One of the features that was missing in my firmware was the sleep/power save mode. Since I already spent all my GPIOs on the LEDs and the sensor, I had to build a logic with the reset button.

The solution I found was to use the EEPROM.

When the AVR resets, it reads the first byte from the EEPROM, negates it, and writes it back. When the value read is 1/true, the program runs. When it is not, the MCU is put to sleep.

All of this logic was simply put at the top of the main function body.

#include <avr/interrupt.h>
#include <avr/eeprom.h> 
#include <avr/sleep.h> 

int main(void) {
    /* When reset, read whether program shall be run */
    uint8_t run = eeprom_read_byte (EEPROM_ADDR);
    /* invert value and write back to eeprom */
    eeprom_write_byte (EEPROM_ADDR, run ? 0 : 1);
    if(!run) {
        /* disable all interrupts and go to sleep */
        cli();
        sleep_mode();
    }
    while(1) {
        /* control loop code here */
    }
}