System and Device Power States (Compact 7)
3/12/2014
Power Manager maps system power states that you define for your platform to Windows Embedded Compact device power states. System power states define the overall power usage modes of your platform. These states have names such as On, Suspend, and UserIdle, and each state represents a specific power consumption profile for the devices on your platform. You can define any number of system power states for your platform, and you can specify how your platform transitions between these states. The transitions between these states can occur in any way that makes the most sense for your platform; these transitions can occur as a result of an event such as docking or undocking from a cradle or moving from AC power to battery power. You can use Power Manager to manage device power states separately from system power states so that devices that are capable of intelligently managing their own power are able to do so. For more information about mapping device power states to system power states on your platform, see Implement System Power States.
Power Manager works with devices that support one or more device power states. There are a limited number of device power states, and each device must inform Power Manager of its power consumption characteristics. Device power states generally trade performance for reduced power consumption. Power Manager does not impose absolute restrictions on device power consumption, responsiveness, or capabilities in any particular power state. Also, Power Manager generally does not request that a device go into a power state that it does not support.
Some applications may require that a device be maintained at a certain device power level. For example, a streaming audio application might require that a network card and audio codec stay powered at a high level while music is playing. A streaming video application might need access to network and audio hardware, and it might want to keep the display backlight powered on and prevent the display from going into a lower-power screen-saver mode. Applications can request that Power Manager set minimum device power-state requirements by calling the SetPowerRequirement and ReleasePowerRequirement functions. For more about how applications use these functions, see Power Management Application Interface.
Power Manager can coexist with device drivers and applications that are not developed to work with Power Manager. Within the power management framework, you define system power states that establish maximum device power states. Within these limits, device drivers can call DevicePowerNotify to regulate their own power levels, and applications can call SetPowerRequirement to verify that the devices they need are running at an acceptable device power level to meet the performance criteria of the application.