A controller includes an input/output circuit and a reference voltage generating circuit. The input/output circuit includes a plurality of receiving circuits. Each receiving circuit receives and processes one data bit and generates an output bit accordingly. The reference voltage generating circuit is coupled to the input/output circuit and includes a plurality of circuit units for providing a plurality of reference voltages. One of the circuit units is coupled to one of the receiving circuits to provide a reference voltage to the corresponding receiving circuit and the receiving circuit processes the data bit according to the received reference voltage.