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White Paper – Energy Harvesting for Low-Power Sensor Systems of 29

White Paper – RX100 Microcontroller Family

Energy Harvesting for Low-Power Sensor SystemsBy: David Squires, Squires Consulting; Forrest Huff, Renesas Electronics America Inc.

Feb. 2015

AbstractThis white paper details important design issues associated with designing the very-low-power remote sensors that play important roles in the feedback control loops of embedded electronic systems. Using a basic configuration for a standalone sensor system as a reference, the discussion covers sensor/transducer types, power budgets, power sources (especially devices that harvest ambient energy) and energy-storage solutions. It also summarizes microcontroller requirements and highlights the latest power-management chips and wireless ICs. Special emphasis is placed on the concept and reality of energy harvesting as a viable method for powering standalone embedded systems for extended periods of time. An example security alarm design is described and explained to provide engineering context and perspective.

Index

I. Introduction – Very-Low-Power Sensor Products 2

II. Basic System Design Issues 2

• Sensors 3

• PowerBudgets 3

• EnergyStorageDevices 3

• EnergyHarvestingSolutions 4

• Microcontrollers(MCUs) 6

• PowerManagementDevices 6

• WirelessConnectivityModules 6

III. Additional Insights on Sensor Components 7

• Sensors 7

• EnergyStorageDevices 7

• EnergyHarvestingSolutions 12

• MCUs 15

• PowerManagementDevices 18

• WirelessConnectivityModules 20

IV. Example Design: Glass Break Sensor 22

V.Summary 28

VI.Appendix 28


I. Introduction – Very-Low-Power Sensor ProductsModernlow-powersensorsenablepreciselocal,remoteorautonomouscontrolofavastrangeofproducts. Their use is rapidly proliferating in vehicles, appliances, HVAC systems, hospital intensive care suites, oil refineries, and military and security systems. They are key components for a vast range of applications with global markets.

Importantly, energy harvesting (EH) technology allows small, standalone sensors to function continuously for extended periods of time — decades, even — without power-line connections or battery replacements. This technology greatly enhances the problem-solving capability of low-power sensors and its use is growing rapidly. For that reason, an energy-harvesting function is shown as the power-source element in the block diagram of a typical very-low-power sensor product shown below in Figure 1.

Figure 1: Components in a Typical Very-Low-Power Sensor Product.

II. Basic System Design IssuesElectronic engineers developing standalone sensor systems must address design issues associated with the following system elements, among others:

• Sensors (transducer type, performance characteristics, operating requirements, etc.)

• Power budget (operating voltage; peak, quiescent and average operating currents)

• Energy storage devices (capacity, leakage, temperature performance, etc.)

• Energy harvesting chips (capability, requirements, limitations, etc.)

• Microcontroller (processing performance, power efficiency, DSP, I/O, low-power modes, etc.)

• Power management devices (features, performance, etc.)

• Wireless connectivity device (peak power, range, frequency, protocols, etc.)

Other technical issues may include size limitations, challenges of operational environments, life-time costs, long-term reliability, safety, and more.

Aspects of the main design areas listed above are presented in the sections that follow.

MCUSensor

PowerMgmt.

Energy Storage

Energy Harvest

RF

Optional elements including actuators, displays, keys, etc.

Ambient Energy

Events


Sensors

Eight physical characteristics are most commonly monitored in embedded system applications. They are listed below with the types of sensors and transducers typically used for measuring them:

•Motion Ultrasound position detectors

•Temperature Silicon diodes or thermistors

•Presence of people Passive IR detectors

•Pressure MicroElectro-Mechanical(MEMs)chips

•Acceleration MicroElectro-Mechanical(MEMs)chips

•Picture LCD cameras (like those used in cellphones)

•Location Geomagnetic sensors (like those used in cellphones)

•Proximity Sensors similar to cellphone components

Power Budgets

For standalone sensor designs that cannot connect to the AC power mains—especially those that must operate for long periods of time between battery changes or cannot be accessed for main-tenance—the amount of power consumed by every component in the system must be carefully identified and minimized. If the data reveals that a design can be powered by a reasonable-sized battery, or a battery + energy-harvester solution, that’s great! If it doesn’t, then it may be neces-sary to implement a top-down design approach.

The top-down approach starts with the embedded system’s total power requirement. A primary battery must be sized to fit in the available space, and that size and the battery type will determine the maximum power-source capability, usually measured in mAh.

The Energy Storage Devices sections of this white paper provide technical data on different types of batteries. For discussion purposes here, however, let’s assume that a hypothetical design has enoughspacetoaccommodatetwoAA-sizebatteriesoccupyingavolumeof16cm3.Tofunctionproperly,theelectronicsinthisdesignrequiresasupplyvoltageof1.8V,sotwo1.5Vbatteriesmust be connected in series.

Thetotalbatterycapacityis16cm3/1000cm3/l*450Wh/l=7.2Wh.ToconvertthistomAh,weknowthatthenominalvoltageis3V,therefore7.2/3=2.4Ah=2400mAh,whichisconsistentwithpub-lished numbers for AA batteries.

This hypothetical sensor product mandates a continuous battery-operating lifetime of 5 years. Thus,theaveragecurrentconsumedbythesensor’scircuitsmustnotexceed2400mAh/(5yrx24hr/dayx365days/yr)=2400mAh/43800h=55µA.Inpractice,thisvalueshouldbederatedsome-what—perhaps by 10% to 20% or more over a 5-year timeframe, depending on the self-leakage of the particular battery. In this hypothetical design, that would impose an upper limit on the average currentofabout45µA.

Althoughthe45µAaverageoperatingcurrentlimitmightseemtobeverylow,it’smorethansufficientfordesignsbuiltwithvery-low-powerMCUs.That’sthecasefortheexampledesigndescribedlaterinthispaper,whichappliestheRenesasRX111MCU.

Energy Storage Devices

There are many component choices for storing the energy needed to power a standalone sensor product (see Figure 2). Conventional batteries are inexpensive, readily available, well understood and relatively easy to incorporate into a design. They would be the first choice if they meet a prod-uct’s target energy budget.


If conventional primary cells can’t be used, however, the alternatives are either a rechargeable batteries, supercapacitors, solid-state batteries, or a combination thereof. Solid-state batteries are rechargeable, but they are so new that for the purposes of this document it was deemed best to put them into a separate category.

Figure 2: Comparison of types of energy storage devices.

MoreapplicationinformationaboutbatteriesiscontainedinSectionIII.

Energy Harvesting Solutions

Energy harvesting technology is exciting, beneficial and—thanks to developments in ultra-low-power semiconductors—meets the needs of and facilitates exciting classes of new embedded system applications.

Free Energy Available in the Environment

The major types of energy normally wasted in the environment can readily be captured are described below:

• Energy from light—Sunlight and indoor and outdoor lighting can be converted into electricity by photoelectric energy cells; i.e., solar cells. To work efficiently, those cells have to be optimized for the characteristic spectra of the incident illumination. Despite the fact that indoor lighting is a good power source for wristwatches and handheld calculators, it doesn’t provide enough energy to be useful for most harvesting applications.

• Mechanical energy—Objects that vibrate or move can be made to produce electricity. Vibrations generate considerable voltage when they are applied to piezoelectric materials. Also, the mechanical energy of pressing or moving an object such as a switch can generate a current if the action changes the flux of a magnetic core situated within an internal coil.

• Thermoelectric energy—If the temperature at one point of the surface of an object is different than what it is at a nearby point, that temperature difference can be converted directly into an electric current via a physical phenomenon called the Seebeck effect. Certain semiconduc-tors and metals with high Seebeck coefficients transform temperature differentials into useful electric energy.

Conventional Batteries Supercapacitors Solid State Batteries

+ High discharge current

+ High energy density

+ Inexpensive

– Limited life

– Replacement labor cost

– Unsafe, polluting

– Form factor

+ Peak power delivery

+ Long life

+ Inexpensive

– High leakage

– Very low energy density

– High temperature degradation

– Form factor

+ Moderate energy density

+ Near zero leakage

+ Long life / Permanent

+ Low cost of ownership

+ Form factor

+ Safe / Eco-friendly

+ Broader temp. range


• Electromagnetic waves—Radio emissions are pervasive everywhere today, and only a small fraction of the energy in those transmissions is consumed by the intended receivers. Radio receivers with antennas can convert some of that wasted RF energy into electricity.

Two aspects of the power sources listed above should be pointed out:

• Thesmallamountofenergyharvestedforpoweringtheelectronicdevicesandsystemsdoesn’t negatively impact the phenomenon or application that produced the energy because the miniscule quantity captured would otherwise be wasted. Essentially, energy harvesting technology adds value to a situation; it doesn’t remove value. It’s an eco-friendly power- generation solution.

• Theenergyavailableintheenvironmentoftenisintermittent,asitiswhenroomlightingisturned on and off. Therefore, if an energy-harvesting enabled product must operate continu-ously or on a time schedule, it must incorporate power-control circuitry and a rechargeable storage device to ensure continuous operation.

