Power Management under Windows CE

Embedded devices are usually powered by batteries and the power have to be correctly managed to improve the device autonomy and by the way the user experience. Windows Embedded CE is provided with a Power Manager that will interact with the different drivers of the system to reduce power consumption when required.

System Power States

The OS have 4 different power states that will be the four states of the internal power manager state machine. The state transition will be linked to system conditions : user activities, system activities, application requests and timers.

The four states are :

• On : in this state the device is full active and user can use all the different peripherals, and by consequence the current consumption is maximum

• User Idle : this state is reach when user is not using the device after a time out, the power consumption can be reduced by, for example, decreasing the display backlight intensity and disabling peripherals.

• System Idle : after application inactivity, the power manager switch to this state.

• Suspend : in this state the device is consuming the less power as possible, but the device cannot be used, the processor clock is decreased (or turned in a suspend state if supported), the peripherals are usually turned off and the SDRAM is still refreshed.

The transition from On to User Idle and User Idle to Suspend is done after timers expirations, those timers value can be setup through the registry to customize your device. You can managed timers when device is powered by a battery and/or by AC, to optimize your device current consumption depending on the power source.

[HKLM\SYSTEM\CurrentControlSet\Control\Power\Timeouts]
"ACUserIdle"=dword:5 ; in seconds

This will set a timeout of 5 seconds to enter the User Idle state when the device is powered by AC. The status of the power source is not identified by the Power Manager, itself, it will require the development of a battery driver. The battery driver will provide power source transition notifications to the power manager.

Drivers Power States

On the system drivers can support 5 different levels of power according to the peripheral capabilities:

• D0 : Full on, the peripheral is fully functional

• D1 : Low On, fully functional the device consumption is reduce comparing to D0 state

• D2 : Standby, device is partially powered, and able to wake up on request

• D3 : Sleep, the device is consuming as less power as possible and can be used to wake up the system

• D4 : Off, the device is off an do no consume any power

Each state is optional and a driver can support only few of them. The power manager will request to the driver the different supported power state at initialization.

System Power States vs Drivers Power States

The Power Manager is fully configurable through the registry, so default configuration is provided to at least manage power by its own without any pre-requisite. Each power state of the drivers will be associated to a system power state as follow :

• On -> D0

• User Idle -> D1

• System Idle -> D2

• Suspend -> D3

The Power Manager will request device driver to change their power state at system power state transitions, according to this list. As explained before, those values can be overridden using the registry by creating new entries like this :

[ HKLM \SYSTEM\CurrentControlSet\Control\Power\State\UserIdle]
"bkl1:"=dword:4 ; backlight off

[ HKLM \SYSTEM\CurrentControlSet\Control\Power\State\SystemIdle]
"bkl1:"=dword:4 ; backlight off

[ HKLM \SYSTEM\CurrentControlSet\Control\Power\State\Suspend]
"bkl1:"=dword:4 ; backlight off

In this example, the driver named bkl1: will be turned off when the system power state will switch to User Idle, System Idle and Suspend states. You can either define power state for drivers individually in the registry or group them into IClass definition. When defining a driver in the registry you can specify which class of driver this driver is associated to.

[ HKLM \SYSTEM\CurrentControlSet\Control\Power\State\SystemIdle\{ A32942B7-920C-486b-B0E6-92A702A99B35 }]
"Default"=dword:4 ; D4

This configuration will turn off all the device from the Iclass ( A32942B7-920C-486b-B0E6-92A702A99B35). This IClass value is the GUID used by the power manageable devices.

That’s all for today, but two additional articles are following in the next few days. I will talk about the device driver interface requirements, and application interface used to communicate with the Power Manager and receive notification from him.

[Updated] : Access to part 2/3

- NicolasPosted byNicolas BESSON [MVP]

In the last article we saw that the Power Manager is in charge of the management of the power on a Windows CE device, and interact with the system to change the system power consumption according to the user and system activity. Each driver can support 5 different levels of power states that can be setup from the registry.

Device Drivers Interface

The Power Manager is using IoControls to communicate with the drivers, to set, get the current power state of the driver. So drivers have to implement the support of those IoControl codes if they can manage the power of the managed device. The command codes are the following:

• IOCTL_POWER_CAPABILITIES: used by the Power Manager to identify the power capabilities of the device driver. This command is called once when Power Manager enumerates the device on the system.
  
• IOCTL_POWER_GET: used to get the current power state of the device.
  
• IOCTL_POWER_SET: used to request a power transition.
  
• IOCTL_POWER_QUERY: used to validate that the driver will support a power transition from its current state to the specified state.
  

Power Manager interface for device drivers

The device drivers cannot change their power state by themselves, without notifying the power manager; otherwise the power manager is not aware of state modification and cannot optimize the power on the device. When drivers require power level modification, they should request the power manager for that by callingDevicePowerNotify from the driver. Then the power manager will use the IoControls to request user to change is power state.

Next time we will identify how the applications can request system power transition.

[Updated] : Access to part 3/3

- Nicolas

Posted byNicolas BESSON [MVP]

Power Management under Windows CE [part 3/3]

Applications can also interact with the power manager to change the current power state of the system or to be notified on power state transitions.

Application System Interaction

Using the Power Manager, applications can request system power state using theSetSystemPowerState. This is true for the system but applications can also interact with drivers. Imagine that a device driver is powered off after being used, this for power consumption reason, when applications requires an access to this driver, the driver must be turned on again. Using SetPowerRequirement, applications can ask drivers through the Power Manager, to switch to a specific power state. Then usingReleasePowerRequirement, the power requirement will be release and the driver will switch back to his original power state.

Request power notification

Using message queue, applications can be notified of power transition occurring on the system. When system is entering suspend, this is transparent for applications, and the only way for applications to be notified of resuming from suspend is to register to this message queue using the RequestPowerNotification function. The messages read from the message queue provide information on the status of the system.

Modify the Power Manager

The source code of the power manager is provided with Windows Embedded CE and can be located in%_WINCEROOT% \PUBLIC\COMMON\OAK\DRIVERS\PM . This gives you a chance to adapt the behavior of the Power Manager to your requirements. Most of the time, you do not have to make any modification in the code; instead use the registry to change the default behavior.

In conclusion the power management is straightforward for Windows Embedded CE when the power requirement of your device is simple, but can start to be tricky when more complicated. In any case, the system can be fully customized through the registry or for advanced usage by modifying the behavior of the Power Manager by directly modifying the source code. Applications can be notified of power transition and act if required, but it can also interact with the system to change the power state of drivers or the system. On their side device drivers can implement power management to support 5 different levels, and optimize the power consumption of your device. The Power Manager is the only one on the system to manage the power, applications and drivers always have to send request to the Power Manager that will relay the requests to the correct system component according to his internal state machine.

-Nicolas

Posted byNicolas BESSON [MVP]