Motorola USB08 - Manual (Page 146)

Designer Reference Manual USB08 Evaluation Board
146 Universal USB Device Driver (USBIO) MOTOROLA
Universal USB Device Driver (USBIO)
E.4.3 Power Management
Windows 98, Windows Millennium, and Windows 2000 support system
power management. That means that if the computer is idle for a given
time, some parts of the computer can go into a sleeping mode. A system
power change can be initiated by the user or by the operating system
itself, on a low battery condition for example. An USB device driver has
to support the system power management. Each device which supports
power switching has to have a device power policy owner. It is
responsible for managing the device power states in response to system
power state changes. The USBIO driver is the power policy owner of the
USB devices that it controls. In addition to the system power changes the
device power policy owner can initiate device power state changes.
Before the system goes into a sleep state the operating system asks
every driver if its device can go into the sleep state. If all active drivers
return success the system goes down. Otherwise, a message box
appears on the screen and informs the user that the system is not able
to go into the sleeping mode.
Before the system goes into a sleeping state the driver has to save all
the information that it needs to reinitialize the device (device context) if
the system is resumed. Furthermore, all pending requests have to be
completed and further requests have to be queued. In the device power
states D1 or D2 (USB Suspend) the device context stored in the USB
device will be lost. Therefore, a device sleeping state D1 or D2 is
handled transparent for the application. In the state D3 (USB Off) the
device context is lost. Because the information stored in the device is
known to the application only (e.g. the current volume level of an audio
device), the generic USBIO driver cannot restore the device context in a
general way. This has to be done by the application. Note that Windows
2000 restores the USB configuration of the device
(SET_CONFIGURATION request) after the system is resumed.
The behaviour with respect to power management can be customized by
registry parameters. For example, if a long time measurement should be
performed the computer has to be prevented from going power down.
For a description of the supported registry parameters, see
E.9 Registry
Entries .