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