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Some Power Considerations 1 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Embedded Systems and Software
Some Power Considerations
Some Power Considerations 2 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Energy/Power Considerations
• Terms – Cell, Battery – Energy (Joule) – Power (J/s or Watt) – Ampere-hour (Ah) – Deep-cycle – MCU – Sleep Modes – ADC – Data rate (BPS)
Some Power Considerations 3 Embedded Systems and Software, 55:036. The University of Iowa, 2013
An Embedded System
Some Power Considerations 4 Embedded Systems and Software, 55:036. The University of Iowa, 2013
An Embedded System
Radio-Serial Interface
Serial Interface Ultrasonic Sensor
Some Power Considerations 5 Embedded Systems and Software, 55:036. The University of Iowa, 2013
An Embedded System
Creek or Small River
Some Power Considerations 6 Embedded Systems and Software, 55:036. The University of Iowa, 2013
An Embedded System
Attach Distance Sensor
Some Power Considerations 7 Embedded Systems and Software, 55:036. The University of Iowa, 2013
An Embedded System
Wake up, Measure Distance to Surface
Some Power Considerations 8 Embedded Systems and Software, 55:036. The University of Iowa, 2013
An Embedded System
Wake up, Measure Distance to Surface
Some Power Considerations 9 Embedded Systems and Software, 55:036. The University of Iowa, 2013
An Embedded System
Relay Data Back to Servers on Internet
To UI Via Cell Modem
Some Power Considerations 10 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Simplified System Diagram
Some Power Considerations 11 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Electronics
Ultrasonic Sensor
Cell Modem (Note Green Dot)
“Electronics” Solar Charge Controller
Battery
Cell Antenna
GPS Antenna Serial-Radio
Interface Antenna
LED
Serial-Interface Connector
Some Power Considerations 12 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Electronics
LP3500 Development PCB
GPS SD Card External Memory
Glue Board
Cell Modem
Some Power Considerations 13 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Where Does The Power Go?
Typically contains it own MCU + firmware
May be part of microcontroller
Some Power Considerations 14 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Principles
fVCP 2α=
Power dissipated Capacitance Voltage
Switching frequency
Number of active gates
The following is a simple model for the power dissipated by a CMOS-based system
(turn off unused systems)
(lower clock)
(Lower voltage) (Use simpler uC)
Some Power Considerations 15 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Digital Switching Levels
Time
Reducing power consumption is the main reason for push towards lower voltage levels
Some Power Considerations 16 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Microcontroller Unit (MCU)
• Intel’s StrongARM, Atmel AVR, PIC, … • Several low power modes
– Active, idle, nap, shutdown, sleep modes Various MCU subsystems are turned off Different clock speeds
– For some MCUs, in deep sleep modes, the power consumption can almost be negligible
– Takes longer to wake from a deep sleep than just a nap – Wakeup time also takes power – Wakeup impacts processing
Some Power Considerations 17 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Radio
• Radio typically contains an embedded controller that provides many functions – Uses RSSI to adjust transmit power – Error detection and correction in hardware/firmware
• Several modes – Receive only, transmit + receive, idle, etc.
• In general, transmit requires most power • Carefully consider radio spec and modes • Mode change can consume a lot of power
– May be better to shutdown completely rather than go into idle mode
Some Power Considerations 18 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Sensors
Passive & low power (~mW and smaller) – Soil moisture, temperature, light, humidity
• Active & high power – Anemometers, disdrometers, cameras
• Many sensors are inherently analog, but some sensors have digital interfaces (provided by embedded controllers)
• Conditioning/wakeup times need to be considered • Analog-Digital Converters (ADC)
– Can be a major power consumer – More bits and high conversion rate requires more power – Don’t over-specify
Some Power Considerations 19 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Batteries
• Uses chemical reaction to provide electrical energy • Battery vs Cell • Batteries are often the most bulky part of a device • Capacity measured in Ampere-hours (Ah) or mAh • Note that the capacity does not consider voltage… • The capacity is the nominal number of hours it can
supply a given current. Normally specified at given load conditions and temperature. For example, 100 mA constant current discharge, 70% of open circuit voltage and 20 oC.