Energy harvesting is still in its infancy. Implementations are evolving rapidly as harvesters are improved, better power management chips are introduced, and engineers acquire application experience with the technology.

Whatcircumstancesfavortheuseofenergyharvestingtechnology?Powerbudgets,packagingrequirements and operating lifetimes are among the determining factors.

As previously stated, if a battery will provide sufficient power to operate a sensor product for its desired operating lifetime, then that will probably be the best design choice. If it can’t, though, a good alternative design approach is to use an energy-harvesting device to supplement a recharge-able battery.

Figure3providesaquickreferencefordeterminingwhetheraprimarybatterywillsufficeorwhether energy harvesting is needed to obtain the necessary lifetime. The horizontal axis of this graph shows days of operation required over the lifetime of the product, while the vertical axis shows the average load current of the electronics on a logarithmic scale.

Figure 3: Battery Lifetimes versus Average Load Current.

1 10 100 1000 10K 100K (3yrs) (30yrs)

Ave

rag

e Lo

ad C

urr

ent

Energy harvesting & secondary (rechargeable)

battery required

2800mAhAA Cell(Primary)

220mAhCR2032Coin Cell(Primary)

Days of Operation

100mA

10mA

1mA

100µA

10µA

1µA

100nA

Primary battery will suffice


The top line moving from upper left to lower right shows the lifetime energy provided by an alka-line AA cell. To use the graph, engineers must first determine how long the product must survive in the field. Then they draw a vertical line at that data point on the horizontal axis (the example shows 2 years). Next, they take the intersection of this vertical line and the preferred battery (this exampleshowsaCR2032Lithiumcell).Finally,theydrawahorizontallinebacktotheY-axistoreadthemaximumaveragecurrentthatcanbesupportedbythatbattery.FortheCR2032,thegraphshowsthatthelimitis10µA.

If the embedded system consumes less than that current, a conventional battery will provide all of the required power. If it consumes more current than what the graph indicates, two design choices can be considered:

• Usealargerprimarycell,anoptionthatonlyworksifenoughspaceisavailable

• Combineanenergy-harvestingdevicewitharechargeablebattery

Figure3’sgraphshowsthatenergyharvestingisrequiredfordesignmustoperatebeyond5years.This design recommendation is based on the author’s personal experience that when conventional primary cells are over 5 years old, they have about a 75% chance of leaking and corroding the bat-tery compartment. That often compromises wiring connections, causing a system failure.

Microcontrollers (MCUs)

ThechoiceoftherightMCUiscriticallyimportantforavery-low-powerembeddedsystems.Ide-ally,thebestMCUwillhavethefollowingthreefeatures,atminimum:

• Multiplepower-downmodestomaximizebatterylife

• Goodperformanceforfast,efficientprocessing

• Veryfastwake-uptimesfrompower-downmodes.

The latter is important because it allows the electronic circuits to spend as much time as possible in a low-power state before transitioning to an operating mode that consumes more current.

SectionIVofthiswhitepaperrevealsthattheRenesasRX111MCUdeliversthesefeaturesandmore that facilitate the design of remote sensor products and other power-efficient embedded systems.

Power Management Devices

Whenengineersincorporateenergy-harvestingtechnologyintoasensor,thedesignmustincludea power-management function to handle fluctuations in the power generated by the harvester. That function can also manage the charging and discharging of the rechargeable battery, provide a regulated supply voltage to the sensor or transducer and other components, and trigger an alarm when the battery nears the end of its useful life.

Several vendors offer power-management devices with different capabilities and features. One of thosechips,Maxim’sverycapableMAX17710PMIC,isdiscussedintheglassbreaksensordesigndetailed in Section IV.

Wireless Connectivity Modules

Low-power sensors often utilize wireless communication techniques to send data back to the cen-tral location that manages the security system operation. This implementation saves cost, enhanc-es security and simplifies installations by eliminating external wiring and connections.

Mostofthemanywirelesscommunicationsolutionssoldtodayoperateinthe2.4GHzISMband,usingeithertheZigBee,Z-Wave,orBluetoothSmartprotocols.(ThelatterisalsoknownastheBluetooth Low Energy [BLE] protocol.


ZigBeeandZ-Waveareusedextensivelyincommercialbuildingsandfactories,whileBluetoothSmart serves home automation applications, as well as portable devices such as health and fit-ness monitors. Since all of the latest smart phones support Bluetooth Smart, the number of sensor products applying this wireless connectivity protocol is expanding dramatically.

III. Additional Insights on Components for Standalone SensorsThis section provides more details about the sensor design components mentioned briefly in Section II. The additional information presented here aims to help system engineers create new embedded products.

Because progress in the technology areas covered here is quite rapid, careful reviews of up-to-date information from manufacturer’s datasheets and online forums are recommended before specific design choices are made.

Sensors

There are so many different types and manufacturers of miniature sensors for measuring physical phenomena that a comprehensive discussion of them is well beyond the scope of this white paper. Careful research using the Internet and other resources is highly recommended.

Recent web browsing, for example, revealed that, according to the Solid State Technology site, suppliersofMEMSchipsnowincludeSTMicroelectronics,RobertBosch,TexasInstruments,Hewlett-Packard, Panasonic, Knowles Electronics, Denso, Avago Technologies, Freescale Semicon-ductor,AKM,AnalogDevices,SeikoEpson,Invensense,InfineonTechnologies,Murata,Sensata,Honeywell, GE Sensing, Triquint and Lexmark, among others. Clearly, this is a vibrant and evolv-ing global market.

Energy Storage Devices

As indicated previously, most energy-harvesting designs include an energy-storage device that serves as a buffer between the load and the energy harvester. A battery or SuperCap supplies current to the electronics when or if the harvester cannot produce power. It also does so when the load requires more current than the harvester can provide. The energy-storage device accumu-lates charge and then supplies a burst of current whenever it’s needed.

Figure2onpage4highlightedthefactthatconventionalrechargeablebatteries,supercapacitorsand solid-state batteries can be utilized for storing electrical energy in very-low-power sensor products and other types of embedded systems. Here is some basic design information about those components:

• Rechargeable batteries—Rechargeable batteries come in many different chemistries, and diverse shapes and sizes. Although different types from diverse manufacturers might seem to be familiar and similar, their specific features and specifications—which aren’t standardized—can have important impacts on the performance of the products in which they are used. It’s advisable to read datasheets carefully and also to perform realistic field tests, if possible.

• SuperCaps—Although supercapacitors are similar to regular capacitors, they offer much higher capacities. They’re available in cylindrical and prismatic (rectangular) form, with the latter generally being less expensive. A SuperCap the size of a thumb has a capacitance of one Farad at 2.5 Volts.

Be aware that SuperCaps have high leakage currents. If they are being used with an energy harvester, that harvester must generate a sufficiently large output. Also, the performance of SuperCaps decreases at elevated temperatures.

• Solid-State Batteries—The recent development of solid-state batteries gives engineers anotherusefulenergystoragechoiceforvery-low-powerdesigns.CymbetandSTMicroarecurrently shipping these miniature devices in volume. The tiny power sources have a solid electrolyte, typically Lithium Phosphorous OxyNitride, or LiPON for short.


Solid-state batteries feature low self-leakage and can be recharged 1,000 times—even more if deep discharges are avoided.

Cymbet’s solid-state batteries feature standard surface-mount IC packages, giving them a big advantage over other types of energy-storage elements.

Rechargeable batteries

Figure4showsthedesignparametersthatshouldbeconsideredwhenselectingrechargeable batteries for energy-storage applications.

Figure 4: Selection Criteria for Rechargeable Storage Devices.

Because AA/AAA rechargeable batteries are readily available and relatively inexpensive, they are the top design choice for products that can accommodate their large sizes. The much smaller Lithiumrechargeablecoincells,commonlybuiltwithaLiMnO2chemistry,areviableenergystor-age alternatives, although their performance characteristics aren’t as good as AA and AAA types.

Each time a rechargeable battery is charged, material is physical transported from the cell’s cathode to its anode. In a Lithium-chemistry battery, for instance, Lithium ions make that journey, returning to the cathode as the battery discharges its stored energy. This back and forth transport slowly degrades the device’s internal structure, reducing the total energy it can store. The number of times a battery can be recharged decreases rapidly as the depth of discharge increases (see Figure 5).

Figure 5: Correlation Between Battery Discharge Endurance and Depth of Discharge.

Attribute Description Units

Capacity Howmuchjuiceavailable? mAh, mAh

Current Continuous current available mA, mA, A

Op Temp Range (C) deg C

Size (mm) Some prismatic, some cylindrical mm

CycleLife(80%DepthofDischarge) #rechargecycles* cycles

Price (high volume) $

Self Discharge (%/yr) Internal leakage %/yr, mA

Other metrics that may be important depending on circumstances

– Charge time – Lifetime versus temperature – Internal resistance – Peak current

* Cell is charged/discharged from 10% to 90% of capacity until the cell capacity has been diminished by 20%.

