Blue Bridge Cup single chip microcomputer -- PWM pulse width modulation

One 、 principle

PWM The full name is Pulse width modulation （Pulse-width modulation）, It is a way to reduce the average power transmitted by electrical signals by dispersing them into discrete forms . So according to Area equivalence The laws of , You can change the time width of the pulse by pair , In order to obtain the corresponding synthesis equivalently amplitude and frequency Waveform of .

The details are shown in the following figure ：

  It can be seen from the above figure , Pulse width modulation Use a square wave whose pulse width will be modulated , And the average value of the wave pattern will change .

The average value of the wave pattern is obviously directly related to Duty cycle  D  of . There are three duty cycle states in the graph ,25%,50% and 75% Duty cycle status , It's not hard to find out , Assume that the duty cycle is D( It means in a pulse cycle , Proportion of power on time relative to total time Duty Ratio), Then meet ：, among T yes PWM The cycle of , Usually the same period as the carrier .

utilize PWM control LED brightness ：PWM It is a pulse width modulated signal , Among them “ Width ”, That is, the high-level time of the pulse .PWM Signal conditioning LED When the brightness is high , The signal frequency is constant , What changes is the time of the high level of the pulse , namely LED On time of . This kind of signal adjusting brightness is equivalent to adjusting LED The average current of , So the current will change .

Two 、 Programming

utilize PWM Pulse width signal realizes independent key pressing S7 Yes L1 Control of indicator light brightness change .

specific requirement ：

1.PWM The frequency of the pulse width signal is 100Hz.

2. After the system is powered on L1 The indicator light is off .

3.L1 Indicator light yes 4 Two brightness modes , They are completely extinguished 、10% The brightness of 、50% The brightness and 90% The brightness of .

4. Press down S7 Key , Cycle switching L1 Four brightness modes of the indicator light .

First the J5 Jumper cap of 2 and 3 On the pin

#include <reg52.h>

sbit L1=P0^0;
sbit S7=P3^0;
unsigned	char count=0;
unsigned	char pwm=0;
void delay(unsigned int t)// The time delay function 
{
 	while(t--);
}
void InitT0()// Timer initialization function 
{
	 TMOD=0X01;
	 TH0=(65536-100)/256;
	 TL0=(65536-100)%256;

	 ET0=1;
	 EA=1;

}
void serviceT0() interrupt 1// Timer service function 
{
    TH0=(65536-100)/256;
	 TL0=(65536-100)%256;// The timing of each interrupt is 100μs
	 count++;
	 if(count==pwm)
	 {
	  	L1=1;
	 }
	 else if(count==100)
	 {
	  	L1=0;
		count=0;
	 }
}
unsigned char flag=0;
void key()// Key related functions 
{
	if(S7==0)
	{
	 delay(100);// Anti shake treatment 
	 if(S7==0)
	 {
	 	  switch(flag)
		  {
		  	 case 0:L1=0;TR0=1;pwm=10;flag=1;break;
			 case 1:pwm=50;flag=2;break;
			 case 2:pwm=90;flag=3;break;
			 case 3:L1=1;TR0=0;flag=0;break;
		  }
		  while(S7==0);// Avoid touching by mistake 
	 }
	}
}
void main()
{
	 P2=0xA0;P0=0x00;P2=0x80;P0=0xff;// Turn off the peripherals 
     P2=0x80;L1=1;
	 InitT0();
	 while(1)
	 {
	  key();
	 }
}