Managing Power

The primary function of power management is to increase the battery life of a Handheld PC (H/PC). This is accomplished by providing accurate estimates of remaining battery life and notifying the user when the batteries are nearing depletion. Power management for an H/PC is based on the following assumptions:

• The computer is used less than two hours per day in bursts from five minutes to one hour.
• The display is powered 100 percent of the time during use.
• The CPU runs less than 10 percent of the time during typical use.
• The computer uses both main batteries and a backup battery.
• The device has no nonvolatile writable memory.
• Maximum battery life is obtained without PC Cards or with PC Cards that require very little power from the computer batteries. PC Cards drawing appreciable power from the internal batteries significantly reduce battery life.

An H/PC manages power by allowing the operating system to automatically select one of three operating states based on user and program activities. These states are:

• Dead

  The computer uses no power. It has no batteries, or all batteries are completely dead. All contents of RAM are lost. The user purchases the device in this state.

• Suspend

  The computer uses minimal power. The CPU internal and external clocks stop and the CPU uses extremely low power. Peripherals and DRAM are usually off. The CPU might take as long as 100 milliseconds to wake up from this state.

• On

  The computer uses more power than in the other two states. Any time the display, keyboard, or touch panel are active, the device is in the on state. When in the on state, the computer can switch between two CPU modes: full speed and idle. In full-speed mode, the CPU runs at normal operating frequency. In idle mode, the CPU internal clock stops and the CPU uses little power. Peripherals and DRAM may be on or off. The CPU can enter and exit this state in approximately 10 milliseconds.