This is the number of cycles recorded for this measurement

0 20 40 60 80 100

104

103

102

10

Depth of discharge (%)

Rec

har

gea

ble

cyc

le

nu

mb

er /

(cyc

les)

Cut-off voltage of charge: 3.25V Temperature: 20ºC


Energy-harvesting systems that experience daily charge/discharge cycles require batteries with extra storage capacities. The extra capacity reduces the depth of discharge (DOD), thereby increas-ing the total number of charge/discharge cycles that the energy storage device can sustain—and, thus, its lifetime.

Another important design criteria for selecting rechargeable batteries is temperature performance. Manychemistriesperformpoorlyatlowtemperatures.LithiumIonbatteriesareamongthebestinthis regard, as a review of battery datasheets will confirm.

SuperCaps

SuperCaps, also known as electric double-layer capacitors, are gradually being used more often to meet the energy-storage requirements of products that utilize energy-harvesting technology. A typical 1-Farad SuperCap can be charged up to 2.5 Volts. To handle higher voltages, SuperCaps can be stacked in series, provided that they are connected to circuits capable of balancing the volt-age between them during charge and discharge cycles.

Hypothetically,a1FSuperCapwitha2.5Vinitialchargecanpoweranaverageloadof10µAforabout19hoursbeforeitsvoltagedropsto1.8V,whichisthelowestoperatingvoltageformanysemiconductor devices.

A SuperCap has a significant initial self-leakage, however. For a 1F SuperCap, that leakage is likely tobeabout100µAforthefirstfewhours.Thisdrasticallyshortensthetimethatthecapacitorcouldpowerthe10µAload,reducingittobetween1and2hours.Accommodatingthishighleak-age current mandates a sizeable energy harvester.

Figure6showsleakagecurrentinaGZ115,arelativelysmall0.15FSuperCapofferedbyCap-XX.Although the device’s critical initial leakage current isn’t specified, tests have shown that it exceeds 100µA.

Figure 6: Leakage Current of a 0.15-Farad SuperCap.

The high initial leakage of a SuperCap means that this type of component might not be suitable for an energy harvesting application unless it can be recharged at least every hour. This requirement canbegreatlyreducedbyensuringthatthecapacitorisfullychargedfor3to4daysbeforeitisdeployed. Proper conditioning causes the self-leakage to drop dramatically, enabling the SuperCap to hold most of its charge over time.

0 20 40 60 80Time (hrs)

Leak

age

Cu

rren

t (m

A)

100

20

16

12

8

4

0

Most energy harvesting apps occur in this region with at least one discharge per hour


Figure 7 shows another critical performance aspect of SuperCaps: aging. Their storage capacity diminishes over time, especially at higher temperatures.

Figure 7: Variations of SuperCap Capacity with Time and Temperature.

The dotted line (the bottom one in Figure 7) plots the operating lifetime of a SuperCap charged to 2.5V as a function of temperature. The data also assumes that the capacity of the energy storage devicehasdeclinedby30%.Forexample,a1FSuperCapchargedto2.5Vandplacedina50°C environmentloses30%ofitscapacityafterabout25,000hours.ThatsameSuperCaploses30% ofitscapacityafteronly8,000hoursat60°C,andafterjust2,800hours(17weeks)at70°C. These storage capacity degradations must be considered for systems that operate at elevated temperatures.

For designs that will operate in cold temperatures, though, SuperCaps have a major advantage over batteries. For instance, in products used in cold storage distribution, SuperCaps are likely to be the best energy-storage choices because the performance of battery chemistries decreases in cold environments.

Derating is one way to address SuperCap aging at elevated temperatures. The dashed line (the top one in Figure 7) reveals that when a SuperCap is derated to 50% of its storage capacity, its lifetime increasesbyafactorofabout5x.Asindicated,thelifetimeat70°Cincreasestoabout90weeks.It’s recommended that a SuperCap used in an energy-harvesting design be oversized when the system will have to operate in high-temperature environments.

Aging is also reduced somewhat when lower voltages are stored in the SuperCap. This is indicat-edinthegraphbythealternatingdotsanddashesofthe1.8Vline.Itshowsalifetimeabout1.8xlonger than the lifetime plotted by the dotted 2.5V line.

A major design feature of SuperCaps is that because they have very low internal impedance, they can supply surges of high current. This is beneficial especially for remoteapplications in which radio modules briefly consume hundreds of mA while transmitting short data messages over long communication ranges.

On the other hand, SuperCaps should receive a substantial initial current when they are being charged. Otherwise, the charging process may stall and the SuperCap will never become fully charged.ThisdegradedperformanceishighlightedinFigure8bythelowercurvederivedfromthedataobtainedwitha35µAchargecurrent.WhenSuperCapsareappliedinenergy-harvestingapplications, the manufacturer’s recommended charging procedure should be followed.

0 20 40 60 80

106

105

104

103

Temperature (ºC)

Life

(H

ou

rs)

70503010

90 weeks

17 weeks

30% drop in capacitance @ 2.5V continuous



Oversize by 50% if design at high temp


Figure 8: Initial SuperCap Charge Currents.

To emphasize: It’s essential to ensure that energy-harvesting circuits deliver enough current, for a sufficient length of time, to fully charge SuperCaps used for energy storage.

Solid-State Batteries

Solid-state batteries are also called Thin-Film Batteries, or TFBs. Unlike SuperCaps, they have verylowleakagelevels,about4%peryear.Thisislowenoughtobeignoredforallbutthemostdemanding applications.

As their name implies, solid-state batteries do not have a liquid electrolyte like conventional bat-teries. Instead, Lithium ions traverse the Lithium Phosphorous OxyNitride (LiPON) layer, which is a thin glass layer, about 1 micron thick. The details of the charging and discharging operations are somewhat complicated, but the following descriptions summarize them:

• Duringthechargingoperation,LithiumionsarefreedintheLiCoO2cathodeaselectronsaremoved around the external circuit to the anode. The Li ions move across the glass electrolyte and, after passing through the electrolyte, they recombine with electrons that have flowed through the external circuit and subsequently plating out on the anode as Lithium atoms.

• Whenasolid-statebatteryisbeingdischarged,Lithiumatomsareionizedintheanode.Thenthe Lithium ions traverse back through the LiPON layer, recombining with CoO2 to form LiCoO2 again.

Cymbet Corporation is one of the few companies actively selling energy storage devices based on this technology. Its product portfolio includes the EnerChip CBC050 solid-state battery. This small 50µAhdevicecandriveanaverageloadof1µAfor50hourswithoutbeingrecharged.

Such solid-state batteries have a major advantage over conventional rechargeable batteries: they deliverahighercyclelife.Forinstance,theCB050achieves1000cyclesat25°Cwitha50%dis-charge depth. Additionally, they come in interesting form factors. Symbet’s tiny EnerChip CBC050 measures just 1.7 x 2.25 x 0.20mm, for example.

In most standalone sensor applications, energy harvesting takes place at least daily. If solar cells generatethechargingcurrentatleast6hoursperday,asolid-stateEnerChipCBC050batteryhastobeabletoprovidepowerfortheother18hours.Duringthattime,theEnerChipcoulddeliveranaveragecurrentofabout3µAbeforerequiringarecharge.Formanyenergy-harvestingbasedapplications, such constraints are quite acceptable: data logging and memory backup being two important examples.

Solicore, Inc. is another solid-state battery supplier (see Figure 9). It has developed extremely thin and flexible lithium-polymer based products for powering remote sensors and other products in which space and form-factor issues mandate unique sets of attributes. Solicore’s thin-film lithium batteriesfitwhereexistingcoincellsorotherrigidbatteriescannot.Theyprovidea3Voutputsatcapacities ranging from 10 to 25 mAh.

0 10 20 30 40 50

2.5

1

0.5

0

Time (hrs)

Volt

age

(V)

Supercapacitor Charging

500µA

200µA

100µA

50µA

35µA

1.5

2


Figure 9: Solid-State Batteries offered by Solicore, Inc.

Solicore’smaterialsexpertiseenablestheextended-life(3-5year)powersourcesnowusedinthemillions of credit cards that can generate a one-time-pass (OTP) codes or display cash balances or reward points. The company’s embedded power solutions also make possible smart medi-cal patches, in which the solid-state battery bends to conform to the area on which the patch is applied. Additionally, the firm’s technology can be applied to implement smart shipping labels capable of tracking the temperature or humidity of items as they pass through a supply chain.

The attractive features of solid-state batteries, combined with the performance of very-low-power chipsliketheRenesasRX111MCUareenablingengineeringteamstocreateextended-liferemoteand portable products for global markets. As the Internet of Things (IOT) continues to evolve, the need for improved energy storage solutions will increase at an accelerated pace.

Energy-Harvesting Solutions

As was explained earlier, energy harvesting collects the ambient energy available for free in the application environment and makes it available for powering electronic circuits. In most situations, the optimum source for harvested energy will be obvious, based on the situations in which the product is used.

Solar power and thermal energy are popular sources, as is energy reaped from mechanical vibra-tions. But RF energy can be tapped, as well; Powercast Corp. offers a range of solutions for doing that.Moreover,manufacturersofwaterfaucetsnowareusingsmallturbinestocollectenergyfrom moving tap water and using it to power electrically operated valves.

