A universal serial bus device includes: a core circuit having a first pin and a second pin, and having an input impedance looking into the core circuit from the first pin and the second pin; and a charging control circuit, coupled to the core circuit, arranged for selectively providing a voltage source to one of the first pin and the second pin; wherein the input impedance of the core circuit is configured to make the voltage source substantially intact when the voltage source is coupled to one of the first pin and the second pin.