Power State Transitions (Windows Embedded CE 6.0)
1/6/2010
Power-on reset
A power-on reset initiates the boot loader. A power-on reset occurs when connecting the battery or AC power to the target device when the target device is turned off. During a power-on reset, the following happens:
- Power is applied to the microprocessor and storage.
- Non-persistent storage is cleared.
- If needed, the file system is initialized to its default state by copying its initial files from persistent storage.
- The target device transitions to the On power state.
Cold boot
A cold boot completely resets the device by forcibly closing all applications, but leaving the file system intact. A cold boot is typically initiated by removing all power, including any backup batteries, from the device. When power is reapplied to the device, a power-on reset happens.
Clean boot
A clean boot forces the OS to boot from a clean file system. In software, you can initiate a clean boot with the following:
Warm boot
A warm boot is a transition from the On, Idle, or Suspend power state. A warm boot closes all applications and clears RAM, but preserves the file system. A warm boot can happen due to a software interrupt or by manually resetting the target device.
On-to-Idle
When transitioning from the On power state to the Idle power state, a timer in the microprocessor stops. This transition requires approximately 10 microseconds. While in the Idle power state, the microprocessor uses a small amount of power only to wait for an interrupt to occur. **
Idle-to-On
An Idle-to-On transition occurs in conjunction with an interrupt, typically a timer interrupt. When the kernel receives an interrupt, the microprocessor is returned to full-power operation.
On-to-Suspend
An On-to-Suspend transition can occur when any of the following happen:
- The user input inactivity timer times out.
- The OFF switch on the target device is pressed.
- An application calls **the SetSystemPowerState function.
Besides the OAL, device drivers are notified of On-to-Suspend and Suspend-to-On transitions. During an On-to-Suspend transition, the device driver can respond to notifications by saving the state of the hardware in RAM, and then shutting down the hardware components in the appropriate order. Then, as the Suspend-to-On transition occurs, the device driver can re-initialize the hardware and reset any necessary power states.
Device drivers are called while the OS is not scheduling other tasks, this limits the actions that a device driver can take in these power states. During the transitions, most function calls are not available, and no other threads are running. **
Suspend-to-On
A Suspend-to-On transition can occur when any of the following happen:
- The ON switch is pressed on the target device.
- An alarm signals an event.
- A wake-up event signals, for example, carrier detection on the target device, or an OEM-specific wake-up source, such as the ring indicator on a PCMCIA modem.
Besides the OAL, device drivers are notified of On-to-Suspend and Suspend-to-On transitions. During an On-to-Suspend transition, the device driver can respond to notifications by saving the state of the hardware in RAM, and then shutting down the hardware components in the appropriate order. Then, as the Suspend-to-On transition occurs, the device driver can re-initialize the hardware and reset any necessary power states.
Device drivers are called while the OS is not scheduling other tasks, this limits the actions that a device driver can take in these power states. During the transitions, most function calls are not available and no other threads are running. **
Critical-to-Off
A Critical-to-Off transition occurs when battery voltages for the primary batteries decrease to a critically low level that prevents the target device from performing a safe shutdown.
Instead of running an On-to-Suspend transition, during which power is shut down in a timely manner, the Critical-to-Off transition bypasses the usual steps of turning off power to any peripherals or devices by immediately shutting down power to them and applying refresh voltage to the RAM. This preserves the file system and sets the microprocessor into the suspend power state.
Recovery from a critical-to-off transition occurs when adequate power is applied to the device. The process of a target device recovering from a Critical-to-Off transition is equivalent to a warm boot transition.