Apr 092009

Some applications for mobile phone need to be seen. One example that immediately comes to mind is Google Maps. What is use of maps if your mobile fades away after less than a minute of inactivity. It may prove to be rather distracting to press a button or two every ten seconds while driving.

Fortunately there is a way of changing this behavior. But take great care. Backlight is turned off for a reason. Having it on all the time isn’t kind to battery.

Whole secret lies inside of SetPowerRequirement function. This function, when called, switches selected device to one of D states with D0 being full power and D4 being full sleep. Since this change is made on system level, be sure to call ReleasePowerRequirement as soon as you are done. If you fail to call that function (e.g. your program crashes), there is no way to return it to lower power state other than rebooting your machine. Notice also that if your program specifies that D3 is needed and other program specifies D0, device will go in D0 mode – when conflicting modes are selected, higher power always wins.

In example we will just set backlight, but this function can be used for other devices too (first check their name in registry at HKLM\Drivers\Active with MobiReg).

Here is code sample in C#.

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