An interrupt must be initialized in the following order:
      
         - 1.Configure the CPUINT if the default
            configuration is not adequate (optional):
               - Vector handling is configured by
                  writing to the respective bits (IVSEL and CVT) in the Control A register
                  (CPUINT.CTRLA).
 
               - Vector prioritizing by round-robin
                  is enabled by writing a '1' to the Round-Robin Priority Enable bit (LVL0RR) in
                  CPUINT.CTRLA.
 
               - Select the priority level 1 vector
                  by writing its address to the Interrupt Vector (LVL1VEC) in the Level 1 Priority
                  register (CPUINT.LVL1VEC).
 
            
 
         - 2.Configure the interrupt conditions
            within the peripheral, and enable the peripheral's interrupt.
 
         - 3.Enable interrupts globally by writing a
            '1' to the Global Interrupt Enable bit (I) in the CPU Status register (CPU.SREG).