Windows Mobile Devices and Power States

Posted July 3, 2008 2:00 AM Categories: .NET | Hardware | Windows Mobile | Compact Framework

I’ve been doing some Windows Mobile development with the .NET Compact Framework recently and ran into a scenario where I needed the device to be in “full power” mode at all times with the back-light on.  The device is constantly powered, so battery life is not a concern.

The obvious choice is to go into the Brightness and Power control panels and turn off the appropriate settings, but I learned that there is a way to handle this at an application level so the behavior only occurs while the application is running.

Power State

An application can force a specific power state using the SetPowerRequirement method, and release that state using the ReleasePowerRequirement method.  Using P/Invoke, these methods look like the following:

   1: public enum CEDevicePowerState
   2: {
   3:     D0 = 0,    // Full On
   4:     D1,        // Low On
   5:     D2,        // Standby
   6:     D3,        // Sleep
   7:     D4,        // Off
   8: }
  10: [DllImport("coredll.dll", SetLastError=true)]
  11: static extern IntPtr SetPowerRequirement(string device, CEDevicePowerState ceDevicePowerState, uint deviceFlags, IntPtr systemState, ulong stateFlags);
  13: [DllImport("coredll.dll", SetLastError=true)]
  14: static extern int ReleasePowerRequirement(IntPtr handle);

SetPowerRequirement will allow you to set a specific power state on a specific device.  In my scenario, I wanted to set the back-light to full power.  The name of the back-light on most (not all) Windows Mobile devices appears to be “BKL1:”.  So, to set the back-light to full power (the D0 state), you would call the method as follows:

IntPtr handle = SetPowerRequirement("BKL1:", CEDevicePowerState.D0, 1, IntPtr.Zero, 0);
The power state will be returned to its default settings when the application exits, or you may call ReleasePowerRequirement, passing in the handle returned from the call to SetPowerRequirement to reset it yourself.
The above will leave the back-light on at all times, but it will not stop the device from going into a suspended state as configured in the control panel.  To stop this from happening, simply call the SystemIdleTimerReset method at a short, regular interval:
   1: [DllImport("coredll.dll", SetLastError=true)]
   2: static extern void SystemIdleTimerReset();
   4: // reset the system's idle time every 10 seconds so it doesn't suspend
   5: Timer timer = new Timer(IdleReset, null, 0, 10000);
   7: private static void IdleReset(object state)
   8: {
   9:     // no suspend
  10:     SystemIdleTimerReset();
  11: }

Lines 1-2 contain the P/Invoke signature.  Line 5 sets up a timer that will be called every 10 seconds (10000ms) to reset the idle timer, and lines 7-11 are the timer callback method which actually calls the SystemIdleTimerReset method.

And that’s that.  With both of these methods in place, my application remains running with the device at full power, never suspending, and with the back-light always on.

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