The USB specification defines a link-layer suspend mode in which both the USB host and the device enter a no-activity phase to save power. The decision to take the USB host into the suspend mode is determined solely by the software. Once the host enters the suspend mode, all devices connected to that host are required to enter the suspend mode within 3 ms. A USB device could not enter the suspend mode by itself; however, when the link power management (LPM) extension is supported, the USB device can request the USB host to enter the suspend mode. When the USB host enters the suspend mode, all USB devices will follow.
A USB host can exit the suspend mode either through interrupts such as timers or through a remote wake-up request by a device with special USB signal leveling. A USB device can similarly wake up through interrupts or remote wake signaling from host or additionally through host reset signaling.
When the USB is in a suspend mode, the USB ULPI link protocol provides a standard method for the PHY to power-down during a time when the D+/D- signaling is directed to the USB link. In this case, a subsection of the USB IP that is always on, detects the wake signaling and generates the wake interrupt to the PMU to proceed with powering up the USB block and the processor that is responsible for its device driver.