Processor, host device, power saving method of USB device, and computer program

A power saving method of a USB device including a main CPU and a device side USB interface for communicating with a USB host, the method comprising:

• when the main CPU proceeds to a power saving mode in which power consumption is reduced, requesting transmission of a suspend command to the USB host, without the device side USB interface proceeding to a suspend state, by setting a power saving flag indicating that the main CPU is in the power saving mode in a memory of the device side USB interface, which is able to be monitored by the USB host;
• when the device side USB interface receives the suspend command from the USB host, allowing the device side USB interface to proceed to the suspend state;
• when the device side USB interface receives a return command from the USB host, allowing the device side USB interface to proceed to a normal state, and switching the power saving flag to be in a flag off state in which the power saving flag is not set;
• when the device side USB interface proceeds to the normal state, allowing the main CPU to proceed to a normal mode in which power consumption is higher than in the power saving mode;
• and
• in a case where the main CPU proceeds to the normal mode when the device side USB interface is in the suspend state, retaining a flag on state in which the power saving flag is set, and retaining the device side USB interface in the suspend state.

A non-transitory recording medium storing a computer readable program used in a processor capable of executing predetermined processing, the processor comprising a main CPU and a communication interface, and the program causing the processor to execute:

• a notification process in which the communication interface, without proceeding to a power saving state, executes notification processing for allowing a host device, which requests execution of the processing to the processor, to detect that an operating mode of the main CPU is a power saving mode, when the main CPU which controls the execution of the processing proceeds to the power saving mode in which power consumption is reduced;
• and
• a state switching a process to allow the communication interface to proceed to the power saving state in which power consumption is smaller than in a normal state when the communication interface, which communicates with the host device, receives a power saving command from the host device, and to allow the communication interface to proceed to the normal state from the power saving state when the communication interface receives a return command from the host device,
• wherein the notification process sets a power saving flag indicating that the operating mode of the main CPU is in the power saving mode in the memory, which is able to be monitored by the host device, as the notification processing,
• wherein the notification process switches the power saving flag to be in a flag off state in which the power saving flag is not set when the state switching process allows the communication interface to proceed to the normal state,
• wherein the main CPU proceeds to a normal mode in which power consumption is larger than in the power saving mode when the communication interface proceeds to the normal state, and
• wherein, in a case where the main CPU proceeds to the normal mode when the communication interface is in the power saving state, the notification process retains a flag on state in which the power saving flag is set, and the state switching process retains the communication interface in the power saving state.