Energy in sensor nets

Where does the power go Components: –Battery -> DC-DC converter –Sensors->ADC->MCU+Memory  Radio

Micro-controller unit MCU Intel strong arm – 400mW Atmel AVR – 16.5 mW Of course, strong-arm can accomplish more processing in a shorter amount of time Intel strong arm – 50mW in idle and 0.16mW in sleep Battery 3000mAh –.16mW=>781 days –16.5mW=>7.5 days –400mW=>7.5 hours

MCU continue Active –All clocks running to all subsystems Idle –Halt CPU, preserve context, able to respond to interrupts. –When an interrupt occurs, processor returns to active Sleep –Turn off power o most circuits. –Able to monitor wake-up event Advanced configuration and power management interface (ACPI) allows the OS to interface with the power saving modes –ACPI MCU has 5 states of various power, SystemStateS0 – fully working, to SystemStateS4 –ACPI devices have similar 4 states

Sleep state transition Going to sleep and waking up is not free – it uses power. When transitioning, power is used that cannot be used for any processing etc. It is wasted (why? Clocks are not stable. Why? PLLs have not stabilized.) Define power usages in the four power levels as P_i. And  _d,k to be the time used to go from the active state to power level k, and  _u,k to go from low power state k to active. The power usage decreases linearly when going to sleep Going to low energy is deemed useful only is more energy is saved during the procedure than is expended by going in and come out of the low power state. The energy save is The threshold for going to sleep power k is state P_kTau (ms) T S S S S S41050

Active power management Variable voltage processing – dynamic voltage scaling (DVS) –The voltage and clock frequency can be decreased to save power. –We can assume that the power decreases quadratically with voltage and linearly with frequency. –Of course, decreasing clock freq. Decreases the MIPS so the decrease in clock does not change the power required for a computation. On the other hand, a lower voltage might be possible at lower clock speed, resulting in a large saving in power. Clock freq power Clock only Clock and voltage freqvoltactiveidlesleep mi croA mi croA

Active power management Sleep has the most power saving. Maybe getting there fastest is the best thing. E.g, 59MHz = 1V, 221MHz=1.75 Reduction in speed is 59/221 = 0.26 (so 1/.26 more time is needed). Reduction in power is (1/1.75)^2 = Total change in energy is 0.32/0.26 > 1 => more energy is used. It is better to use full power and go to sleep ASAP (assuming there is very little power used at sleep, which is true) On the other hand, if one is merely waiting for something to happen, then low power is useful. Also, if events occur frequently, then it is not useful to go to sleep and best to finish one task just as the next event has occurred. Running NOPs is a complete waste of energy. Clearly, the programs must be written with power in mind, with the processor in mind. A power aware OS can help

radio The radio can use a large fraction of the total power MCUsensorradiopower activeonTransmit=36.3mW activeOnTransmit=19.1mW986.0 ActiveOnTransmit=13.8mW942.6 ActiveOnTransmit=3.47mW815.5 ActiveOnTransmit=2.51mW807.5 ActiveOnTransmit=0.96mW787.5 ActiveOnTransmit=0.30mW773.9 ActiveOnTransmit=0.12mW771.1 ActiveOnRX751.6 ActiveOnIdle727.5 ActiveOnSleep416.3 ActiveOnremoved383.3 SleepOnRemoved64 ActiveremovedRemoved360

radio mcusensorradiomodulationData ratepower activeon0.7368mWOOK mWOOK mWOOK mWOOK mWASK mWASK mWASK mWASK RXAnyany22.20 idle22.06 Off9.72 IdleOnOff5.92 sleepoff 0.02 Not shown is that when the radio is turned on and off, large amount of power are required

battery Batteries are specified in terms of mAh, milliamp hours. An AA has about mAh. The battery also has a maximum current drain to meet the specified lifetime. If the current is beyond that, then the lifetime is greatly reduced in that one does not receive the 3000mAh as promised. The problem is that this current is very small, smaller than what is required to keep the system running. Relaxation effect –If the system is turned on and current brought to nearly zero, then the battery can catch-up and the full lifetime can be acheived