NXP Semiconductors QorIQ LS1012A Reference Manual

and device. The Stream ID field in the events indicate which stream within the endpointis generating the events. If software receives an XferNotReady event with theEventStatus field indicating XferNotActive, it means that the host is attempting to initiatea stream that has not been added to the hardware's cache through Start Transfer. Inresponse to this event, software should add the requested stream if it is available.33.4.3.6 Low power operation33.4.3.6.1 Low power operation of USBThe USB 3.0 protocol allows a SuperSpeed host and device to negotiate with each otherand decide when to bring their link into a lower power state (U1 or U2). This isaccomplished by each side making individual decisions about whether they have anytraffic currently pending or if they expect any traffic soon. The device requests lowpower entry by transmitting an LGO_U1 (or LGO_U2) Link Command and exits lowpower by using an LFPS handshake. The USB 3.0 Device Controller uses informationfrom the host (bus side) and from software (application side) to decide when the best timeto request low power entry is and when to exit from low power. The same calculation isused when the controller decides whether to accept a low power request from the host.The controller maintains state information about each endpoint and whether it is "active"or "paused". If all enabled endpoints are paused from either the application side or thebus side, low power entry is allowed. If at least one endpoint is not paused, low powerentry is not allowed. To indicate that it does not have any pending traffic and allow thelink to go into a low power state, the software may pause all enabled endpoints using oneof the following mechanisms:• Allow transfers to complete and not start any new ones• Not setting the HWO bit in the TRBs to '1'Exception: Software may prepare a Setup TRB for control endpoints without affectingthe ability of the link going into low power. Otherwise, an endpoint is considered activeon the application side. For these endpoints, the host indicates that it is paused in one ofthe following ways:• Setting the Packet Pending (PP) flag to '0' in the DPH• Setting the PP flag to '0' and NumP to '0' in the ACK TP• For interrupt endpoints, by not polling the endpoint for two service intervals• Note: After an endpoint is configured using the DEPCFG command and before thehost sends any packets to it, the endpoint is considered to be paused on the bus sideFunctional DescriptionQorIQ LS1012A Reference Manual, Rev. 1, 01/20182288 NXP Semiconductors