) interrupt 1 using 2   {  
        static unsigned char pwm_index=0;   //t?????????????????  
       
        pwm_index++;    //?15????1
       
        if (pwm_index==PWM_PERIOD) {   //1.5???????  
                pwm_index=0;                //reset pwm_index
                PWM_ON(PWM_OUT);        //turn on pwm_out               
        }
        if (pwm_index==PWM_DC)  //???????????????  
                PWM_OFF(PWM_OUT);        //turn off pwm
}  

void mcu_init(void) {
        PWM_ON(PWM_OUT);         //drive pwm_out off
        IO_OUT(PWM_DDR, PWM_OUT);        //pwm_out as output
}

void tmr0_init(void) {
        TMOD = (TMOD & 0xf0) | 0x02;   //???0,????2,8?????  
        TH0=-PWM_STEPPING;     //??????241????0,?15??????(11.0592MHz)  
        TL0=TH0;     //?????  
        TR0=1;       //?????  
        ET0=1;       //?????0??
        EA=1;        //?????
}

//////////////////?????//////////////////////

void main(void) {  

        volatile unsigned short i;

        mcu_init();         //reset the mcu
        tmr0_init();         //set up tmr0
             
        while(1) {      
                for (i=0; i<0x02ff; i++)
                        continue;                                //waste some time
                PWM_DC++;                                        //update pwm duty cycle
                if (PWM_DC==PWM_PERIOD) PWM_DC=0;
                //(PWM_DC == PWM_PERIOD)? PWM_DC=0: PWM_DC++;
        }         
}  
============end code============

here is the waveform.



(原文件名:C51 PWM.PNG)

as PWM_ON() is defined as a logic low, and PWM_OFF() as a logic high, the code starts with DC=0% and ends with DC=100%. if you connect an led + resistor from the output pin (p2.0) to Vcc, you will see the light goes brighter.

you can change the pin by redefining PWM_PORT, PWM_DDR and PWM_OUT.

and the code can be easily ported to any other processor, as long as you reconfigure the timer and interrupt portion of it.