System Power States
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4/8/2010
A system power state explicitly specifies the maximum device power state for all devices in the system. OEMs define these system power states, which are referred to by OEM-defined names such as On, SystemIdle, OnBattery, InCradle, OutOfCradle, and so on. There is no limit on how many system power states OEMs can define, and the names of the power states are not necessarily ordered in a linear manner.
The system power state names must be defined as registry keys in the system configuration. OEMs are also responsible for creating explicit mappings between the predefined device power states and the system power states. Define these mappings in the registry, as well.
The following table describes some examples of Power Manager system power states.
| Power state | Description |
|---|---|
BacklightOff |
State after a short period of user inactivity. The backlight is off. Other things could be turned off, as well. |
On |
Normal operating state when a user is interacting with the device. |
Resuming |
Initial state after the device wakes up from the Suspended state. This state has a fifteen-second timeout before it transitions to another state. The system must transition to another state, or it reverts to the Suspended state. There is no way to reset the timer. |
ScreenOff |
Backward-compatible state for applications that must turn off the screen through the display driver ExtEscape function. A typical case is Windows Media Player, which can turn off the screen through a user-defined key press. |
Suspended |
Sleep state. No threads are running. The CPU is idle. The system can wake up only by means of a hardware wake-source interrupt. |
Unattended |
State in which a program runs without the user's knowledge or the need for interaction with the user, such as the Always-Up-to-Date synchronization program. It has a reference count (UREF). Upon exiting such an application, the device goes directly to the Suspended state without waiting for the system idle timer. |
UserIdle |
State after a longer period of user inactivity. The screen is either in low-power mode or off (depending on the OEM configuration). Other things could be turned off, as well. |
The UserIdle state is intended for when the user is using the device, but not actively interacting with it; for example, when the user is looking at the display.
The SystemIdle state is intended for when system processes are active, but the user is not directly using the system and might consider the device to be idle; for example, during a file transfer.
The sample Power Manager implementation makes various decisions about user and system activity based on the UserActivity and SystemActivity timers. The time-outs for transitioning between these system power states is different when the system is on AC power from when the system is on battery power.
The sample run-time images that are provided with Windows Embedded CE are all AC-powered. OEMs can choose to implement a separate set of power states for use when the system is on battery power, in a cradle, and so on. Implement these customizations by copying the sample Power Manager platform device driver (PDD) to the platform directory, and then modifying the PDD appropriately.
See Also
Concepts
Power States
Mapping of System Power States to Device Power States
System Power State Transitions
Example System Power States