Input Impedance

The DC input impedance of CMOS logic gates is infinite, so long as the voltage is between the supply rails. They do have a small input capacitance, up to 5pF. But watch out, sometimes devices have internal pullup resistors that can be enabled. Pullup resistors are useful for a variety of purposes, such as detecting closed switches, receiving open-collector opto-isolator signals, or fault outputs from power-driver ICs, for use with wired-OR logic, etc.

The ATmega64A does have internal pullup resistors, although it also has a global pullup disable. The pullup resistor can also be turned on or off individually for each pin. The port-pin schematic above is from page 264 of the datasheet.

Under the topic DC Characteristics, on page 327, the datasheet says the pullup resistor value is from 20 to 50k.