In step S508, it is determined whether or not a power low signal SRC_LOSS outputted from the data latch is change. Generally Speaking, the power low signal SRC_LOSS outputted from the data latch would be changed according to the state of the power voltage of the power input terminal. When the power voltage of the power input terminal is charged/discharged to the common voltage, the power low signal SRC_LOSS outputted from the data latch may be changed from the logical high voltage to the logical low voltage or from the original logical low voltage to the logical high voltage. Since the mention above is design of selectiveness, the detailed description is omitted. When the determination is positive, the step S509 is performed. When the determination is negative, the step S511 is performed to re-detect.