Lab 5 – Power/Energy

Larry, Ravi

• What is Power? • What is Energy?• A Real World Example

• Why do we care?

• ATMega328p Demo Setup• ATMega328p Interactive Demo

• Low Power Wireless Communication Methodologies• ATMega328p & Xbee Demo Low Power Wireless Demo

• Summary

• Questions

What will be discussed today

What is Power?• Physic’s Definition:

• Electric Power:

– Using Ohm’s Law, we can get the other forms– What is the SI unit for power?

• Examples:

– What is the power dissipatedthrough the 100Ohm resistor?

R

VRIIVP

22

What is Energy?• Physic’s Definition

– Power definition again:

– Definition of work (Energy):

• Mechanical Definition:

• Electric Definition:

– What is the SI unit for energy?

• Examples– Circuits in previous slides run for 1 minutes

How much energy was dissipated through resistor?

– What is the Ah in the iphonebattery?

• ZeroTracer[1]

– ~750W = 1HP– What is the Amp-hour of the battery?

– What is the charge current of the 9.9KW charger?

A Real World Example

Why do we care?• HydroWatch Berkeley[2]

– Low power– Self-charging– Ultra Low maintenance– Runs forever

• Home power consumption monitoring– Has to be low power otherwise monitoring is not useful

• Helps enable your smartphone to run for a day without recharging

• Extend Battery life

ACME module[3]

Discharge graph of NCR18650 Cells[4]

Demo Setup• Circuit layout

– ATmega328P instead of ATmega88PA

– 10x 1000uF Caps inparallel as C1

• Capacitors as Simulated battery– Energy stored in Capacitor

– What is the energy stored in our capacitors assuming 5V DC supply in our circuit?

– Given the energy above, what is nominal Ah?

• Serial Communication at 9600 baudrate externally powered– We are only concerned about how long will the ATmega328P be able to

generate the TX signal

[5]

Atmega328P background

• 8Mhz internal Calibrated RC Clock• 128Khz internal RC Clock• 1.8V-5.5V Operating voltages• Low Power Consumption

• On Right is the Block Diagram

ATmega328P Datasheet[6]

• Overall Goals:– Take ADC– Do 1000 statement processing– Transmit something over UART– Wait 1 sec– Repeat

ATMega328P Power Demo

• I – No Optimization– 8Mhz Clock– BOD (Brown Out Detection) = 2.7V

• II – Turn off things we don’t need and change IOs – Same as I– Turn off SPI/I2C/Timers/etc– Make all IOs as inputs and pull-up (see datasheet on why)

• III – Slow down the clock– Same as II– Slow clock to 1 Mhz– BOD = 1.8V because we are using only 1Mhz (see datasheet)

• IV – Sleep while waiting– Same as III– Turn off ADC when not being used– Setup interrupt to wake up after 1 sec

• Other Optimization?

Power Optimization Steps

Atmega328P background followup

• Atmega328P power mode:

– Why do we use Power-save mode?

Power Optimization Performance

Optimization Number of seconds before shutdown

% change

I 4 seconds ---

II 5 seconds 25% increase from I

III 14 seconds 180% increase from II

IV >200 seconds > 14000% increase from III

Low Power Wireless Communication Methodologies

• Adjust power base on distance from objects

• Send only when necessary, i.e. save data and then send all at once

• Send as fast as possible and then go to sleep

• Synchronized listening cycle, i.e. wake, listen, and transmit for 1 sec then go to sleep for 1 sec.

• Use error correction that does not require resubmission of data• Tolerate error in data (TCP vs UDP)

• Use Multi-hop network

ATMega328p & Xbee Demo Low Power Wireless Demo

• Notice the low “On” state so power consumption is minimized

Summary

• Talked about Power– P=IV

• Talked about Energy

• Gave a real world example of Power/Energy– ZeroTracer

• Why low power is important

• ATMega328P Demo

• Low Power communication methodologies– Send when you can, etc

• Xbee Demo

Questions/Comments?

References• [1] - http://www.zerotracer.com• [2] - http://www.cuahsi.org/pageFiles/CUAHSI%20Cyberseminar%20Bode%

20Final.pdf• [3] - http://acme.cs.berkeley.edu• [4] -

http://industrial.panasonic.com/www-data/pdf2/ACA4000/ACA4000CE240.pdf

• [5] - http://www.atmel.com/Images/doc8349.pdf• [6] - http://www.atmel.com/Images/Atmel-8271-8-bit-AVR-Microcontroller-

ATmega48A-48PA-88A-88PA-168A-168PA-328-328P_datasheet.pdf