Книга: Microsoft Windows Embedded CE 6.0 Exam Preparation Kit

Reducing Power Consumption in Idle Mode

Reducing Power Consumption in Idle Mode

To switch the device into reduced-power mode, Windows Embedded CE relies on the OEMIdle function, which the kernel calls when the scheduler has no threads to run. The OEMIdle function is a hardware-specific routine that depends on the capabilities of the platform. For example, if the system timer uses a fixed interval, then the OEMIdle function cannot really provide the expected power saving functionality because the system wakes up every time a timer interrupt occurs. On the other hand, if the processor supports programmable interval timers, you can use the kernel's dwReschedTime variable to specify the amount of time spent in reduced-power mode.

On waking up from reduced-power mode, the system must update the kernel global variables used by the scheduler. This is particularly important for the CurMSec variable, which the system uses to keep track of the number of milliseconds since the last system boot. The wakeup source can be either the system timer or another interrupt. If the wakeup source is the system timer then the CurMSec variable is already updated before execution is passed back to the OEMIdle function. In other cases, the CurMSec does not contain an updated value. To learn more about the OEMIdle implementation details, refer to the Idle.c source code file, located in the %_WINCEROOT%PlatformCommonSrcCommonTimerIdle folder.

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