AVR Timer Overflow ISR not running

Question

MCU: Atmega328P IDE: Microchip Studio v7.0.2594 Toolchain: Atmel AVR 8-bit (C language) Native I am trying to run Timer0 Overflow interrupt. However, it seems like ISR is not getting executed. The Pin toggles inside while(1) but stays LOW when operated inside ISR Same kind of issues happen when I try to run Atmega2560 Timers. Here is my basic code. I have worked on many Atmega MCUs but this has never happened before. Note: Observing the signal pin using a Logic Analyzer.

#define F_CPU 16000000UL
#include <avr/io.h>
#include <avr/interrupt.h>
#include <util/delay.h>


ISR(TIMER0_OVF_vect)
{
    PORTB ^= (1<<5);  //Toggle once in every 8.12ms
    TCNT0 = 127;
}
int main(void)
{
    /* Replace with your application code */
    DDRB = (1<<5);
    PORTB = 0;
    
    TCCR0A = 0;
    TCNT0 = 127;
    TIMSK0 = (1<<TOIE0);
    TIFR0 = 0;
    TCCR0B = (1<<CS02)|(1<<CS00); // CLKio Divide by 1024
    //Duration of 1 count = 16MHz/1024 = 15.625KHz = 64us   
    //Duration of 127counts = 64*127 = 8.12ms

    sei();
    
    while (1);
    {
        PORTB ^= (1<<5);
        _delay_ms(10);
    }
}

When I disable the interrupt using //sei(); or //TIMSK0 = (1<<TOIE0); The code inside while 1 works fine

Answer 1

After going through the datasheet of ATmega2560, it said: " The Interrupt Vectors can be moved to the start of the Boot Flash section by setting the IVSEL bit in the MCU Control Register (MCUCR). Refer to “Interrupts” on page 101 for more information. The Reset Vector can also be moved to the start of the Boot Flash section by programming the BOOTRST Fuse"

When I unprogrammed(set to 1) the BOOTRST bit of High Fuse Byte, it resolved the issue and the ISR is working now with the above code.