How to implement PWM with MCS-51 single chip microcomputer

About the method of implementing PWM in MCS-51 microcontroller
PWM  stands for "pulse width modulation".
PWM  actually outputs a series of rectangular waves on a pin of the microcontroller, and its period is generally fixed; while the time occupied by its high level and low level can be controlled and adjusted. The ratio of the time occupied by the high level and the low level is called the duty cycle, and its value is between 0 and 1. PWM is  widely used. Simply put, it can control the brightness of lights, the speed of DC motors, and even output voice signals.
New microcontrollers often contain hardware  PWM  modules, so it is very simple to write programs; but the old MCS-51 series does not have this condition. In fact, it is not difficult to implement PWM on a microcontroller 
without  PWM  hardware . All you need is a timer to make it work in interrupt mode. The following is an example of a question on the Internet to explain the method of implementing  PWM  . Question link: http://zhidao.baidu.com/question/170790790.html?push=core Question: AT89C52 microcontroller outputs adjustable  PWM  wave. Requirements: 1. The crystal oscillator of the microcontroller is 12MHz, and the output cycle is 1s. 2. Use two switches to control the increase and decrease of the duty cycle, divided into 20 levels. In response to this question, Zuoerlundao designed a simple experimental circuit: the 8  lines 　of  the P2  port  output  PWM  waveforms; two buttons are connected 　to  /INT1 and /INT0  to adjust the duty cycle; two BCD  digital displays are connected  　to  the P0  port to display the current duty cycle level. 　During the experiment, an oscilloscope can be used to observe  the waveform output by the P2  port. 　If  the frequency of the PWM  waveform is increased,  the LED  can also be used to observe the effect of gradually brightening and dimming. At present, only the flashing time changes. //The program is as follows: //======================================== #include unsigned  int  a,  b; //------------------------------------------ void  main() { TMOD  =  0x01;  //T0 timing mode 1 TH0  =  (65536-50000)  /  256;  //50ms@12MHz TL0  =  (65536-50000)  %  256; TR0  =  1;  ET0  =  1; EX0  =  1;  EX1  =  1; IT0  =  1;  IT1  =  1; EA  =  1;  a  =  0; b  =  10; while(1); } //------------------------------------------ void  time0()  interrupt  1 { TL0  =  (65536-50000)  %  256; TH0  =  (65536-50000)  /  256;  //50ms@12MHz a++; if(a  ==  20)  a  =  0;  //Adjust the period here. if(a  <  b)  P2  =  0xff;  //Adjust the duty cycle here. else  P2  =  0x00; P0  = 


















                         
          
         
                 
         
         
                 
         
         




         
          
         
                   
                
           
         ((b  /  10)  <<  4)  +  b  %  10;  //Display duty cycle level
}
//------------------------------------------
void  X0_INT()  interrupt  0
{
         b++;
         if(b  >  19)  b  =  19;            //The maximum duty cycle level is 19
}
//------------------------------------------
void  X1_INT()  interrupt  2
{
         b--;
         if(b  <  1)  b  =  1;              //The minimum duty cycle level is 1.
}
//===========================================