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42073A-MCU Wireless-02/13
ATmega2564/1284/644RFR2
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
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" onpage 241. 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 IVSEL bit in the MCU Control Register
can also be moved to the start of the Boot Flash section by programming the
When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts
are disabled. The user software can write logic one to the I-bit to enable nested
interrupts. All enabled interrupts can then interrupt the current interrupt routine. The I-bit
is automatically set when a Return from Interrupt instruction – RETI – is executed.
There are basically two types of interrupts. The first type is triggered by an event that
sets the Interrupt Flag. For these interrupts, the Program Counter is vectored to the
actual Interrupt Vector in order to execute the interrupt handling routine, and hardware
clears the corresponding Interrupt Flag. Interrupt Flags can also be cleared by writing a
logic one to the flag bit position(s) to be cleared. If an interrupt condition occurs while
the corresponding interrupt enable bit is cleared, the Interrupt Flag will be set and
remembered until the interrupt is enabled, or the flag is cleared by software. Similarly, if
one or more interrupt conditions occur while the Global Interrupt Enable bit is cleared,
the corresponding Interrupt Flag(s) will be set and remembered until the Global
Interrupt Enable bit is set, and will then be executed by order of priority.
The second type of interrupts will trigger as long as the interrupt condition is present.
These interrupts do not necessarily have Interrupt Flags. If the interrupt condition
disappears before the interrupt is enabled, the interrupt will not be triggered.
When the AVR exits from an interrupt, it will always return to the main program and
execute one more instruction before any pending interrupt is served.
Note that the Status Register is not automatically stored when entering an interrupt
routine, nor restored when returning from an interrupt routine. This must be handled by
software.
When using the CLI instruction to disable interrupts, the interrupts will be immediately
disabled. No interrupt will be executed after the CLI instruction, even if it occurs
simultaneously with the CLI instruction. The following example shows how this can be
used to avoid interrupts during the timed EEPROM write sequence.
Assembly Code Example
in r16, SREG ; store SREG value
cli ; disable interrupts during timed sequence
sbi EECR, EEMPE ; start EEPROM write
sbi EECR, EEPE
out SREG, r16 ; restore SREG value (I-bit)