This document discusses power management techniques for mobile phones. It explains that power management aims to provide optimal performance while minimizing power consumption through automated processes without user intervention. It describes various techniques like dynamic power management (DPM) and dynamic voltage and frequency scaling (DVFS) that can dynamically adjust the system power modes and operating frequencies/voltages based on activity levels. The role of hardware support and applications in enabling these techniques is also covered. Specific examples of how power management can be applied during media playback are provided.

1. Power Management in Mobile Phones Need and Techniques

2. Agenda What is Power Management? Why Power Management? Power Management Techniques HW support .. Role of Applications .. Power Management and Media use cases

3. What is power Management? Technique for effective use of energy Desired performance at minimal power consumption Automated without user intervention A little help from Applications is always welcome!!

4. Why Power Management? Applications and usage have become sophisticated Requiring More complicated chips Freq of operation directly proportional to power consumed Main chip is not the only consumer Bigger color touch screen consumes more power More RAM requires more power Each new peripheral like WLAN and BT requires power But resource & energy constrained! Breakthrough in battery technology is not at same pace of the Most phones will have about 1000 mAh battery Let us see how long this battery last? Only 80% of the battery is usable At 500mA it will last less then two hours Some figures 0.6mW/MHz (0.13µm, 1.2V) including cache controllers for ARM11 This means processor only consumes .5mA/MHz or ~300mA at full 600MHz

5. Why Power Management?.. All capabilities are not always used Most applications do not require 100% MIPS Most HW accelerators or component are not used often Most peripherals not used often There is an Idle time between active use How to handle such cases Its like complicated switch board with multiple switches Some On/Off some variable dimmer Effective management of this required

6. Power Management Technique Simple energy management directives: Power off component when not in use (USB) Slow down whenever possible (processor) Efficient schemes: Dynamic power management (DPM) system switches dynamically between high- and low-consumption system power modes based on activity Dynamic voltage and frequency scaling (DVFS) DVFS is a technique through which the idle time of the system is minimized. It uses the optimal operating frequency and voltage to allow a task to be performed in the required amount of time. This reduces the active power consumption (power consumed while executing a task) of the device while still meeting task requirements.

7. DPM

8. Power Management Technique… Decision: DVFS or DPM? DPM - High latency for Power Off to Power On Voltage stabilization time Clock stabilization time Dependencies chain make it even more DVFS also require clock stabilization There are other techniques like cutting clock - low latency Not that useful as in 45 nm considerable Power is lost even when not switching – leakage current ARM in sleep will also consume power But until we have sufficient sleep time we cannot switch off

9. HW Support .. Support Multiple Voltage Levels (Voltage domain) Power = V*I , lower voltage means lower power Chip is divided in multiple Voltage domains Voltage level controlled by SW “DVFS” Support Multiple Power Domains Having switches for controlling power to sections of chip But the number and dependencies is unmanageable HW helps here by managing dependencies Support for Clock Gating (Clock domain) Switching consumes power Some power can be saved by cutting clock This is quickest thing to do Here also number of clocks and there dependencies are complex HW helps here in managing dependencies

10. Role of Applications.. Allow processor to go to sleep For longer sleep duration DPM kicks in Avoid periodic polling This impacts the sleep duration and type Unnecessary wakeups increases power consumptions Instead waiting for events/interrupts If required and possible try using deferrable timers Some OS provides it e.g. Symbian Optimize computation to reduce MIPS For higher MIPS, HW chooses high freq/voltage Utilize HWA as much as possible Use OEM supplied libraries and routines for complex algorithms E.g. Video decode, these are profiled and Tuned for optimal power consumption

11. Power Management and Media UC Media Play (MP3) Scenario Classification: Final frame rendering at fixed rate Much slower then time it may take to decode a frame Recommendation Fetch and Decode frames in chunk While decoded frames are rendered only audio peripheral, system DMA , memory etc. will be powered and not processor Impact High power consuming resource remain idle for a large chunk of time With single frame decode total idle for processor is X but it wakes-up frequently inhabiting deeper sleeps With multiple frame total idle time still remains X but system wakes-up less frequently So while Media is playing processor does go to sleep App should still refrain from periodic polling

12. Thanks

13. Power Management Value chain(Backup)