Power Management

Outline Why manage power? Power management in CPU cores Power management system wide Ways for embedded programmers to be power conscious

Why power management? Desktops: Lower power bill, lower heat, lower fan noise Laptops, mobile systems: Above + battery life!

Power management tradeoffs Usually, power consumption is proportional to performance Low powerHigh power High performanceLow performance

Main power mgmt goals Lower I, V If you ’ re not using something, turn it off Try to meet deadlines exactly instead of being too fast

Core power I Inside the core, we are basically charging and discharging capacitances C + V core - Q = charge, C = Capacitance, V core = Voltage

Core power II Let

Voltage and frequency scaling I Intel Speedstep AMD PowerNow! Scale voltage and frequency dynamically based on need

Voltage and frequency scaling II Voltage and frequency settings for Intel Pentium M processor

Why scale V & f together? Oscillators are not stable at all voltages for a given frequency Increasing f Increasing Vcc Operation OK

Processor power modes Many CPUs/systems have several modes of operation Active, Power-save, Idle, Halt Latency vs. power tradeoff Active Power-save Idle Halt Overhead Power, Performance Latency

Active mode Processor executes instructions normally Entire CPU core active

Power-save Clock circuitry may be disconnected from unused CPU subsystems CPU not executing instructions Can usually return to active mode very quickly (several times per ms)

Idle CPU still not executing instructions Context may be stored in system memory, CPU registers and cache disabled Must restore context before resuming execution

Sleep Entire system context stored to nonvolatile memory upon entering sleep Power to main memory disabled Latency very high

PIC 16F877A SLEEP mode By executing the SLEEP instruction, the PIC goes into power down mode and draws very little current (μA vs. mA) Will wake via WDT or external interrupt

Palm OS Run, doze, sleep Never really turned off Run – Executing instructions, returns to doze when finished Doze – Main clock running, LCD on, CPU not executing instructions Sleep – Main clock off, LCD off, only user generated interrupt will wake system

Windows XP Active – computer running normally Standby – Just enough power to keep contents of RAM, CPU off Hibernate – Makes image of RAM contents on hard disk Alter policies through Control Panel -> Power Options on Windows XP

Embedded power mgmt I Use low power modes whenever you can int main(void){ while(1){ … do_stuff(); delay_ms(DELAY); … } int main(void){ while(1){ … do_stuff(); power_save(); … }

Embedded power mgmt I Idle system instead of using delays Instead of polling lines, configure system to wake up from idle upon an interrupt from that line If unable to use pin as interrupt, still idle and wake periodically to check status of pin

Embedded power mgmt II Shut down peripherals when possible int main(void){ … if (userinput==FALSE) set_lcd(LCD_BACKLIGHT_OFF); //Turn backlight off if (userpresent==FALSE) set_lcd(LCD_OFF); //Turn LCD completely off … }

Embedded power mgmt III Don ’ t allow CMOS inputs to float! GOOD Very small current BAD Not so small current

Summary Buy the right core for the job Go to low power processor states whenever possible Power peripherals only when necessary

References “ Dynamic Power Management for Embedded Systems, ” IBM and Montavista, November 2002 “ Embedded Power Management, ” “ Palm OS Power Management, ” “ AMD PowerNow! Technology, ” AMD, November 2000 “ Low Power Microcontrollers, ” NEC, April 2004 “ Microarchitecture and Performance, ” Intel, e.htm “ Inside DSP on low power, ” June 2004, “ Power Management Discussion Document, ” 3G Lab, February 2001 “ Enhanced Intel SpeedStep Technology for the Intel Pentium M Processor, ” Intel, March 2004 “ Bios and Kernel Dev. Guide for AMD Athlon 64 and AMD Opteron Processors, ” AMD, April 2004