A decoding circuit includes a pre-trigger signal generating unit, a comparing unit, and a starting signal generating unit. The pre-trigger signal generating unit receives the former encoded data and generates a pre-trigger signal when the former encoded data of the received command matches the corresponding former encoded data of a predetermined command. The comparing unit generates a match signal when the latter encoded data of the received command is the same with the latter encoded data of the predetermined command. The starting signal generating unit outputs a starting signal according to the pre-trigger signal and the match signal. The starting signal starts a corresponding operation of the predetermined command.