power management. The kernel calls the OEMIdle function, part of the OAL, when no threads are ready to run. This action switches the processor into idle state, which includes saving the current context, placing the memory into a refresh state, and stopping the clock. The processor idle state reduces power consumption to the lowest possible level while retaining the ability to return from the idle state quickly.
It is important to keep in mind that the OEMIdle function does not involve Power Manager. The kernel calls the OEMIdle function directly and it is up to the OAL to switch the hardware into an appropriate idle or sleep state. The kernel passes a DWORD value (dwReschedTime) to OEMIdle to indicate the maximum period of idle time. When this time passes or the maximum delay supported by the hardware timer is reached, the processor switches back to non-idle mode, the previous state is restored, and the scheduler is invoked. If there still is no thread ready to run, the kernel immediately calls OEMIdle again. Driver events, as in the response to user input via keyboard or stylus, may occur at any time and cause the system to stop idling before the system timer starts.
The scheduler is, by default, based on a static timer and system ticks at a one-millisecond frequency. However, the system can optimize power consumption by using dynamic timers and by setting the system timer to the next timeout identified by using the scheduler table content. The processor will then not switch back out of idle mode with every tick. Instead, the processor switches only to non-idle mode after the timeout defined by dwReschedTime expires or an interrupt occurres.
Lesson Summary
Windows Embedded CE 6.0 R2 provides a default Power Manager implementation with a set of power- management APIs that you can use to manage the power state of the system and its devices. It also provides the OEMIdle function, which it executes when the system does not have any threads scheduled in order to provide original equipment manufacturers (OEMs) a chance to put the system into a low power idle state for a specified period of time.
Power Manager is a kernel component that exposes a notification interface, an application interface, and a device interface. It acts as a mediator between kernel and OAL on one side and device drivers and applications on the other side. Applications and device drivers can use the DevicePowerNotify function to control the power state of peripheral devices at five different power levels. Device power states may also associate with default and custom system power states to keep system and devices synchronized. Based on activity times and corresponding events, Power Manger can automatically perform system state transitions. The four default system power states are On, UserIdle, SystemIdle, and Suspend. Customizations for system-state-to- device-state mapping take place in the registry settings of individual devices and device classes.
In addition to Power Manager, the kernel supports power management by means of the OEMIdle function. Switching the processor into idle state reduces power consumption to the lowest possible level while retaining the ability to return from the idle state quickly. The processor will return to non-idle state periodically or when interrupts occur, as when it responds to user input or when a device requests access to memory for data transfer.
You can significantly reduce the power consumption of a device if you implement power management properly using Power Manager and OEMIdle, thereby increasing battery life, decreasing operating costs, and extending device lifetime.
Lab 3: Kiosk Mode, Threads, and Power Management
In this lab, you develop a kiosk application and configure a target device to run this application instead of the standard shell. You then extend this application to run multiple threads in parallel in the application process and analyze the thread execution by using the Remote Kernel Tracker tool. Subsequently, you enable this application for power management.
To help you successfully master the procedures presented in this lab, see the document 'Detailed Step- by-Step Instructions for Lab 3' in the companion material for this book.
> Create a Thread
1. Using the New Project Wizard, create a new WCE Console Application named HelloWorld. Use the Typical Hello_World Application option.
2. Before the _tmain function, implement a thread function named ThreadProc:
DWORD WINAPI ThreadProc( LPVOID lpParameter) {
RETAILMSG(1,(TEXT('Thread started')));
// Suspend Thread execution for 3 seconds
Sleep(3000);
RETAILMSG(1,(TEXT('Thread Ended')));
// Return code of the thread 0,
// usually used to indicate no errors.
return 0;
}
3. By using the CreateThread function, start a thread:
HANDLE hThread = CreateThread( NULL, 0, ThreadProc, NULL, 0, NULL);
4. Check the returned value of CreateThread to verify that the thread was created successfully.
5. Wait for the thread to reach the end of the thread function and exit:
WaitForSingleObject(hThread, INFINITE);
6. Build the run-time image and download it to the target device.
7. Launch Remote Kernel Tracker and analyze how threads are managed on the system.
8. Start the HelloWorld application and follow the thread execution in the Remote Kernel Tracker window, as illustrated in Figure 3–11.
Figure 3-11 Tracking thread execution in Remote Kernel Tracker tool
> Enable Power Management Notification Messages
1. Continue to use the HelloWorld application in Visual Studio.
2. Generate power-management notifications in more frequent intervals by going into the subproject registry settings and setting the registry entry for the UserIdle timeout in AC power mode (ACUserIdle) to five seconds:
