Synchronous Sensing in ATmega2560

From Table 1, interrupts INT7:4 are registered synchronously. The value on the INT7:4 pins are sampled before detecting edges. If edge or toggle interrupt is selected, pulses that last longer than one clock period will generate an interrupt. Shorter pulses are not guaranteed to generate an interrupt. For both types of sensing, if low level interrupt is selected, the low level must be held until the completion of the currently executing instruction to generate an interrupt.