• Battery Chemistries: Alkaline, Lead-Acid, Li-ion, NiMH, etc. Each chemistry has its own characteristics
Some Power Considerations 20 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Constant Current Discharge Curve
0 200 400 600 800 1000 1200 1400
0.25
0.5
0.75
1
1.25
1.5Constant Current Discharge (100 mA)
Volta
ge (V
)
Discharge Time (min)
Duracell AA Energizer NH15-2500 Eveready Gold Kodak AA Panasonic HHR-160AAB RadioShack Enercell Rayovac Maximum Plus
Some Power Considerations 21 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Battery Capacity
• AA Size batteries – Few hundred mAh (Alkaline) to ~ 3,000 mAh (Li-ion)
• Factors – Previous number of discharges. Capacity decreases as the cycle
number increases. – Temperature. Overall, capacity decreases with decreasing
temperature, but over some temperature ranges, capacity my increases
– Load. Higher loads reduce apparent capacity
Some Power Considerations 22 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Battery Capacity
0 200 400 600 800 1000 1200
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6 Rayovac Maximum Plus ( Alkaline, AA)
Volta
ge (V
)
0 200 400 600 800 1000 1200
50
100
150
200
250
300
350
400
Current (m
A)
Discharge Time (min)
70%
Capacity is Area
Some Power Considerations 23 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Capacity Dependence on Load
• Peukert’s Law is often used as a model:
• t is discharge time, I is discharge current, and n is Peukert’s number that depends on battery chemistry and battery history. Values for n range between 1.1 – 1.2. C is the capacity removed from the battery.
• Empirical formula • Dimensionally inconsistent, and often interpreted
and used incorrectly.
tIC n=
Some Power Considerations 24 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Capacity Dependence on Load
• Better formulation of Peukert’s law:
• Here C0 is the capacity measured at I0, and CI is the capacity at a load I.
• The state of charge (SOC) of a battery is defined as:
Where CI is the capacity at a load I
10
0
−
=
nI
II
CC
ICtI ×
−=1SOC
Some Power Considerations 25 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Example
• An NiMH cell has Peukert’ number n = 1.1 and has a capacity of 2,500 mAh measured at 50 mA and 20 oC. (a) What is the cell’s capacity at a load current of 300 mA? (b) What is the SOC after 2 hours at a load of 300 mA?
mAh 090,2836.0500,2
836.030050 11.11
0
0
=×==>
=
=
=
−−
I
nI
CII
CC
%7171.0090,2
230011SOC ==×
−=×
−=ICtI
Some Power Considerations 26 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Battery Capacity
50 100 150 200 250 300 350 400 4501500
2000
2500
3000
Load Current (mA)
Cap
acity
(mAh
)
Capacity vs Constant Load Current
EAEBEDEEEFEGEHEIEJEX
Linear fit (no Peukert)
Some Power Considerations 27 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Effects of Temperature
• Temperature affects rate of chemical reactions and influences battery capacity
• Lower temperatures => lower (apparent capacity) • Broadly speaking, temperature effects are
reversible – Some manufacturers specify “% Service vs.
Temperature” rather than “Capacity vs. Temperature” • At very high and very low temperatures damage
to battery occur • Temperatures affect self-discharge & shelf life
Some Power Considerations 28 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Effects of Temperature S
ervi
ce (%
)
Cap
acity
(%)
Temperature (oC) Storage Time (days)
Ser
vice
(%)
Cap
acity
(%)
Temperature (oC) Storage Time (years)
Some Power Considerations 29 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Modeling Temperature Effects
• Simple linear model for “%Service”
• Here S(T) is “% Service”, T0 is where Service = 100%, and α is the temperature coefficient of the “%Service”. For batteries that exhibit a plateau, this equation would apply to the relevant part of the plot.
• Simple linear model for self discharge
( ))(1100)( 0TTTS −+= α
( )TdCdTC β−= 1),( 0
Where C(T,d) is the capacity of the battery after d days of storage at temperature T, and β has units day-1 oC-1
Some Power Considerations 30 Embedded Systems and Software, 55:036. The University of Iowa, 2013
0 200 400 600 800 1000 1200 1400
0.25
0.5
0.75
1
1.25
1.5Constant Current Discharge (100 mA)
Volta
ge (V
)
Discharge Time (min)
Duracell AA Energizer NH15-2500 Eveready Gold Kodak AA Panasonic HHR-160AAB RadioShack Enercell Rayovac Maximum Plus
Estimating Battery Capacity
May be possible to use curve to gauge battery state. Must be under load conditions.
