// Arduino clock frequency is 16 MHz
// Debayan Sutradhar

#include <avr/io.h>
#include <avr/interrupt.h>

unsigned volatile int n_compare_matchs = 0;


ISR(TIMER2_COMPA_vect)
{
  // TASK: Use n_compare_matchs to toggle D12 on each timer interrupt 
  // and Toggle D13 at 1 sec interval.

  if (n_compare_matchs >= 124) {
    n_compare_matchs = 0;

    // One second done
    // Toggle PB5 (D13)
    PORTB ^= (0b00100000);
  } else  {
    n_compare_matchs++;
  }

  // Toggle PB4 (D12)
  PORTB ^= (0b00010000);
}


// Datasheet used: 
// https://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-7810-Automotive-Microcontrollers-ATmega328P_Datasheet.pdf

int main()
{

    // TASK: set the prescaler for Timer2 to 1024
    // We need to set TCCR2B bit 0,1,2 (CS20, CS21, CS22) as 1 to set prescaler to 1024
    // Source: Page 131
    TCCR2B |= (0b00000111);


    // TASK: set Wave Generation mode to CTC(clear Timer on Compare match)
    // Source: Page 127, 130
    // We need to set:
    // TCCR2A Bit 0 (WGM20) as 0
    TCCR2A &= ~(0b00000001);
    // TCCR2A Bit 1 (WGM21) as 1
    TCCR2A |=  (0b00000010);
    // TCCR2B Bit 3 (WGM22) as 0
    TCCR2B &= ~(0b00001000);

    // TASK: Set upper limit for CTC with OCR2A (remember: ticks start form 0)
    // Set to 124 (0 - 124 = 125 ticks)
    OCR2A = 124;

    // TASK: Enable Timer2 interrupt on compare match
    // Source: Page 132
    // Set TIMSK2 Bit 1 (OCIE2A) to 1 
    TIMSK2 |= (0b00000010);

    // TASK: Set pin modes for D13 and D12
    // Source: Page 66, 72
    // Mark Bit 4 and 5 (PB4, PB5) as high on DDRB
    DDRB |= (0b00110000);

    // TASK: Enable Interrupts
    // Source: Page 11, 16
    // Mark Bit 7 of SREG as 1 to enable global interrupts
    // We need to use SEI/CLI to set/clear
    // Direct register manipulation is not allowed
    sei();

    while (1);
}