Controlling the frequency of PWM in AVR

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#ifndef F_CPU                   // if F_CPU was not defined in Project -> Properties
#define F_CPU 1000000UL         // define it now as 1 MHz unsigned long
#endif
 
#include <avr/io.h>             // this is always included in AVR programs
#include <avr/interrupt.h>
 
///////////////////////////////////////////////////////////////////////////////////////////////////
int main(void) {
    
    DDRC |= (1 << PC5) | (1 << PC4);            // set PC5 (pin 28) and PC4 (pin 27) for output
    
    /*
    TCCR1A - Timer/Counter 0 Control Register A
    
    bit           7         6         5         4        3       2        1        0
    name        COM1A1    COM1A0    COM1B1   COM1B0    FOC1A   FOC1B    WGM11    WGM10
    set to        0         0         0         0        0       0        0        1
    
    COM1A1 = 0    normal port operation, OC1A disconnected
    COM1A0 = 0
    
    com1B1 = 0    normal port operation, OC1B disconnected
    com1B0 = 0
    
    bit 3 = 0
    bit 2 = 0
    
    WGM11 = 1     CTC (Clear Timer on Compare match) mode, see TCCR0B also
    WGM10 = 0     TCNT0 will count up to value in OCR1A, then signal timer 0 compare interrupt
    */
 
    TCCR1A = 0b00000001;
    
    /*
    TCCR1B - Timer/Counter 0 Control Register B
    
    bit           7          6        5       4         3         2         1        0
    name        ICNC1      ICES1      -      WGM13      WGM12      CS12      CS11     CS10
    set to        0          0        0       0          1          1         0        1
    
    ICNC1 = 0     
    FOC0B = 0
    
    bit 5 = 0
    WGM13 = 0
    
    WGM02 = 0     CTC (Clear Timer on Compare match) mode, see TCCR0A also
    
    CS02 = 0
    CS01 = 1      clock / 64, should not use less than this to allow ISR enough clock cycles
    CS00 = 1
    */
 
    TCCR1B = 0b00001010;
    
    /*
    TIMSK - Timer/Counter 0 Interrupt Mask Register
    
    bit           7        6        5       4       3       2         1         0
    name          -        -     TICIE1  OCIE1A  OCIE1B   TOIE1       -       -
    set to        0        0        0       1       0       0         0         0
    
    bit 7 = 0     don't use Force Output Compare A
    bit 6 = 0
    bit 5 = 0
    bit 4 = 0
    bit 3 = 0
    OCIE1B = 0    don't enable Timer/Counter 0 Output Compare Match B Interrupt
    OCIE1A = 1    enable Timer/Counter 0 Output Compare Match A Interrupt Enable
    TOIE1 = 0     don't enable Timer/Counter 0 Overflow Interrupt
    */
    TIMSK = 0b00010000;
    
    PORTC |= (1 << PC4);        // turn on PC4 (pin 27) full bright
    
    OCR1A = 0b00000001;     // set compare register to low value to produce fast PWM
    
    sei();              // enable interrupts
    
    while (1) { }
    
    return(0);                  // should never get here, this is to prevent a compiler warning
}
 
///////////////////////////////////////////////////////////////////////////////////////////////////
ISR(TIMER1_COMPA_vect) {
    PORTC ^= (1 << PC5);
}