The Atmel®AVR® provides several different interrupt sources. These interrupts and the separate
Reset Vector each have a separate Program Vector in the Program memory space. All interrupts
are assigned individual enable bits which must be written logic one together with the
Global Interrupt Enable bit in the Status Register in order to enable the interrupt. Depending on
the Program Counter value, interrupts may be automatically disabled when Boot Lock Bits
BLB02 or BLB12 are programmed. This feature improves software security. See the section
“Memory Programming” on page 215 for details.
The lowest addresses in the Program memory space are by default defined as the Reset and
Interrupt Vectors. The complete list of Vectors is shown in “Interrupts” on page 46. The list also
determines the priority levels of the different interrupts. The lower the address the higher is the
priority level. RESET has the highest priority, and next is INT0 – the External Interrupt Request
0. The Interrupt Vectors can be moved to the start of the boot Flash section by setting the Interrupt
Vector Select (IVSEL) bit in the General Interrupt Control Register (GICR). Refer to
“Interrupts” on page 46 for more information. The Reset Vector can also be moved to the start of
the boot Flash section by programming the BOOTRST Fuse, see “Boot Loader Support – Read-
While-Write Self-Programming” on page 202.