Let MindShare Bring "USB 3.2 and xHCI" to Life for You
Each generation of the USB is backward compatible and a USB 3.2 topology may include devices operating at USB 2.0 Low Speed (LS), Full speed (FS), and High Speed (HS) as well as USB 3.x SuperSpeed (SS) Gen 1 and Gen 2. Prior to USB 3.2, USB cables, plugs, and receptacles were based on single-lane serial bus communications with an independent set of signals for USB 2.0 and SS Gen 1/Gen 2 communications. USB 3.2 adds an option for doubling the bandwidth of single-lane 5 Gb/s SS Gen 1 or 10 Gb/s SS Gen 2 by employing the extra Tx/Rx signal pair already present in USB Type-C (USB-C) interfaces. The result: a dual-lane SS Gen 1 or Gen 2 interface (referred to in the USB 3.2 Specification as Gen 1x2 or Gen 2x2, respectively). In this scheme, data is alternately “striped” across two lanes and sent in parallel—it effectively doubles bus bandwidth for a given clock speed. The peak transfer rate under USB 3.2 is now Gen 2x2: 10 Gb/s x 2 lanes = 20 Gb/s. Note that traditional single-lane SS Gen 1 (Gen 1x1) and SS Gen 2 (Gen 2x1) are still available and that the scheme has no impact on legacy USB protocol on the USB 2.0 signals.
In addition to the new Enhanced SuperSpeed (ESS) Gen 1x2, Gen 2x2 features, USB 3.2 carries forward the many SuperSpeed protocol optimizations first introduced in USB 3.0/USB 3.1 and designed to mitigate some of the disadvantages of USB 2.0 bus operations. These include unicast instead of broadcast packets, device asynchronous messages, packet bursting, end-to-end and link level flow control, link level error handling, link-level power management, and many others. Several significant enhancements to SuperSpeed Plus (SSP) hubs have also been added in the USB 3.2 specification. All of the above topics are covered in the USB 3.2 part of the class.
All generations of USB rely on platform host controllers to manage devices attached to each bus instance. USB 2.0 employed UHCI/OHCI and EHCI compliant host controllers to handle low, full, and high speed devices. The advanced capabilities of USB 3.x require new generations of USB host controllers. Course topics on the last day include Intel’s eXtensible Host Controller Interface (xHCI). A single host controller based on xHCI can manage both USB 2.0 and Enhanced SuperSpeed (ESS) topologies as well as attached devices of any USB speed. The xHCI operational model, software interface (registers, memory data structures), doorbell-based work notification, and hardware transaction scheduling are all described.
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USB 3.2 with xHCI Course Info