2. avr timer/counter 0 --TC0 -- fast PWM output

PWM: Pulse Width Modulation. In the figure, T is the pulse period, t is the high level time, and the ratio of t to T (t/T) is called the duty cycle. Pulse width modulation refers to adjusting the size of t, that is, changing the duty cycle of the pulse
. The larger the duty cycle value, the higher the output voltage. Changing the duty cycle changes the output voltage, which is often used to implement D/A, adjust voltage or current, change the speed of the motor, etc.

Fast PWM mode: Its counting method is that TCNT0 starts counting from 0 to 255, counts by 1 and returns to 0, and then continues to count by 1. Compared with the phase PWM correction mode (counting from 0 to 255, and then counting from 255 to 0), there is only one slope, so the PWM output frequency is high. In fast PWM mode, the maximum value of the counter determines the frequency of PWM, and the comparison register OCR0 determines the size of the duty cycle. T/C0 is an 8-bit counter, and the output PWM frequency = system clock frequency/(division factor*255).

PWM generation process: After the timer is started, when the values ​​of TNCT0 and OCR0 are equal, the output pin OC0 (PB3) is cleared to 0, and when TNCT0 reaches 255, OC0 is set. Changing the value in OCR0 during program execution changes the duty cycle of OC0 output.

//Function: Control the light-emitting diode on the PB3 pin from bright to dark, and then from dark to bright (code from Easy to Play AVR MCU C Language CD)

#include 
#define uchar unsigned char 
#define uint unsigned int
unsigned int count;
/*********The following is the delay function*********/
void Delay_ms(uint xms)
{
int i,j;
for(i=0;i{ for(j=0;j<1140;j++) ; }
}
/********The following is the port initialization function********/
void port_init() 
{
DDRB|=(1<PORTB&=(0<}
/********Timer 0 initialization********/
void timer0_init()
{
TCCR0=(1<// T/C0 works in fast PWM mode, without frequency division
// When compare match, OC0 is set, and when count reaches 0XFF, OC0 is cleared 
OCR0=0; //Initial value of compare match register, 

SREG=0x80; //Enable global interrupt
}
/*********The following is the main function*********/
void main(void) 
{
port_init();
timer0_init(); 
while(1)
{
for(count=0;count<256;count++) //When OCR=0, the LED is the brightest, and then gradually dims
{
OCR0=count; //Assign compare match register value
Delay_ms(20); //Delay for a period of time to observe the effect
} 
Delay_ms(3000); // When the LED is the darkest, delay for a while
for(count=255;count>0;count--) //When OCR=255, the LED is the darkest, and then gradually brightens
{
OCR0= count; 
Delay_ms(20); 
} 
Delay_ms(3000); //When the LED is the brightest, delay for a while
} 
}