Interrupt Response Time

The interrupt execution response for all the enabled AVR interrupts is 4-clock cycles minimum. After 4-clock cycles, the Program vector addresses for the actual interrupt handling routine is executed. During this 4-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 3-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 AVR MCU is in Sleep mode, the interrupt execution response time is increased by 4-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 4-clock cycles. During these 4-clock cycles, the Program Counter (two bytes) is popped back from the Stack, the Stack Pointer is incremented by 2, and the I-bit in SREG is set.