《USB2_LinkPowerMangement_ECN[final]》.pdfVIP

USB Engineering Change Notice ENGINEERING CHANGE NOTICE Title: USB 2.0 Link Power Management Addendum Applies to: Universal Serial Bus Specification, Revision 2.0 Summary of ECN This ECN defines a power management feature for USB called Link Power Management (LPM) which is similar to the existing suspend/resume, but has transitional latencies of tens of microseconds between power states (instead of three to greater than 20 millisecond latencies of the USB 2.0 suspend/resume). It has several parts: (a) it introduces formal terminology for line (bus) states in terms of power states, (b) defines a new, fast mechanism for transitioning the bus on a root port from an enabled state (called L0, see below) to a new Sleep state (called L1, see below), (c) defines new, fast timing requirements for resuming a bus from L1 back to (L0), and (d) defines the method for extending the existing USB protocol to accommodate explicit L1 entry. The L0 (enabled) to L1 (link Sleep) transition is actively initiated by the host using a new USB-defined transaction. This allows a device to immediately detect the intent of the host and respond immediately, instead of the USB 2.0 defined method of ‘discovery by loss of activity’, which requires 3 orders of magnitude more elapsed time. The L1 to L0 transition is similar to the USB 2.0 resume, as it can be initiated by either the host port or the device (i.e. remote wake). However, the duration of resume signaling is redefined to be three orders of magnitude faster (i.e. tens of microseconds, not tens of milliseconds). The LPM feature may be utilized at any depth in a valid USB connection hierarchy. However, it is expected that the most value to be obtained will be when it is used with root-ports and possibly a single level of hub beyond the root-ports. This is motivated primarily by the fact that the goal is to minimize the transitional latencies. Supporting multiple layers in th