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ATmega8A [DATASHEET]
8159E–AVR–02/2013
handling routine, and hardware clears the corresponding Interrupt Flag. Interrupt Flags can also be cleared by writ-
ing 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 nec-
essarily 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.
When using the SEI instruction to enable interrupts, the instruction following SEI will be executed before any pend-
ing interrupts, as shown in the following example.
Assembly Code Example
in
r16, SREG
; store SREG value
cli
; disable interrupts during timed sequence
sbi
EECR, EEMWE
; start EEPROM write
sbi
EECR, EEWE
out
SREG, r16
; restore SREG value (I-bit)
C Code Example
char
cSREG;
cSREG = SREG; /* store SREG value */
/* disable interrupts during timed sequence */
_CLI();
EECR |= (1<<EEMWE); /* start EEPROM write */
EECR |= (1<<EEWE);
SREG = cSREG; /* restore SREG value (I-bit) */
Assembly Code Example
sei
; set global interrupt enable
sleep
; enter sleep, waiting for interrupt
; note: will enter sleep before any pending
; interrupt(s)
C Code Example
_SEI(); /* set global interrupt enable */
_SLEEP(); /* enter sleep, waiting for interrupt */
/* note: will enter sleep before any pending interrupt(s) */