void PWM_Init()
{
   TCCR0 = (0<<WGM01)|(1<<WGM00)|(1<<CS00);//setup PMW, no prescaler
   OCR0 = 0xC0; //set 75% duty cycle
   DDRB |= (1<<DDB3);
}
void main()
{
   unsigned char PWM_on;
   unsigned char PWMON[]="PWM on Pin 4";
   unsigned char PWMOFF[]="PWM off Pin 4";
   PWM_on = 0;
   PWM_Init();
   while (1)
   {
      switch (Received_Command)
      {
        case '3':         
        if (PWM_on == 1)
        {
        TCCR0 ^=(1<<COM01); //disable OC0
        PWM_on = 0;
        UART_putstring(PWMOFF);
        USART_Transmit(LF);
        Received_Command = '0';
        }
        else
        {
        TCCR0 ^=(1<<COM01);
//Clear OC0 on compare match when up-counting. Set OC0 on compare match when downcounting.
        PWM_on = 1;
        UART_putstring(PWMON);
        USART_Transmit(LF);
        Received_Command = '0';
        }
   }//end while(1)
}

#include <avr/io.h>

int main (void)
{
   // Speaker as output
   DDRB   |= (1 << 1);
   
   // DAC inputs as input, pullup
   PORTD  |= 0xFF;
   
   // Set on match, clear on TOP
   TCCR1A  = ((1 << COM1A1) | (1 << COM1A0));

   // Phase + Frequency Correct PWM, Fcpu speed
   TCCR1B  = ((1 << CS10) | (1 << WGM13));

   // TOP is 0xFF for 8-bit audio
   ICR1    = 0xFF;

   for (;;)
     OCR1A = ((int8_t)PIND + 0x7F);
}

#include<avr/io.h>
#include<avr/interrupt.h>
#include<util/delay.h>
#define Delay( x ) _delay_loop_2( x )

#define F_CPU 1000000UL


pwm_init()
{
     TCCR1A |= (1<< COM1A1) | (1<< COM1B1) | (1<< COM1A0) | (1<< COM1B0) | (1<< WGM10)| (1<< WGM11) ;// 10 bit PWM phase correct
     TCCR1B |= (1<< CS10); // no prescaling
     //TCCR1B |= (1<< CS10) | (1<< CS12) ;
     TCNT1 = 0x0000;
}

void pwm_set( int left_speed, int right_speed )
{
     OCR1A = left_speed;
     OCR1B = right_speed;
}
int main ()
{
   unsigned int i;
   DDRD = 0xff;
   PORTD = 0x00;
   DDRB = 0xff;

// PD0 and PD1 - right motor
// PD7 and PD6 - left motor
//  1       0 forward
//  1       1 brake (both outputs low)
//  0       1 reverse
//  0       0 coast (both outputs off =high impedance?)

     PORTB |= (1 << PB1) | (1 << PB2);
     PORTD |= (1 << PD7) | (0 << PD6) | (1 << PD0) | (0 << PD1);

pwm_init();
while(1)
{
for(i=1;i<256;i++)
{
pwm_set(i,i);
Delay(5000); //20000 cycles
}
for(i=255;i>0;i--)
{
pwm_set(i,i);
Delay(5000);
}
}
}