Energy harvesting offers the following important advantages over alternative solutions:

• WithEHtechnology,very-low-powerproductscanbepoweredfortheirlifetimes,cuttingservicing and battery replacement costs.

• Incommercialapplications,energyharvestingdoesawaywiththeneedforperiodicshut-downs for sensor maintenance. This is critical in large refineries and petrochemical process-ing plants that operate continuously for years or decades and have to incur huge costs during shutdowns.

• Energyharvestingisenvironmentallyfriendlybecauseitreducestheflowofreplacementbatteries into landfills, thereby decreasing the waste stream.


• EH-basedsensordesignsallowmeasuringdevicestobefitteddeepinsideaircraftwingsandother structures, places very difficult to access for maintenance after final assembly.

Basic Energy-Source Design Information

Engineering teams aiming to harvest the free energy in the environment should consider the fol-lowing facts about devices that can be used to capture the available power:

• Photovoltaic(solar)cellshaveanoperatingvoltagebetween0.5Vand0.7V.Individualcellsare frequently connected in series or parallel combinations to meet the required voltage and current requirements. Different materials are used to obtain maximum efficiency from indoor or outdoor light.

Single-crystal solar cells typically have lifetimes exceeding 10 years. However, some of the newer organic solar cells degrade dramatically from the sun’s ultraviolet rays and hence have lifetimes of 5 years or less if used outdoors. Datasheets should be studied thoroughly and if system engineers have any doubt about which type of solar cell to use in an energy harvesting application, consulta-tions with experts are recommended.

• Vibrationandmotionharvesterscomeintwomaintypes,asdiscussedinSectionII:thosethat use piezoelectric materials and those that move a magnetic core in a coil of wire. Piezo-electric devices undergo small deformations as they vibrate and those deformations cause produce a voltage across the material. The impedance of such energy sources is generally quite high.

Motionharvestersinswitchesandotheractuatorscontainaninternalcoilandmagneticcorepo-sitioned such that operator actions induce a current to flow through the coil that can be captured. Miniatureversions(MEMSdevices)typicallyhaveamasssecuredtotheendofabeam.Thatassembly is designed to resonate at the primary vibration frequency of the object to which it is attached. As the tiny beam moves, a coil attached to it swings back and forth though the field of a small magnet, causing an alternating current in the coil that is subsequently rectified and stored.

• Thermoelectricgenerators(TEGs)usethepreviouslymentionedSeebeckeffect.Asmallpieceof semiconductor material, typically Bismuth Telluride (BiTe), is attached to a heat source on one side, and to a heat sink on the other side. The amounts of heat flux that can pass through the TEG and the temperature differential between the hot and cold side determine the power produced.

There is a common misconception that if you place a thermoelectric generator into a hot environ-ment, it will product lots of energy. But in fact, the TEG only generates power if one side of the material is kept cooler than the other side. TEGs can generate quite high currents, but usually at low voltages, between 50 and 500mV. A boost converter is usually required to transform this out-put into a voltage high enough to drive electronic circuits.

• RFsourcescanbeharvested,butdoingsoproducessuchsmallamountsofenergythattheyare useful only in special circumstances. A notable exception is Near Field Communication. NFC conforms to a standard created by Sony and other companies for low-data-rate trans-missions over short distances—about 1 inch. It provides power to transaction cards when theyareplacedwithintheelectromagneticfieldproducedbya13.56MHzcoil—inapoint-of-sale (POS) terminal, for example.


Amounts of Power Available from Key Sources

Figure 10 illustrates the amounts of energy available for harvesting from different sources in typical application environments.

Figure 10: Power available from Energy Sources in the Environment.

The data in Figure 10 shows that the best sources to tap for energy harvesting are the thermal en-ergy available in industrial plants, particularly manufacturing and chemical processing operations, and the energy from sunlight, provided that a suitable location can be found for a solar cell.

Characteristics of Energy Harvested from Different Sources

Earlier discussions mentioned that harvested energy often is inconsistent. In fact, it might be described as being ‘badly behaved’. This fact is explained in the paragraphs below.

• Solar—Illumination intensity of can vary from 150 lux (in warehouses, homes and theaters), to 500 lux (in office environments), to 1000 lux (in detailed drawing shops or outside on an overcast day), and up to 10,000 lux (in direct sunlight). Energy-harvesting efficiency depends on the spectral content of the light shining on the cells. Obviously, these factors must be con-sidered when selecting and sizing a solar cell. But an important fact is that in some instances the amount of output may vary tremendously, even when the sun is shining.

An outdoor solar cell may be generating many mA of current at 5V in bright sunlight. However, if clouds obscure the sun, or if a shadow falls on the cell, the current it produces may drop to tens of µAorless,andthevoltagemightdropbelow4V.Undersuchacondition,thecellwouldhavedif-ficulty charging a Lithium battery. This scenario typifies the sort of ‘badly behaved’ energy source that a power management ICs must handle, and it indicates the range of currents and voltages for which that chip must compensate.

• Piezoelectric—A piezoelectric generator uses mechanical vibrations to produce open-circuit voltages that can easily be hundreds of volts. However, this type of energy source has a very high internal impedance; in fact, it’s so high that a piezoelectric device can’t drive much cur-rent. Also, the polarity of the voltage and current reverses as the deflection or direction of vibration changes. Thus, it’s another example of a ‘badly behaved’ source.

• Thermal—Thermoelectric generators (TEGs) generate sizeable currents—they can be tens or hundreds of mA—but only at a few hundred millivolts. A boost circuit is needed to increase the voltage up to levels sufficiently high to charge a battery.

Energy Source Harvested Power

Photovolataic – Office – Outdoor

10µW/cm2

10µW/cm2

Vibration/Motion – Human – Industry

4 µW/cm2

100µW/cm2

Thermal Engergy – Human – Industry

25 µW/cm2

1-10 mW/cm2

RF – GSM (900MHz) – Wi-Fi (2.4GHz)

0.1 µW/cm2

0.01 µW/cm2


Power Levels Required for Typical Applications

It’s instructive to compare the amount of power obtainable from photovoltaic, vibration/motion, thermal energy and RF sources shown in Figure 10 with the peak power requirements of typical applications that apply energy-harvesting technology. The latter are shown in Figure 11.

Clearly, it’s very important to know both the peak power (current) and the average power (current) required by a product.

Figure 11: Peak Power Levels of Typical Embedded System Applications.

The storage element in an energy-harvesting sensor is usually sized according to the average power (current) requirements and the time between charges. However, if the peak power (current) is much larger than the average power, (say 5x or more larger), then the capacity of the storage device might have to be increased so that its internal resistance becomes low enough to properly power the load. This is commonly the case with sensor designs that incorporate radio modules that can draw 20mA or more during transmissions and receptions, while the rest of the embedded systemmayconsumeonly3or4mA.

In the paragraph above, “current” is shown in parentheses for the following reason. Although power is ultimately the parameter of concern, most batteries are sized in mAh. Therefore, it is usually easiest to tabulate the total current consumed by all of the devices in a design. Knowing the total current draw, it is a simple task to multiply that number by the supply voltage to obtain the power needed to operate the sensor product.

MCUs

Themicrocontrolleristhedigitalbrainofanyembeddedsystem.Thus,choosingtherightMCUforanEH-basedsensorproductisacriticallyimportantdesigndecision.Ideally,theoptimumMCUforbattery and remote applications such as security sensors will offer the following features, among others:

• Avery-low-powerarchitectureprovidingmultiplepower-downmodesformaximizing battery life

• Goodperformanceforfast,efficientprocessing

• Veryfastwake-uptimesfrompower-downmodestoensurethatthesystemspendsthe greatest possible amount of time in a low-power state, yet responds quickly to deliver essential system operational capabilities.

• AhardwareDigitalSignalProcessor(DSP)forrapidlyandconditioningrawsignals,filteringsensor outputs, determining a signal’s spectral content, and eliminating false signals.

PeakPower

EHF Low Power Zone

10nW

1µWRFID Tag

Sensors/Remotes

Hearing Aid/Wireless Sensor

Bluetooth Transceiver

100mW

1W LaptopComputer

10W

100nW

Real Time Clock (RTC)

Watch/Calculator

10µW

100µW

1mW

10mWGPS

GSM Cell Phone

Power Tools

100W

These lower power consumption devices are ideal candidates for Energy Havesting

solutions


True Low Power™ capability of the RX111

The32-bitRenesasRX111MCUisanidealdesignchoiceforsensorproductsthatapplyenergy-harvesting technology. This inexpensive chip combines a breakthrough power-control technol-ogy— True Low Power™ capability— with exceptional features such as ultra-fast wake-up times, zero-wait-state flash memory and enhanced DSP capability. It also provides multiple safety func-tions and a host of advanced peripherals, including USB 2.0 support, LCD Drive capability, Real Time Clock (RTC) and a Capacitive-Touch Sensing Unit (see Figure 12).

Figure 12: Key Features of Renesas’ RX111 MCU.

