Embodiments of the invention improve power management in a data storage device which executes serial data communication with a host. In one embodiment, a HDD executes a reset process from a sleep mode (clock inactive) as being triggered by a control signal that is not based on a clock from the host. Even during a period in which the system clock is suspended, the HDD is able to receive the control signal from the host. In responding to the control signal and by activating a circuit required for serial data transmission, the HDD is enabled to receive a software reset command from the host via a serial data transmission circuit. With such an arrangement, the HDD can receive the software reset command from the host, and in responding to the reception of the command, the HDD can be reset to the normal active mode.