Universal serial bus device and universal serial bus system

A universal serial bus (USB) system, comprising:

• a host;
• a USB cable;
• and
• a USB device, to be connected to the host by the USB cable and comprising:
• an electrical physical layer module, reading voltages of first and second transmission lines to extract a recovery clock and data;
• a logical physical layer module, detecting the recovery clock and data from the electrical physical layer module to output an indication signal, and, when no recovery clock is available, the logical physical layer module sets a voltage value of the indication signal to be a predetermined value;
• and
• a link layer module, having a state machine defining link connectivity and link power management of the USB device,
• wherein the link layer module compares the voltage value of the indication signal with the predetermined value and makes the state machine leave a normal operation state when the voltage value of the indication signal has been maintained at the predetermined value over a predetermined time period;
• wherein the indication signal comprises a first bus signal and a second bus signal, and when no recovery clock is available, the logical physical layer module sets a voltage value of the first bus signal to be a first value and sets a voltage value of the second bus signal to be a second value, and the predetermined value is formed by the first value and the second value.