USB Complete fourth- P43

USB Complete fourth- P43: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- P43Chapter 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: &GXKEG #EVKQP &GVGEVGF %JCTIGT 6[RG 5RGGF 8QNVCIG 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 require- ments and other details for implementing charger detection. %JCTIKPI &GCF $CVVGTKGU 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.*WD 2QYGT 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.2QYGT 5QWTEGU 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 2.0 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 2.0 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 you shouldn’t connect two bus-powered hubs in series. The upstream hub can guarantee no more than 100 mA to each downstream port, and that amount doesn’t leave enough current to power a second hub that also has one or more downstream ports that each require 100 mA. An exception is a bus-powered compound device, which consists of a hub and one or more downstream, non-removable devices. In this case, the hub’s config- uration descriptor can report the maximum power required by the hub’s elec- tronics plus its non-removable device(s). The configuration descriptors for the non-removable device(s) report that the devices are self-powered with bMax- Power = 00h. The hub descriptor indicates whether a hub’s ports are removable. Like other high-power, bus-powered devices, a USB 2.0 bus-powered hub can draw up to 100 mA until configured and up to 500 mA after being configured. During configuration, the hub must manage the available current so its devices and the hub combined don’t exceed the allowed current. Like other self-powered devices, a self-powered USB 2.0 hub may also draw up to 100 mA from the bus so the hub interface can continue to function when the hub’s power supply is off. If the hub’s power is from an external source such as AC power from a wall socket, the hub is high power and must be capable of supplying 500 mA to each port on the hub. If the hub uses battery power, the hub may supply 100 or 500 mA to each port on the hub. USB 3.0 raises the current limits. USB 3.0 hubs can provide up to 900 mA per port if high power and 150 mA per port if low power. If the upstream port isn’t connected, the hub doesn’t provide power to the downstream ports unless the hub supports the USB battery charging specification.1XGTEWTTGPV 2TQVGEVKQP As a safety precaution, hubs must be able to detect an over-current condition, which occurs when the current used by the total of all devices attached to the hub exceeds a set value. On detecting an over-current condition, a hub’s port circuits limit the current at the port, and the hub informs the host of the prob- lem. Windows warns the user when a device exceeds the current limit of its hub port (Figure 16-3). The current that triggers the over-current actions must be less than 5A. To allow for transient currents, the over-current value should be greater than the 397Chapter 16Figure 16-3. When a device exceeds the current limit of its hub’s port, Windowswarns the user and offers assistance. total of the maximum allowed currents for the devices. In the worst case, seven high-power, bus-powered, USB 2.0 downstream devices can legally draw up to 3.5A. So a supply for a self-powered hub with up to seven downstream ports would provide much less than 5A at all times unless something goes very wrong. A hub can implement multiple over-current gangs. A device can briefly draw a larger inrush current on attachment to the bus. The over-current protection circuits typically don’t see the inrush current because a capacitor downstream from the protection provides the stored energy. If the inrush current is too large, the device will fail compliance tests.398 ...