TheRX111’sthreepower-controlledRunmodes(High-Speed,Middle-Speed,andLow-Speed)andthree Low-Power modes (Sleep, Deep Sleep and Software Standby) can be programmed to keep or make different combinations of on-chip functions operational, allowing excellent system design flexibility. In sensor applications, for instance, a common requirement is to wake up the system when an event has occurred, or on a periodic basis, using the built-in Real Time Clock (RTC).

Renesasmanufacturesthischipwiththesame130nmlow-power,low-leakageprocesstechnologyusedsuccessfullyinthepopularRL78seriesofMCUs.

Design Features of the RX111 MCU

KeyfeaturesandcharacteristicsoftheRX111MCUincludethefollowing:

• ExceptionalRun-modepowerefficiency:100µA/MHz

• Sleep-modepowerconsumptionaslowas310nA

• Ultra-fastwake-uptime:4.8µs

• Superiorarchitecture:3.08CoreMarks/MHzperformance

• Sixoperatingmodes,plusnumerousotherdesignoptionsforsavingpower

• Standardandadvancedon-chipperipherals:ADC,LVD,RTC,USB,andmore.

Power-Controlled Operating Modes

TheRX111MCUletssystemengineerstailortheavailableprocessingcapabilityandthechip’spower consumption to match the computational requirements of diverse application tasks. As previously mentioned, each of the CPU’s three power-controlled Run modes makes available a dif-ferent set of on-chip peripheral modules (see Table 1). Restrictions apply, though. The availability of some oscillators, the PLL, Flash memory programming and certain peripheral clock frequen-cies depends on the Run mode selected. Note that the graphic below is TABLE 1, not Table 2, as indicated.

Efficient RX Architecture

Advanced Clock System

Module Power Shut-off

Zero-wait-state Flash

Multiple Run Modes

Multiple Power Modes

Low Leakage Process


Table 1: Clock Sources Usable in RX111’s Power-Controlled Run Modes.

Mode PLL HOCO LOCO Main Osc. Sub Clock

High-Speed Usable* Usable Usable Usable Usable

Middle-Speed Usable* Usable Usable Usable Usable

Low-Speed Not Usable Not Usable Not Usable Not Usable Usable

* VCC ≥ 2.4V

TheMCU’ssupplyvoltagerequirementsaren’taffectedbythepower-controlledRunmodes. Operationisalwaysallowedoverthedevice’sfull1.8Vto3.6Vrange.However,theclock frequenciesusableintheHigh-,Middle-,andLow-Speedmodesdodependonthesupply voltage (see Table 2).

Table 2: Maximum Clock Frequencies in Run Modes.

Power- controlled Operating Mode

Operating Voltage Range

Operating Frequency Range

ICLK FCLK PCLKD PCLKB

High-Speed

3.6to2.7V Upto32MHz Upto32MHz Upto32MHz Upto32MHz



Middle-Speed 3.6to1.8V Upto8MHz Upto8MHz Upto8MHz Upto8MHz

Low-Speed 3.6to1.8VUp to 32.768kHz

Up to 32.768kHz

Up to 32.768kHz

Up to 32.768kHz

Details of RX111’s Low-Power Operating Modes

IntheMCU’slow-powerSleep,DeepSleepandSoftwareStandbymodes,differenton-chipfunc-tions are stopped or powered down, saving various amounts of current. Here are the details:

• Sleepmode—TheCPUisstoppedwithdataretained.ThisreducestheCPU’sdynamiccurrentconsumption,whichisasignificantcontributortotheMCU’soveralloperatingcurrent.TheCPUwakesupfromSleepmodeintotheRunmodeinonly0.21µsat32MHz.

• DeepSleepmode—TheCPU,RAMandFlashmemoryarestopped,withdataretained.At32MHzwithmultipleperipheralsactive,thetypicaloperatingcurrentisonly4.6mA.Ittakesjust2.24µsfortheCPUtowakeupfromDeepSleepmodeandenterRunmode.

• SoftwareStandbymode—ThePLLandalltheoscillatorsexceptthesub-clockandIWDTarestopped.AlmostalloftheRX111’smodules—CPU,SRAM,Flash,DTCandperipheralblocks—are stopped, with data retained. The Power-on-Reset (POR) circuit remains opera-tional,however.Also,ifnecessarytheIWDT,RTC,andLVDmodulescanbeoperated.Currentconsumptioninthismodeisfrom350nAto790nA,dependingonwhetherornottheLVDandRTCfunctionsareused.Whenwakingupinthe4MHzRunmode,CPUoperationbeginsaftera4.8µsdelay.Whenwakingupinthefast32MHzRunmode,thewaittimeextendsto40µs.


Table3showsthepowerconsumptionlevelsandwake-uptimesoftheRX111MCU.

Table 3: Power Consumption and Wake-Up Times of the Renesas RX111 MCU.

Additional RX111 Power-Saving Capabilities

AlthoughtheSleep,DeepSleepandSoftwareStandbymodesoftheRX111MCUareveryhelp-ful for decreasing current consumption, other techniques can achieve further power reductions. For instance, various clock-signal frequency-division ratios can be set individually. This capability applies to the system, peripheral module, S12AD and Flash clocks. It’s a very useful control feature when application requirements differ between on-chipfunction blocks.

Additionally, each peripheral module in the RX111 has a separate Stop control bit. This allows softwaretoindividuallycontroltheMCU’son-chipfunctionstoobtainfurtherreductionsindy-namic current.

Power Management Devices

As stated earlier, very-low-power remote sensor products that utilize energy-harvesting technol-ogy need a power-management device, due to the variable nature of the voltage and current produced by the harvester. That device converts that unregulated voltage and current into regu-lated electrical energy that can be accumulated in a storage device. It can also supply power to the system load at the proper voltage.

Typically the power-management chip includes circuits for protecting both the load and the energy-storage device, the most common of which are the following:

• Under-VoltageLockout—Thisprotectioncircuitturnsoffpowertotheloadandconservesthecharge in the battery when the output voltage becomes too low. This capability is important because if the battery is discharged too much, it can experience permanent damage and lose some or all of its storage capacity.

• Over-VoltageProtection—Thisfunctionmonitorsthechargevoltage.Ifthevoltagerises too high, the power-management chip either shunts the excess charge to ground, or electronically inserts a high impedance between the harvester and battery that prevents additionalchargefrombeingpushedintothebattery.Whichevermethodisused,thebatteryis protected.

MCU Configuration

Wake-up Time

Current Consumption (25C, 3.3V)

Power Mode

CPU Clock

Peripheral Clocks Mode Regulator LVD HS OCO HS Ext

Osc PLL LS OCO32KHz

Ext Osc (RTC)

RAM State

I/O Pin State

Code Source at Wake-up

Wake-up Sources Frequency Min

(mA)Typ

(mA)

Active (Run) ON ON/OFF

High ON (NVHC)

ON Clock ON (32MHz) OFF OFF Clock OFF ON Active Active Flash

Any Interrupt, LVD, POR, Ext Reset

–

32MHz 3.2 10.6

High ON (NVHC) 8MHz 1.7 3.7

Middle ON (LVHC)

8MHz 1.32 3.5

4MHz – 2.15

1MHz 0.74 1.2

Low ON (LVLC) 32KHz 0.00396 –

Sleep OFF ON/OFF

High ON (NVHC)

ON Clock ON (32MHz) OFF OFF

Clock OFF

ON Active Active Flash


0.21µs 32MHz 1.8 6.4

Middle ON (LVHC) Clock OFF 0.875µs 8MHz 0.9 2.2

Low ON (LVHC) Clock OFF 7µs 1MHz 0.7 1

Deep Sleep OFF ON/OFF

High ON (NVHC)

ON Clock ON (32MHz) OFF OFF

Clock OFF

ON Active Active Flash


2.24µs 32MHz 1.2 4.6

Middle ON (LVHC) Clock OFF 3.55µs 8MHz 0.7 1.8

Low ON (LVHC) Clock OFF 15.80µs 1MHz 0.6 0.9

Software Standby OFF OFF Standby ON (LVLC)

ONPower

OFF Clock OFF

OFF

Power ON

Clock OFF

Power On

Clock OFF

ON

Retain RetainFlash

(Powered On)

Any External Interrupt Pin, POR,

RTC Alarm, Wake-up, Ext Reset

4.80µs (4MHz)

40µs (32MHz)

790nA

OFF 450nA

OFF

ON 690nA

OFF 350nA

Min:Peripheralclocksallstop,CPUNOOP-Loop,Flashaccess25%,Peripheralmodulesallstop Typ:Peripheralclocksallrunningnodivider,CPUallcommandoperation,Peripheralsmoduleson–DTC/RSPI1channel,MTU1channel,CMT1channel


• Over-CurrentProtection—Thisprotectioncircuitperformsafunctionsimilartoacircuitbreakerinahouse.Whenanexcessiveamountofcurrentisdeliveredtotheload,theover-current circuit isolates the load from the battery, ensuring that heavy loads don’t pull the batteryvoltagedowntoolow.Whenthisfunctionactivates,thereusuallyisafaultconditionin the system that must be corrected before normal operation can resume.

