7.8.1 Interrupt Response Time
The interrupt execution response for all the enabled AVR interrupts is five clock cycles minimum. After five clock
cycles the program vector address for the actual interrupt handling routine is executed. During these five 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 five 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 five clock cycles. During these five clock cycles, the Program
Counter (three bytes) is popped back from the Stack, the Stack Pointer is incremented by three, and the I-bit in
SREG is set.