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again. If the controller receives a data transfer request for a card that has been powered down due to a normal power down event it automatically applies power and initializes the stack rst.
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Universal Serial Bus (USB) is a bus standard for connecting peripheral and memory devices to a host computer. It supports hot insertion and removal of devices from the bus devices may be attached or removed at any time. The bus consists of four signals: two carrying differential data and two carrying power to the USB device. The USB speci cation revision 2.0 de nes three data rates: Data rate USB High Speed USB Full Speed Limited capability low speed Data transfer rate Up to 480 Mbits/sec 12 Mbits/sec 1.5 Mbits/sec
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13.6.1 USB overview
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The USB system consists of a single host controller connected to a number of USB devices. The host includes an embedded root hub that provides one or more attachment points. The host is the bus master and initiates all data transfers over the bus. Each USB device passively responds to requests addressed to it by the host. The host is often a desktop computer, but a supplement to the USB speci cation introduces a dual-role USB device. As well as being a normal USB device, this kind of device is also able to take on the role of a limited USB host, without the burden of having to support full USB host functionality. This is the On-The-Go (OTG) supplement aimed at portable devices. Many USB devices implement just a single function USB keyboards and data storage devices are examples of these but multi-function devices are also possible. These are called composite devices, an example being a USB headset that combines a USB headphone and microphone. Likewise, although the functionality of most devices remains static, some devices can alter the USB function or functions they implement. A mobile phone is an example of this it may use various different USB functions to exchange data with a host computer. Related USB devices that provide similar functionality are grouped into USB device classes, and standard protocols are de ned to communicate with them. This means
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that a generic device class driver on the host machine can control any compliant device. Many classes are further subdivided into subclasses. The USB Implementers Forum assigns unique codes to each class and subclass, and USB devices report these codes for each function they support. Examples of USB classes include the USB Mass Storage class for devices such as MultiMediaCard readers, and the Communications Device class for modem devices. A USB device is made up of a collection of independent endpoints. An endpoint is the terminus of a communication ow between host and device that supports data ow in one direction. Each endpoint has its own particular transfer characteristics that dictate how it can be accessed. Four transfer types are de ned: Transfer type Bulk Description Used for transferring large volumes of data that has no periodic or transfer rate requirements (for example, a printer device). Used to transfer speci c requests to a USB device to con gure it or to control aspects of its operation. Used where a constant delivery rate is required (for example, an audio device). Given guaranteed access to USB bandwidth. Used to poll devices that send or receive data infrequently, to determine if they are ready for the next data transfer.
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Every USB device contains at least one input and one output control endpoint both with endpoint number zero (ep0). The host uses this pair to initialize and control the device. Full speed devices can have a maximum of 15 input and 15 output endpoints, in addition to ep0. Each USB function on a device has an associated set of endpoints, and this set is known as an interface. Before a device can be used, it must rst be con gured. This is the responsibility of the host, and is normally done when the device is rst connected. In a process known as bus enumeration, the host requests information on the capabilities and requirements of the device. The data returned speci es the power requirements of the device. It also describes each interface, in terms of its class type, the endpoints it contains and the characteristics of each endpoint. This is the device con guration. Certain devices offer alternative con gurations. This information is contained in a set of device descriptors once more, these are not to be confused
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