USB Complete fourth- P44

USB Complete fourth- P44: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- P44Chapter 16 To resume communications with a suspended function, a host issues a Set Port Feature(FUNCTION_SUSPEND) request for normal operation. Note that exiting function suspend uses Set Port Feature rather than Clear Port Feature. If the device’s link isn’t in U0, the downstream-facing hub port that is the device’s link partner uses low frequency periodic signaling to initiate the transition to U0. The hub then resumes communicating with the function. A function with remote wakeup enabled can request to wake by sending a DEV_NOTIFICATION Transaction Packet with a Function Wake notifica- tion. If the device’s link isn’t in U0, before sending the notification, the device uses low frequency periodic signaling to transition the link to U0. The signaling propagates upstream from the device until reaching a hub that isn’t in U3 and then propagates back downstream to the device requesting the wakeup. If the host places a device in the Suspend state when one or more functions are suspended, the functions remain suspended when the device wakes. The host or device must then initiate exiting function suspend for the individual func- tion(s). Both composite and non-composite devices can use function suspend. Informing the Host of Delays Hubs help manage bus traffic by informing the host of delays due to a device’s being in a low-power state. On receiving a header packet addressed to a port in a low-power state, the hub sends a deferred header packet to the host, which halts communication attempts with the device. When the target port has transi- tioned to U0, the hub sends the header packet to the device with the Deferred bit set in the Link Control Word. To inform the host that the device is ready to communicate, the device sends an ERDY Transaction Packet. .CVGPE[ 6QNGTCPEG /GUUCIGU USB 3.0 hosts can save additional power by obtaining information about the maximum delay each device can tolerate between sending an ERDY Transac- tion Packet and receiving a response from the host. The host can use more aggressive power management with devices that can handle long delays. The p r o t o c o l s f o r o b t a i n i n g t h i s i n f o r m a t i o n i n c l u d e t h e S e t Fe a - ture(LTM_ENABLE) and Set SEL requests and DEV_NOTIFICATION Transaction Packets with Latency Tolerance Message Device Notifications. The SuperSpeed USB device capability descriptor indicates whether a device sup- ports Latency Tolerance Message notifications.406 Managing Power 7UKPI 2+0) If a host initiates an isochronous transaction with a device in a low-power state, the device might be unable to transition to U0 in time to send or receive data in the scheduled service interval. To prevent this problem, the host uses PING and PING_RESPONSE Transaction Packets. Before beginning the isochronous transfer, the host sends a PING Transaction Packet, which causes all links between the device and host to transition to U0. The device returns a PING_RESPONSE Transaction Packet when the device is ready to transfer data. The host must send the PING far enough in advance of a scheduled trans- fer to enable the transfer to take place on time. This use of PING is unrelated to the high-speed PING protocol described in Chapter 2.2QYGT /CPCIGOGPV WPFGT 9KPFQYU Recent PCs manage power according to the Advanced Configuration and Power Interface Specification (ACPI). A system that implements ACPI power manage- ment enables the operating system to conserve power by shutting down compo- nents, including suspending the USB bus, when the computer is idle. PCs support these low-power, or sleeping, states: In the S1 state, the display is off and drives are powered down. USB buses are suspended, but VBUS remains powered. In the S3 state, the PCI bus’s main power supply is off and memory isn’t accessed, but system memory continues to be refreshed. USB buses are sus- pended. In older systems, USB’s VBUS is not powered in the S3 state. In newer systems, VBUS is powered by the PCI bus’s auxiliary supply (Vaux). In the S4 state, the system context is saved to disk and the system, including the USB bus, is powered off. You can view and change a system’s power-management options in Control Panel > Power Options. Under Windows Vista, you can specify when the sys- tem enters S3, called sleep (Figure 16-4). The Advanced Settings tab includes options to enable or disable selective suspend for USB devices under USB set- tings and to select hibernation (S4) under Battery > Critical battery action. Under Windows XP, the Power Schemes tab specifies when the system goes into standby and hibernation. Standby is either S1 or S3. On a system that has no USB devices that can wake the system, standby is S3. On a Windows XP sys- tem that has a USB keyboard, mouse, or another USB device that can wake the 407Chapter 16Figure 16-4. Windows Vista enables users to specify power-saving options thatdetermine when USB devices enter the Suspend state. system, the standby state is S1 due to problems in using S3 with some BIOSes and device hardware. The problems include loss of VBUS in the S3 state, false device removal and arrival notifications on resuming, resetting of devices during suspend and resume, and failure to resume fully. For devices that have problems resuming from S3, a possible fix is to force the host controller to reset on resuming by adding a ...