USB Complete fourth- P51

USB Complete fourth- P51: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- P51Chapter 20 Returning to Operation as a Peripheral When finished communicating, the B-device returns to its role as a peripheral using the following protocol: 1. The B-device stops all bus activity and may switch in its pull-up resistor. 2. The A-device detects a lack of activity for at least 3 ms and switches out its pull-up resistor or removes VBUS to end the session. 3. If VBUS is present and the B-device didn’t switch in its pull-up in Step 1, the B-device switches in its pull-up to connect as a peripheral. The bus is in the J state. 4. If VBUS is present, the A-device resets the bus. The A-device can then enu- merate and communicate with the B-device, suspend the bus, or end the session by removing VBUS. 6JG 5GUUKQP 4GSWGUV 2TQVQEQN If the A-device has turned off the VBUS voltage, a B-device can use the Session Request Protocol (SRP) to request the host to restore VBUS and begin a new session. The two SRP methods are data-line pulsing and VBUS pulsing. The B-device must try data-line pulsing first, followed by V BUS pulsing. An A-device that supports SRP must respond to one of the methods. An A-device must respond to SRP if the device ever turns off VBUS while a micro-A plug is inserted. A B-device must support initiating SRP if the device wants to request communications with an OTG device when VBUS is off. A B-device whose targeted peripheral list has no devices that support SRP will have no need to initiate SRP. In data-line pulsing, the device switches in its pull-up (on D+ or D-, depending on device speed) for 5–10 ms. In VBUS pulsing, the device must drive the VBUS line long enough for the host to detect the VBUS voltage but not long enough to damage a non-OTG host that isn’t designed to withstand a voltage applied to VBUS. Because VBUS capacitance is much higher on a non-OTG host, the voltage rises more slowly. Within 5 seconds of detecting data-line pulsing or VBUS pulsing, the A-device must turn on VBUS and reset the bus. Standard hubs don’t recognize SRP signaling, so if there is a hub between the B-device and the A-device, the B-device can’t use SRP. Non-OTG USB periph- erals also have the option to support SRP.476 Hosts for Embedded Systems5WRRQTV HQT 4GOQVG 9CMGWRWhen V BUS is present and the bus is suspended, an OTG device can useremote wakeup to request communications from an OTG device or other USBhost.%CDNGU CPF %QPPGEVQTUA device with a Micro-AB receptacle is an OTG device. Every OTG devicemust have one and only one Micro-AB receptacle, and any device with aMicro-AB connector must function as a OTG device. The Micro-AB receptaclecan accept either a Micro-A plug or a Micro-B plug.Figure 20-1 shows the cabling options. Two OTG devices connect to each othervia a cable with a Micro-A plug on one end and a Micro-B plug on the otherend. It doesn’t matter which device has which plug.A host or upstream hub connects to an OTG device via a Standard-A toMicro-B cable. A peripheral with a Micro-B receptacle connects to an OTGdevice with a Micro-A-to-Micro-B cable. A peripheral with a permanentlyattached cable with a Micro-A plug attaches directly to the OTG device.A peripheral with a Standard-B or Mini-B plug or a captive cable with a Stan-dard-A plug must use an adapter to connect to an OTG device. The adapter hasa Micro-A plug and a Standard-A receptacle. The Micro-A plug attaches to theOTG device. The Standard-A receptacle accepts a Standard-A plug from a cablethat attaches to the peripheral with a Standard-B or Mini-B plug or a captivecable. This adapter is the only approved adapter for standard USB cables.Micro-A, Micro-B, and Micro-AB connectors have an ID pin that enables anOTG device to determine whether a Micro-A or Micro-B plug is attached. In aMicro-A plug, the ID pin is grounded. In a Micro-B plug, the ID pin is open orconnected to ground via a resistance greater than 1M Ω. ( The MicroUSBMicro-B ID Pin Resistance ECN raised this value from its original 100kΩ.) AnOTG device typically has a pull-up resistor on the ID pin. If the pin is a logiclow, the attached plug is a Micro-A. If the pin is a logic high, the attached plugis a Micro-B.The USB 3.0 specification defines a USB 3.0 Micro-AB receptacle and USB3.0 Micro-A plug that include contacts for SuperSpeed traffic. 477Chapter 20Figure 20-1. An OTG device can communicate with a USB host or a device onthe OTG device’s target peripheral list. $WU %WTTGPV The ability to draw up to 500 mA per port from the bus is a convenience for users and a cost saver for device manufacturers. But providing this much cur- rent, or even the 100 mA that USB 2.0 battery-powered hosts must provide, can be a burden for some hosts. Some peripherals, including battery-powered ones, may not need bus power at all.478 Hosts for Embedded Systems For these reasons, OTG devices have more flexible requirements for providing bus current. A USB 2.0 OTG device must provide the greater of 8 mA of bus current or the maximum amount the devices on the targeted peripheral list require, up to 500 mA. To conserve power, an A-device can leave VBUS unpowered until the device detects SRP signaling or launches an application that uses USB. For faster response when a device is attached, an A-device can have an option to power the bus on detecting device attachment. 7UGT /GUUCIGU Every OTG device must have a display or another means to display error mes- sages to users. To pass compliance tests, an OTG dev ...