May not be possible to use curve to gauge battery state.
Some Power Considerations 31 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Question: what type of diodes should D1, D2, be, and why?
Answer: Schottky diodes, because they have lower turn-on voltages than Si diodes. Thus, these diodes dissipate less power.
In normal operation, the 5 V linear regulator provides clean 5 V. D1 drops VD(on) . This is greater than (3 V + VD(on) ), so D2 is reverse-biased and open. Without main power, D2 is forward biased turn on, and powers the controller.
Power Supplies
Some Power Considerations 32 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Coin & Button Cells
• Capacities: 50–300 mAh • Designed for 3–200 µA or few mA pulsed load • Enough to power CMOS • Used for RAM backup • WSN: Power RTC • Li chemistry typical
– => single cell – => very long service
Some Power Considerations 33 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Power Supplies
Linear Three-Terminal Regulator
Quiescent current for LM78xx regulators are large, and can waste lots of energy not suitable for battery-operated equipment.
Some Power Considerations 34 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Power Supplies
MAX604 Linear Regulator
Quiescent current for MAX604 a few micro-amps, thus much more suitable for battery-operated equipment.
Some Power Considerations 35 Embedded Systems and Software, 55:036. The University of Iowa, 2013
DC/DC Converter
Simple Buck Converter
Feedback provides regulation
Some Power Considerations 36 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Power Supplies
MAX724 DC/DC switching Regulator
Very wide input range, efficient conversion to output voltage
Note that switching regulators can step up voltages
Some Power Considerations 37 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Battery Models
• Motivation – Understand battery behavior – Simulate battery behaviors – Predict battery lifetime. Think about the battery-icon
on your cell phone • Physical models • Abstract models
– Electrical circuit models – Discrete time models – Stochastic models – Mixed models
• Empirical Models
Some Power Considerations 38 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Physical Battery Models
• Consists of differential equations that describe electrochemical processes in detail:
• Most accurate, but some can require up to 50 parameters, thus hard to configure model
• Not feasible to embed in an embedded system…
Write sets of differential equations that describe reaction and diffusion processes (at both electrodes). Incorporate effects of temperature on electrolyte, ion mobility, passive film forming at electrodes etc. Solve equations
Some Power Considerations 39 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Empirical Battery Models
• These models attempt to capture/describe behaviors of interest using simple equations
• Examples include Peukert’s law, self-discharge, etc:
• Simplest to configure and use • Generally not very accurate (Peuket’s law) • Model parameters are chemistry- and battery specific. For
example, Peukert’s number for an alkaline AA may be different for a same-brand AAA battery.
• However, a well designed empirical model based on good experimental data for a specific battery model can be very accurate, and simple to embed
10
0
−
=
nI
II
CC ( )TdCdTC β−= 1),( 0
Some Power Considerations 40 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Coulomb Counter/Battery Fuel Gauge ICs Fuel gauge; Q = I×t Coulombs = > Coulomb counter
Example 15 bit (1.6 µV / 78µA) measurement Unique ID DS2740
Li-Io
n ba
ttery
Some Power Considerations 41 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Smart Batteries
Transient Voltage Suppression
Some Power Considerations 42 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Smart Battery
Smartbattery monitor
Some Power Considerations 43 Embedded Systems and Software, 55:036. The University of Iowa, 2013
Question 1. (10 points) Consider a battery-operated consumer electronics device that uses an embedded microcontroller that will accept a 2.7–5.5 V power. The device can also be powered from a power supply that plugs into a mains outlet. Consumers’ expectation is that the switchover between battery- and mains power is transparent. That is, the instant a user inserts the power supply connector, the device switches to the power supply, and the instant the user unplugs the device, it switched to battery power.
Draw a block diagram/schematic that shows how to implement this functionality using diode(s), linear regulator(s), battery, etc. Explain how the circuit works. Provide as much details as you can. For example, specify the type of diodes, indicate voltages on the diagram, include critical capacitors, and so on. You can assume that unregulated 9 V dc power is available.
Question 2. (10 points) List (1 point) and the briefly explain (1 point) five design considerations that one can employ to reduce the power consumption in an AVR-based embedded system.
Sample Exam Questions
Some Power Considerations 44 Embedded Systems and Software, 55:036. The University of Iowa, 2013
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