Common use of USBSUSPEND Clause in Contracts

USBSUSPEND. In this state, the Host Controller is not generating SOF tokens on the USB; nor is it processing any lists that may be enabled in the HcControl register. In fact, the Host Controller ignores most of the operational registers which need not contain any meaningful values; however, the Host Controller does preserve their values. While in this state, the Host Controller monitors the USB for resume signaling, and if detected, changes the state to USBRESUME. Because of this, there is a restriction on how the Host Controller Driver may modify the contents of HcControl while in the USBSUSPEND state: Host Controller Driver may only write to HcControl with the HostControllerFunctionalState field set to either USBRESET or USBRESUME (see exception). After a certain length of time without SOF tokens, devices on the USB enter the suspend state. Normally, the Host Controller Driver must ensure that the Host Controller stays in this state for at least 5 ms and then exits this state to either the USBRESUME or the USBRESET state. An exception is when this state is entered due to a software reset and the previous state was not USBSUSPEND, in which case, if the Host Controller remains in the USBSUSPEND state for less than 1 ms, it may exit directly to USBOPERATIONAL (the timing of less than 1 ms ensures that no device on USB attempts to initiate resume signaling and thus the Host Controller does not attempt to modify HcControl). The only interrupts possible in the USBSUSPEND state are ResumeDetected (the Host Controller will have changed the HostControllerFunctionalState to the USBRESUME state) and OwnershipChange.