ISR(USART_RXC_vect)
{
   ......
   switch ( serial_processing() )
   {
      case ISO_RETURN_CODE_OK: 
      serial_puts_P(PSTR("\n\rOK"));
      print_prompt();
      break;
   ......
   }

}

char serial_processing(void)
{
   ......
   sei();
   unsigned char return_code = iso_send_msg(iso_msg_buf, serial_msg_len ); //използва TIMER0 overflow interrupt.
   ......
   cnt = iso_recv_msg(iso_msg_buf);  //използва TIMER0 overflow interrupt и INT0.
   ......
   
}

Interruptible Interrupt routine
“Rule of thumb”: in the doubt ISR. Those in the following described method use only if one realizes oneself the
different function mode.
#include <avr/interrupt.h>
//...
void XXX_vect(void) __attribute__((interrupts));
void XXX_vect(void) {
//...
}
Here stands for XXX for described the name of the vector the above (thus e.g. void TIMER0_OVF_vect
(void)…). The difference compared with ISR is that when calling the function global the Enable interrupts
bits become certified here automatically again set and thus further interrupts. This can to not insignificant
problems of in the simplest case a stack OVERFLOW up to other unexpected effects