Rainbow Electronics ATmega8HVD - Manual (Page 13)

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8052B–AVR–09/08
ATmega4HVD/8HVD
When using the SEI instruction to enable interrupts, the instruction following SEI will be exe-
cuted before any pending interrupts, as shown in this example.
6.7.1 Interrupt Response Time
The interrupt execution response for all the enabled AVR interrupts is four clock cycles mini-
mum. After four clock cycles the program vector address for the actual interrupt handling
routine is executed. During this four clock cycle period, the Program Counter is pushed onto
the Stack. The vector is normally a jump to the interrupt routine, and this jump takes three
clock cycles. If an interrupt occurs during execution of a multi-cycle instruction, this instruction
is completed before the interrupt is served. If an interrupt occurs when the MCU is in sleep
mode, the interrupt execution response time is increased by four clock cycles. This increase
comes in addition to the start-up time from the selected sleep mode.
A return from an interrupt handling routine takes four clock cycles. During these four clock
cycles, the Program Counter (two bytes) is popped back from the Stack, the Stack Pointer is
incremented by two, and the I-bit in SREG is set.
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) */