TAILIEUCHUNG - USB Complete fourth- P43

USB Complete fourth- P43:This book focuses on Windows programming for PCs, but other computers and operating systems also have USB support, including Linux and Apple Computer’s Macintosh. Some real-time kernels also support USB. | Chapter 16 A device attached to a USB charger can determine the charger type after pulling up D full speed or D- low speed and detecting the voltage on the line not pulled up Device Speed Action Detected Voltage Charger Type Full Pull D high D- is low Host or hub D- is high Dedicated Low Pull D- high D is low Host or hub D is high Dedicated To ensure valid voltages when connecting a low-speed device must draw less than 100mA when it pulls D- high. The specification provides timing requirements and other details for implementing charger detection. Charging Dead Batteries A dead-battery provision allows devices with dead or very weak batteries to draw up to 100 mA from a host or hub until the batteries are charged to a weak battery threshold. A device whose battery has charged to the weak battery threshold is capable of powering up successfully and connecting to the bus by pulling D or D- high. The device defines its weak-battery-threshold voltage. The provision also allows the bus to power a device that normally operates on battery power but has no batteries installed. Hub Power Power use on hubs has special considerations. A hub must control power to its downstream devices and must monitor power consumption and take action when devices use too much current and present a safety hazard. Power Sources The root hub gets its power from the host. Other hubs are either self-powered or bus-powered. If the host uses AC power from a wall socket or another external source a USB root hub must be capable of supplying 500 mA to each port on the hub. If the host is battery-powered the hub may supply either 500 or 100 mA to each port. A hub that supplies 500 mA per port is a high-power hub and a hub that supplies 100 mA per port is a low-power hub. 396 Managing Power All of a bus-powered hub s downstream devices must be low power. A USB hub can draw no more than 500 mA and the hub itself will use some current leaving less than 500 mA for all attached devices combined. Thus

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