The MAX17710 PMIC

Maxim’sMAX17710—shownschematicallyinFigure13—isagoodexampleofthelatestgenera-tionofhighlyintegratedPowerManagementICs(PMICs).ItisdesignedtooperatewithThinFilmBatteries (TFBs) and offers all of the requisite capabilities, not only for protecting and charging the battery, but also for managing the power to the load.

SeveralinnovativefeaturesareincorporatedintotheMAX17710.Aspreviouslydiscussed,onecharacteristic of TFBs is that their internal resistance increases dramatically at low tempera-tures, limiting the amount of current they can supply. To compensate for this phenomenon, the MAX17710PMICcanchargeanenergy-storagecapacitorfromtheTFB.

Ifforanyreasonthethin-filmbatterycannotdeliverthefullcurrentrequiredbytheload,thePMICdischarges the power stored in the capacitor into the load, thus sourcing the necessary supply cur-rent. After the current required by the load diminishes—when the radio transmitter turns off, for instance—the power management device recharges the SuperCap from the TFB.

Fig 13: Maxim’s MAX17710 Power Management IC.

Whensensorproductsincorporateawirelessconnectivitymodule,thatradiodevicedrawsasignificantamountofcurrentwhenittransmitsdata.TheEM9301thatisdiscussedinthenextsection,forexample,requires12mAonTxand13mAonRx.Fortunately,mostmessagescanbelimitedtolessthan5ms.TheMAX17710PMICcouldswitchona32µFstoragecapacitorthatcouldsupplythatcurrentforthefull5ms,dischargingfrom3.8Vdownto1.8Vintheprocess.ThePMICwould then use the TFB to recharge the capacitor.

If that the thin-film battery is Cymbet’s EnerChip CBC050, it can deliver a maximum current of 300µAtoaload.Thiswouldrechargethe32µFcapacitorinlessthanonesecond,quicklymakingthe sensor system ready for the next transmission or reception. At cold temperatures, it would take longer to recharge the capacitor, but this is still a viable design solution.

PCKP

REG

SEL13.3/ 2.3 /1.8V

Select

0.1µF

1µF

MAX17710BATT

CHG

0.1uF

GND

LXBATT

Boost Reg

PGND

FB

REF

SEL2

CPCKP

PATENT PENDING

Output Linear Reg

Linear Charge & Ideal Diode Control

Shunt Protection to Reject Overcharge

Disable

LCEµP

AE Event Detector

*State

Machine

Overdischarge and UnderVoltage protection

Thin Film Battery

Energy Source #3

(Solar, Piezo, Coil, RF

Antennae, etc.)

Energy Source #2

(Solar, Piezo, Coil, RF

Antennae, etc.)

Energy Source #1

(Solar, Piezo, etc.)


TheMAX17710providesaspecialshutdownmodeifthebatteryvoltagedropsto3V.Whenthathappens,thePMICdisconnectsthebatteryfromtheload,reducingthedrainonthebatterytolessthan 1nA and conserving all of the remaining charge in the battery. Again, this protection feature is important because if the TFB’s voltage drops below approximately 2V, the battery will be perma-nently damaged.

AboostfunctionisanotherfeatureprovidedbytheMAX17710PMIC.Itenablesthermoelectricgenerators to charge the thin-film battery without the need for a separate boost regulator chip.

LTC4071 Shunt Battery Charger

Linear Technology is another supplier of devices for managing power within energy-harvesting systems. It offers several chips that can be used together or separately to implement different capabilities.Figure14showsthecompany’sLTC4071shuntbatterycharger.

Figure 14: Linear Technology’s LTC4071 Shunt Battery Charger.

New power-management solutions are being introduced by semiconductor companies besides MaximandLTC.Specifically,IntersilandTexasInstrumentsareamongtheothersthatnowhaveofferings in this area. Again, comprehensive product searches are advised before making design choices.

Wireless Connectivity Modules

Mostwirelesschipsandmodulesoperateinthe2.4GHzISMbandusingZigBee,Z-Wave,orBlue-tooth Smart protocols. The system design trend today for establishing reliable communication links between remotely situated system elements is to apply the latter.

4071 BD

3-STATEDETECT

OSCCLK

ADJ

0.9sec – 7sec

PULSEDDUTY CYCLE = 0.003%

30µs – 200µs

NTCBIAS

NTC

RNOM10k

10kT

–+

–+

–

+HBO

LBSEL LBSEL MUST BE TIED TO V CC OR GND

VCC

BAT

GND

EA

ADC

LTC4071

1.2V 1.2VMP2

MP1

BODYDIODE

Li-IonBATTERY

+

VIN

RIN

SYSTEMLOAD


The EM9301 Bluetooth Smart module

One solution for adding Bluetooth Smart capabilities to an embedded system is the small, low-en-ergyEM9301radiomoduleofferedbyEnergyMicroelectronics.It’sidealforimplementingBlue-tooth Smart wireless communication in portable devices and very-low-power applications.

Figure 15: Simplified Block Diagram of the EM9301 Bluetooth Smart Module.

TheEM9301isoptimizedforultra-lowpowerwirelesssensing,remotecontrol,andmonitoringapplicationsandoperatesonaslittleas0.8V.Itcanbepoweredfromawiderangeofsingle-cellbatteries or energy harvesters.

The device combines the Physical layer, Link layer and Host Controller Interface (HCI) layer func-tions.TheflexiblechipisfullyBluetoothSmartqualifiedforsingle-modeMasterandSlaveap-plications.

InReceivemode,theEM9301consumesonly13mA,muchlessthancompetingproducts,whichrequireupto23mA.AnintegratedDC/DCconverterpowerstheEM9301RFcircuitrywhileconcur-rentlydeliveringupto100mAtoanexternaldevice(e.g.,sensors,systemMCU,LEDindicators,ordisplaysanddrivercircuits).ItsRFoutputpowerisprogrammablefrom+4dBmto–20dBmin6steps.

PuttingtheEM9301intheSleeporIdlemodes,inwhichitconsumeslessthan1µA,savessystempower.Theradio’soverallimplementationbillofmaterial(BOM)andtheitssizecanbereducedsignificantly by the proper design of a 200Ω differential-impedance PCB trace antenna. That design approach eliminates the need for matching, balun, and antenna components.

RST

VCC2

ANTP

ANTN

AV

SS

_PLL2

AVDD_PA

AVSS_PA

AV

SS

_PLL1

AV

SS

_RF

VSS BIAS_RVDD

Power Management

Digital LDO

Bandgaps RF core LDOs

Central Biasing

SEL

VBAT BatteryLevel

Detector

VCC1

SW_DCDCVSS_DCDC

DCDCConverter

XtalOsc

XTAL1

XTAL2

Control Logic

IRQ

WU/CSN

SPI_SCK

UART_RX/SPI_MOSI

UART_TX/SPI_MISO

HostControllerInterface

BluetoothLow

EnergyController

FrequencySynthesizer PA

LNAIF Filter

&Demod

RF Core

4 5 19 6 7

9

21

22

11

24

10

3

2

1

23

8

18

13

16

15

14

12 20 17

EM9301


IV. Example Design: Glass Break SensorTo put into perspective the major design issues related to the development of very-low-power em-bedded system products that utilize energy harvesting, this section of the white paper describes an example project: An electronic glass break sensor that’s a basic element in a building’s security system. The design quickly and accurately detects when a window breaks and wirelessly sends a signal to a central monitoring system to trigger an alarm.

This example design is presented for illustrative purposes only. It is not meant to be a guide for how to build an actual glass break sensor. Instead, the discussion aims to show how to tackle an energy-harvesting problem, while demonstrating some of the analysis and calculations required to implement a successful solution. The practical tips and pointers presented will help design teams avoid pitfalls that might cause schedule delays. However, the guidance offered shouldn’t be fol-lowed exactly.

A better security solution

The example electronic glass break sensor avoids the many negatives associated with the tradi-tional security approach of installing strips of thin metalized tape around the perimeter of a glass pane. If the window breaks, that tape either tears or its resistance changes. The resulting open circuit or change in resistance is detected to sound an alarm. Installing the tape is expensive and it disfigures the perimeter of the window.

By contrast, an electronic glass break sensor system leaves the entire window clear. If produced in volume, this type of effective, reliable security product could be low in cost, small, unobtrusive, maintenance-free, flexible and easy to install.

ThehypotheticalsystemdesignisshownbelowinFigure16.Itusesoftwosensors:oneforwak-ingupthesystemMCUandanotherformeasuringthevibrationsfromthewindowpane.Themicrocontroller that processes the signal from the vibrations, and a wireless transmitter sends an alarm signal to a remotely located security control center.

This sensor system applies energy-harvesting technology to obtain reliable, maintenance-free operation for decades without external wired connections that could possibly be severed.

Figure 16: Block diagram of Example Glass Break Sensor.

Of course, there are multiple challenges in creating such a glass break security sensor. But per-haps the biggest one is to keeping the power needed to operate the embedded system low enough that a small energy harvester can power it indefinitely. This is an extreme case of ultra-low-power design.

