In 2016 data sheets of ATmega328P, the electrical characteristics (Vcc = 5V, 850C) of a port-pin are:
VIL = 1.5V : max voltage that is guaranteed to be taken as LL
VIH = 3.0V : min voltage that is guaranteed to be taken as LH
VOL = 0.9V : max voltage that the port-pin will assume when LL is asserted on this pin
VOH = 4.2V : min voltage that will be sustained by the port-pin when LH is asserted on this pin