USB Complete fourth- P42

USB Complete fourth- P42:This book focuses on Windows programming for PCs, but other computersand operating systems also have USB support, including Linux and AppleComputer’s Macintosh. Some real-time kernels also support USB.
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USB Complete fourth- P42Chapter 15Figure 15-6. A USB 3.0 hub contains a USB 2.0 hub and a hub for SuperSpeed.(Adapted from Universal Serial Bus 3.0 Specification.)$WU 5RGGFU The speed of a hub’s upstream port determines what bus speeds are available to downstream ports. If the upstream port connects at SuperSpeed, the hub can communicate with downstream devices at any speed. If the upstream port con- nects at high speed, the hub can communicate downstream at low, full, and high speeds. If a USB 3.0 hub’s upstream port connects at full speed, the hub can communicate downstream at low and full speeds. A downstream-facing port that connects internally to an embedded device can support a single speed. At the hub’s upstream port, traffic to and from downstream SuperSpeed devices uses the SuperSpeed wires, and traffic to and from downstream low-, full-, and high-speed devices uses the USB 2.0 wires. As with USB 2.0 hubs, all upstream traffic on the USB 2.0 wires uses high speed (unless a USB 1.x hub is upstream from the hub).%QORQPGPVU The SuperSpeed portion of a USB 3.0 hub consists of a repeater/forwarder and a hub controller. Like the hub repeater in a USB 2.0 hub, the repeater/for-386 All About Hubs warder re-transmits received packets, detects device attachment and removal, establishes the connection of a device to the bus, detects bus faults such as over-current conditions, and manages power to the device. A hub may partially store a Data Packet before beginning to forward it, and the hub stores and for- wards all other packets. Buffers help to manage the traffic that passes through the hub. Buffers enable storing packet headers for later delivery to a down- stream device that must exit a low-power mode before receiving traffic. Buffers also enable receiving asynchronous messages from multiple downstream devices at once and holding received payload data to repeat. To enable retrying, after transmitting a Data Packet, the buffer retains the packet until receiving a link-level acknowledgement. As in a USB 2.0 hub, a USB 3.0 hub controller manages communications between the host and the hub. The hub sends status information via an inter- rupt IN Status Change endpoint. A hub with information to report sends an ERDY Transaction Packet to the host./CPCIKPI 6TCHHKE The hub stores and forwards header packets and repeats Data Packets. The hub must be able to store eight header packets directed to the same downstream port and eight header packets received at a downstream port. During hub enumeration, the host sends a Set Hub Depth request to assign a hub-depth value to the hub. The value equals the number of additional upstream hubs that lie in the path between the hub and the root hub. Hubs that connect directly to the root hub have a hub depth of zero. Any hubs that con- nect to downstream ports on those hubs have a hub depth of one. Any hubs that connect to those hubs have a hub depth of two, and so on up to a maxi- mum hub depth of four. The USB 2.0 specification defines the root hub as tier 1 in the bus topology, so hub depth equals the hub’s tier - 2. Unlike USB 2.0 hubs, USB 3.0 hubs don’t broadcast downstream traffic but instead direct traffic only toward the target device. Using routing instead of broadcasting enables ports to enter a low-power state when not communicating with the host even if the bus is carrying traffic to other device. In the upstream direction, hubs route all traffic to the host as with USB 2.0. On receiving a packet from the host, a hub uses its hub-depth value and a Route String in the packet header to determine whether the hub should process the packet or route the packet to a downstream port. The Route String has five 4-bit fields. Each field contains information that applies to one of up to five external hubs in the 387Chapter 15 path that the packet travels. The hub-depth value identifies which 4-bit field in a received Route String applies to the hub. The field contains either a port number to route the packet to or zero if the packet’s destination is the hub itself. Because the Route String’s fields are four bits, a USB 3.0 hub can have at most 15 downstream ports. A hub that isn’t configured assumes all packets are directed to itself.6JG *WD %NCUU Hubs are members of the hub class, which is the only class defined in the main USB specification.*WD &GUETKRVQTU The hub descriptor informs the host of hub-specific capabilities such as sup- ported modes for power switching and overcurrent protection. For USB 3.0 hubs, the hub descriptor has additional fields to support USB 3.0 capabilities. A host can request the descriptor with a Get Hub Descriptor control request. A USB 3.0 hub must have a device capability descriptor with a Container ID that identifies the device instance. The Container ID is the same value for the USB 2.0 and USB 3.0 hub functions in a device.*WD %NCUU 4GSWGUVU A host can use hub-class requests to obtain status information, set and clear hub and port features, and monitor and control transaction translators.2QTV +PFKECVQTU The USB 2.0 specification defines optional indicators to indicate port status to the user. The specification assigns standard meanings to the colors and blinking properties of status LEDs or similar indicators. Each downstream port on a hub can have an indicator, which can be a single bi-color green/amber LED or a sep- arate LED for each color: ...