Minimizing the sensor system’s power consumption

The secret to successfully implementing this security product is to keep circuit elements powered off except when absolutely necessary. The example design is built around the Renesas RX111

Solar Cell Mic

Front Side, Sticks to Window Glass

Double Sided Tape

MCU EM9301

Antenna

Batteries

Chip Cap Mic


MCU,achipthatofferspower-downmodesdrawingaslittleas350nA.Inoperation,thesensorwakes up this microcontroller only when a suspicious event occurs. Under normal circumstances, energy-harvesting technology readily handles the sensor system’s miniscule power levels.

If the glass in the window breaks, the resulting sharp vibrations cause the ‘Alert’ transducer to generateavoltageononeoftheMCU’soneofitsinterruptlines,quicklyputtingtheCPUintoRunmode. As noted above, the Renesas RX111 wake ups within a few microseconds.

Afterit’sinRunmode,theMCUcapturesthesignalfromthewiderbandwidth,moreaccurate‘Data’ sensor using its ADC, and then uses its DSP hardware to analyze the signal’s spectral con-tent to determine if it matches the spectrum of glass breaking. That is, it checks whether or not the signal contains a lot of very high frequency content.

The event analysis capability is a very important feature of this glass break sensor design. It mini-mizes or eliminates false alarms caused by non-catastrophic disturbances such as a pane bending duestrongwindsorthingsbumpingintothewindow.TheMCUignoressucheventsbecausetheycontain less high-frequency content.

Windowpane breakage sensors

As indicated above, the example design gets from two separate sensors: One that wakes up the MCUwhenaneventhappens,andasecondforaccuratelymeasuringthespectrumofthevibra-tion of the windowpane.

Some experimentation is required to find the exact spectral criteria for confirming window break-age. In general, though, when glass breaks it creates an abundance of high frequency energy, rangingfromabout20kHzto1MHz.Breakageisacatastrophiceventthatproducesadistinctspec-tral signature that can be readily separated from the spectra of casual events.

ApiezoelectrictransducersuchasMide’sV20Wwithzero-gramtipmassisusedforthe‘Alert’sensor.Itgeneratesanopen-circuitvoltageof4Vwithaslittleas0.25gofacceleration.It’sagoodchoiceforwakinguptheMCUwhensuspiciousvibrationsaredetected.

BecausetheV20Wpiezoelectricsensorgeneratesbothpositiveandnegativevoltages,itssignalgoestoafull-waveSchottky-diodebridgerectifiertoensurethattheMCUgetsapositivesignalre-gardless of transducer polarity. Specifically, the output of that rectifier goes through a 1kΩ resister toa3.6Vzener-diodeshuntacrosstheIRQpinontheRX111MCU.ThezenerdiodeprotectstheIRQpinagainstdamaginghigh-voltagespikes.

To help eliminate false sensing from non-breakage events, a low-pass R-C filter can be inserted beforethesignalenterstheIRQpin.

The second transducer, the ‘Data’ sensor, is the microphone that accurately measures the vibration signature.AtinyMEMSdeviceisagoodchoicehere.Thedeviceoperatesfrom1.8to3.6Vwhileconsuming0.65mA,acurrentlowenoughtobedrivenfromoneortwooftheRX111’sGPIOpins.Thosepinsareratedtosource4mA,soputtingtwopinsinparallelensuresplentyofcurrentdrive.

ThatMEMSsensortakes10mstowakeup.Someapplicationsmightrequirefasterturn-onperfor-mance. Experimentation with the complete system helps to fine-tune such development issues. BecausethefrequencyresponseoftheMEMSmicrophonerolls-offsignificantlybeyond100kHz,nosamplingbeyond200kHzisnecessary,eventhoughthewindowglassvibratesatupto1MHzwhen it breaks.

Signal acquisition

TheRX111MCU,theheartofthissystemdesign,drawslessthan1µAinapower-downmodeandhasanunder-5µswake-uptimeafteraninterrupt.Also,thechipincorporatesanADCfordigitiz-ing the vibration signature and provides a built-in low-power hardware DSP circuitry that quickly performs spectral-analysis calculations.


Whenthewindowpaneexperiencesabreakageorisotherwisedisturbed,causingtheRX111toreceive an interrupt from the ‘Alert’ sensor, that signal puts the CPU in Run mode if it has been asleep. If it is already awake, the interrupt causes it to gracefully exit the task it’s executing. The ADC is then turned on to initiate acquisition of event’s vibration signature from the ‘Data’ sensor.

During signature acquisition, the ADC operates at 200kHz so that frequency content up to 100kHz canbediscerned.ThisrequirestheMCU’sclocktorunatabout6MHz.ThedesignhasaVCCof3.6V,whichsupportsthefastestconversionspeed.

Itshouldbesufficienttotakeonly0.1secondsofsamples(200,000x0.1=20ksamples)afterasuspicious event. However, experimentation may be necessary to determine exactly how many samples are required and at what frequency they must be acquired.

InRunmodetheRX111’sADCconsumes1mAanditsCPUconsumes0.6mA,foratotalof1.6mAduring the sample acquisition process. Assuming that this takes 0.1 second, the total Ah con-sumedfromthebatteryis0.1sx1.6mA=160µAs=160µAs/3600(seconds/hour)=0.04µAh.This energy expenditure is negligible compared to the power consumed by the rest of the sensor system.

Signal analysis

To analyze the spectral content of the vibration, a Fast Fourier Transform (FFT) has to be per-formedonthe20,000datapointsproducedbytheADC.Thecalculationsinvolveabout80,000multiplies, many additions, subtractions, scaling operations, etc. Using the RX111’s multiplier, di-vider and barrel shifter to handle DSP functions greatly speeds this computational task compared withMCUsthatlacksuchbuilt-inhardwareandthusmustperformtheminsoftware.Theextraspeed decreases power consumption, besides improving the system’s response time.

Intheexampledesign,iftheMCUclockisboostedto32MHzfortheDSPcomputations,thespectral-analysis FFT is calculated in less than a second. By contrast, a typical software computa-tion time is several minutes, long enough for an intruder to disable or destroy the sensor.

To emphasize: Using the RX111 with its DSP hardware both reduces power requirements and dra-matically reduces the time needed to analyze the event’s signal spectra—an advantage that might be critical in some security situations.

The power the RX111 consumes in performing the FFT (assuming the worst case of 10 seconds for completingthecalculation)is3.2mAx10s/3600(seconds/hour)=9µAh.Ofcourse,thisisarareactivity, one that’s only activated when there is a disturbance on the windowpane.

TheMCUisputintoDeepSleepmodewhenitisn’tanalyzinganeventorcommunicatingwiththeremotely located main security system controller; i.e., nearly all the time. In that mode with the real-timeclock(RTC)running,theRX111draws650nA.In24hours,thisamountsto15.6µAh.SothisisthelargestcontributiontotheaveragepowerconsumedbytheMCU.

The reality is that for many types of monitoring equipment, the quiescent power draw dominates theoverallpowerbudget.Quiescentcurrentisconsumed24/7/365,whereastheactivitiesbeingmonitored cause infrequent current spikes.

Wireless communication

For window-break sensor products that will be installed in a households or small buildings or sales and office spaces, Bluetooth Smart is a good protocol for communicating with a central con-trol unit. It uses relatively low power, has sufficient range and can be implemented by a number of certified chips or modules. For the longer-range communications necessary for protecting larger commercialbuildings,theZigBeeorZ-Waveprotocolsmightbebetterchoices.

Forthisexampledesign,theEMMicroEM9301BluetoothSmartdeviceprovideswirelessconnec-tivity.ThedesigncalculationsthatfollowarebasedontheemBeaconexamplefoundontheEMMicroelectronicswebsite.TheenergyprofilefromthatexampleisshowninFigure17.


Figure 17: Power Consumed by the EM9301 Bluetooth Smart During a Transmission.

AlthoughEMMicro’sreferencedesignconfigurestheEM9301inbeaconmode(Txonly),theradiomodule in the example glass break sensor uses both the Tx and Rx modes.

In remote security applications, good design practice mandates regular communication between the sensor and the central controller. Those data exchanges often called a ‘heartbeat’ signal. If the regular communications linkup fails to occur for any reason, the controller can flag this as a problem. Alternatively, it might wait until the heartbeat is absent two or three times in succession before setting a fault alarm.

In the example sensor design, heartbeat communication is assumed to occur every 5 minutes, un-less glass breakage is detected.

Power consumed during transmission and reception

The data within the packet that’s sent to the security controller includes temperature, battery volt-age—and most importantly—whether or not there was a glass-breakage event. Figure 17 shows thatthetotalcurrentusedbytheEM9301fortransmittingthisinformationis92µAsfromstartuptoshutdown. Since transmissions normally occur once every 5 minutes, the total power consumed by the Bluetooth Smart module’s data transmissions in one hour is

92x60(minutes/hour)/5(events/minute)=1,104mAs

=1,104µAs/3600(seconds/hour)=0.3µAh

However, to complete the sensor’s handshake with the central controller, the power consumed bytheEM9301’sreceivermustalsobecalculated.Theradiodraws13mAinreceptionmode.Thisexample design assumes that the receiver has to remain on for 20ms after sending the heartbeat signal in order to receive the acknowledgement signal sent from the central controller. During this intervaltheEM9301consumesatotalof13mAx20ms=260µAs


Because recptions also occur 12 times per hour, the total current consumed in Receiver mode in one hour is

260µAsx12=3.1mAs

=3.1mAs/3600(seconds/hour)=0.9µAh

Intotal,ittakesonly1.2secondseveryhourfortheEM9301toperformBluetoothtransmissionsandreceptions.DuringthattimetheEM9301drainsthesensorsystem’sbatteryby1.2µAh.Whenthesensorsystemisn’tcommunicating,theMCUputstheradiomoduleintoDeepSleepmode,whereitconsumes9µA.Thus,inonehourtheBluetoothSmartmoduleconsumes9.0µAh.

For the purposes of calculating a power budget, the power consumed by an actual glass-breakage event can be ignored, since that happens only rarely and draws a relatively small amount of cur-rent when it does happen.

Therefore,totalpowerconsumedperhourbytheEM9302is

Tx + Rx + Deep Sleep

=0.3µAh+0.9µAh+9.0µAh=10.2µAh

Deep Sleep mode power consumption

Again, it’s worth pointing out that the Deep Sleep component of the radio module’s power us-age dominates its total power consumption, a reality that arises frequently in ultra-low power designs. To eliminate this power usage, a p-channel FET switch could be connected between the positivepowerrailandtheEM9301andturnedonbytheMCUonlywhenawirelesstransmissionisrequired.TheFETcanbecontrolledfromaGPIOpinontheMCU,assumingthatthemicrocon-troller’s quiescent power is sufficiently low, as it is with the RX111.

WhentheRX111wakesup,itturnsontheFET,applyingpowertotheBluetoothSmartmodule,asdescribedabove.EliminatingtheEM9301’sDeepSleepquiescentcurrentmakesthesensorsystemmuch more power efficient.

Energy storage and power management

Intheexampledesign,theRenesasRX111MCUkeepstheaveragecurrentconsumptionbelow1µA.TheEM9301wirelessmodulepoweredviatheFETswitchconsumesanother1µA.There-fore,thesensorsystem’stotalaverageconsumptionisonly2µA.Thatbeingthecase,avenerableCR2032lithiumbatterycouldpowertheexamplesensorforaperiodofabout5years,asFigure3on page 7 indicates.

Because the goal was to create a glass break sensor that can operate for longer time span without maintenance, it applies energy harvesting technology. A successful design requires a low-leakage storageelementthatcansupplytheaveragecurrentforoneday—36µAhiftheenergyharvestercanchargethatbatteryforatleast6hoursperday.

The storage element, in conjunction with the power management chip, must be able to supply thesensor’speakcurrentrequirements.ThecombinationofCymbet’s50µAhCBC050solid-statebattery,Maxim‘sMAX17710powermanagementICandastoragecapacitorcandeliverthe20mApeak currents for the short periods (~20ms) required for Bluetooth Smart handshake transmis-sions.

TheMAX17710usesaclevertechniqueofchargingacapacitorfromthesolid-statebattery,andit’sthecapacitorthatdeliverstherequisitepeakcurrentneededbytheEM9301radiochip.Thecapaci-tor can be recharged in several seconds from the CBC050 after being discharged during wireless transmissions.

AdesignchallengerelatedtotheCBC050batteryisthatifitdischarges36µAhduringthenight,this is about 70% of its capacity. That deep discharge will reduce the battery’s lifetime to less than


1000cycles(3years).OnesolutionwouldbetoconnectfourCBC050batteriesinparallel.Thisreducesthedailydepthofdischargeperbatterytoabout10µAh,ora20%DepthofDischarge,andthat reduction extends the battery lifetime to beyond of 10 years.

Energy harvester

Windowsinstructuresarealmostalwaysreceivelightforsomeportionofaday.Therefore,asolar cell would be a good choice as an energy harvester for the example sensor design. Although many companies make solar cells, most of their products aim at ‘Big Solar’ installations that gen-erate kilowatts or megawatts of power. Fortunately, small, efficient solar cells are available from various suppliers.

Sanyo Amorton, for instance, offers a relatively broad selection in different sizes that are useful for bothindoorandoutdoorlighting.Websearcheswillrevealothersuppliersthatoffersmallphoto-electric cells suitable for harvesting solar energy.

Several criteria should be considered when selecting the optimum solar cell for EH applications, as mentioned in Section III. The choice of the appropriate solar cell should be take into account the wavelength of the incident light (outdoor or indoor); the expected illumination intensity (and the anticipated variations thereof); and the illumination duration per day; as well as the required cell output current. Taken together, these factors determine how big the cell must be.

Other solar-cell selection considerations include lifetime (organic cells degrade with UV exposure), cell shape (circular for watch faces), flexibility (most solar cells are rigid), and, of course, cost and availability.

Tools that accelerate system development

RenesasmakesiteasytoforsystemengineerstocreatenewdesignsthatutilizetheRXMCUsand the many other processors in our extensive lineup of popular embedded system solutions. Comprehensive hardware and software tools—including very low cost and free products—make it possible for customers to move projects swiftly through the product development process from concept stage to final RX-based products.

Available development aids include the following:

• Renesas Customizable Software Library: Applilet is a support tool that makes it easy to generatecodeoptimizedforanRX100MCU.ItfunctionsthroughasimpleGUIwindows application or via an e2studio plug-in. This free tool generates customizable device drivers that compile and work right out of the box. am.renesas.com/applilet

• e2 studio – the new Eclipse based Integrated Development Environment (IDE) from Renesas: Complete development and debug environment based on the popular Eclipse platform and the associated C/C++ Development Tooling (CDT) project. am.renesas.com/e2studio

• RX100 Series Starter Kits: Renesas offers complete RX100-series based hardware and software platforms for in-depth application design including the E1 Debugger, e2 studio, demonstration firmware, and a trial version of the Renesas RX compiler. am.renesas.com/RSKRX111

• 3rd Party Solutions:Renesasofferscompletesupportforawide-rangeof3rdparty developmentoptionsincludingfullsupportfortheIAREmbeddedWorkbenchandthe KPITGNURXCompiler.RTOSandUSBsupportoptionsareavailablefromMicrium,CMXSystems, RoweBots, Segger and Express Logic.

http://am.enesas.com/applilet

http://am.renesas.com/e2studio

http://am.renesas.com/RSKRX111


V. SummaryThis white paper has presented an overview and background material on the design of very-low-powerembeddedsystems,emphasizingenergy-harvestingtechnologyanditsapplication.Manyimportant very-low-power design concepts have been explained and an example design was presented to put them into perspective.

The authors recognize that energy harvesting is an emerging discipline—a technology that’s still in its infancy. However, they believe that holds enormous promise and will eventually become mainstream because it can solve difficult problems associated with applications for rapidly grow-ing ‘Internet of Things’ markets.

VI. AppendixCompaniesMentioned

Cap-XX – SuperCaps www.cap-xx.com

Cymbet – Solid-state Batteries www.cymbet.com

Decawave – Advanced RTLS Solutions www.decawave.com

EnergyMicro–BluetoothSmartModules www.emmicroelectronic.com

EnOcean–WirelessBuildingManagement Solutions Using Energy Harvesting www.enocean.com

Leviton – Licensee of EnOcean Technology, ElectricalOEM www.leviton.com

Linear Technology Semiconductor Manufacturer(PowerManagementChips) www.linear.com

Maxell–ComponentSupplier(Supercaps) www.maxell-usa.com

Maxim–SemiconductorManufacturer (PowerManagementChips) www.maxim-ic.com

MicroStrain–SensorSystems www.microstrain.com

Mide–PiezoTransducers www.mide.com

Perpetuum – Vibration Harvester Powered WirelessSensorSystems www.perpetuum.com

PowerCast – RF-Based Energy Harvesting Solutions www.powercastco.com

Renesas–SemiconductorManufacturer(MCUs) www.renesas.com

Sanyo Amorton – Solar Cells www.panasonic.net/energy/amorton/ en/products/

Solicore – Thin Batteries www.Solicore.com

Tadiran–BatteryManufacturer(LithiumThionylChloride Batteries) www.tadiran.com

Varta–BatteryManufacturer www.varta.com

http://www.cap-xx.com

http://www.cymbet.com

http://www.decawave.com

http://www.emmicroelectronic.com

http://www.enocean.com

http://www.leviton.com

http://www.linear.com

http://www.maxell-usa.com

http://www.maxim-ic.com

http://www.microstrain.com

http://www.mide.com

http://www.perpetuum.com

http://www.powercastco.com

http://www.renesas.com

http://www.panasonic.net/energy/amorton/ en/products/

http://www.panasonic.net/energy/amorton/ en/products/

http://www.Solicore.com

http://www.tadiran.com

http://www.varta.com

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