Energy harvesting-low-power-sensor-systems

White Paper – Energy Harvesting for Low-Power Sensor Systems Page 1 of 29
White Paper – RX100 Microcontroller Family
Energy Harvesting for Low-Power
Sensor Systems
By: David Squires, Squires Consulting; Forrest Huff, Renesas Electronics America Inc.
Feb. 2015
Abstract
This 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
• Power Budgets 3
• Energy Storage Devices 3
• Energy Harvesting Solutions 4
• Microcontrollers (MCUs) 6
• Power Management Devices 6
• Wireless Connectivity Modules 6
III. Additional Insights on
Sensor Components 7
• Sensors 7
• Energy Storage Devices 7
• Energy Harvesting Solutions 12
• MCUs 15
• Power Management Devices 18
• Wireless Connectivity Modules 20
IV. Example Design: Glass Break Sensor 22
V. Summary 28
VI. Appendix 28

I. Introduction – Very-Low-Power Sensor Products
Modern low-power sensors enable precise local, remote or autonomous control of a vast range of
products. 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 Issues
Electronic 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
Power
Mgmt.
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 Micro Electro-Mechanical (MEMs) chips
• Acceleration Micro Electro-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
enough space to accommodate two AA-size batteries occupying a volume of 16cm3. To function
properly, the electronics in this design requires a supply voltage of 1.8V, so two 1.5V batteries
must be connected in series.
The total battery capacity is 16cm3/1000cm3/l*450Wh/l = 7.2Wh. To convert this to mAh, we know
that the nominal voltage is 3V, therefore 7.2/3 = 2.4Ah = 2400mAh, which is consistent with pub-
lished numbers for AA batteries.
This hypothetical sensor product mandates a continuous battery-operating lifetime of 5 years.
Thus, the average current consumed by the sensor’s circuits must not exceed 2400mAh/(5yr x
24hr/day x 365days/yr) = 2400mAh/43800h = 55µA. In practice, this value should be derated some-
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
current of about 45µA.
Although the 45µA average operating current limit might seem to be very low, it’s more than
sufficient for designs built with very-low-power MCUs. That’s the case for the example design
described later in this paper, which applies the Renesas RX111 MCU.
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.
More application information about batteries is contained in Section III.
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:
• The small amount of energy harvested for powering the electronic devices and systems
doesn’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.
• The energy available in the environment often is intermittent, as it is when room lighting is
turned 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.
What circumstances favor the use of energy harvesting technology? Power budgets, packaging
requirements 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.
Figure 3 provides a quick reference for determining whether a primary battery will suffice or
whether 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)
AverageLoadCurrent
Energy harvesting &
secondary (rechargeable)
battery required
2800mAh
AA Cell
(Primary)
220mAh
CR2032
Coin 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
example shows a CR2032 Lithium cell). Finally, they draw a horizontal line back to the Y-axis to
read the maximum average current that can be supported by that battery. For the CR2032, the
graph shows that the limit is 10µA.
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:
• Use a larger primary cell, an option that only works if enough space is available
• Combine an energy-harvesting device with a rechargeable battery
Figure 3’s graph shows that energy harvesting is required for design must operate beyond 5 years.
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)
The choice of the right MCU is critically important for a very-low-power embedded systems. Ide-
ally, the best MCU will have the following three features, at minimum:
• Multiple power-down modes to maximize battery life
• Good performance for fast, efficient processing
• Very fast wake-up times from power-down modes.
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.
Section IV of this white paper reveals that the Renesas RX111 MCU delivers these features and
more that facilitate the design of remote sensor products and other power-efficient embedded
systems.
Power Management Devices
When engineers incorporate energy-harvesting technology into a sensor, the design must include
a 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
those chips, Maxim’s very capable MAX17710 PMIC, is discussed in the glass break sensor design
detailed 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.
Most of the many wireless communication solutions sold today operate in the 2.4GHz ISM band,
using either the ZigBee, Z-Wave, or Bluetooth Smart protocols. (The latter is also known as the
Bluetooth Low Energy [BLE] protocol.

ZigBee and Z-Wave are used extensively in commercial buildings and factories, while Bluetooth
Smart 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 Sensors
This 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,
suppliers of MEMS chips now include STMicroelectronics, Robert Bosch, Texas Instruments,
Hewlett-Packard, Panasonic, Knowles Electronics, Denso, Avago Technologies, Freescale Semicon-
ductor, AKM, Analog Devices, Seiko Epson, Invensense, Infineon Technologies, 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.
Figure 2 on page 4 highlighted the fact that conventional rechargeable batteries, supercapacitors
and 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
another useful energy storage choice for very-low-power designs. Cymbet and ST Micro are
currently 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
Figure 4 shows the design parameters that should be considered when selecting rechargeable
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
Lithium rechargeable coin cells, commonly built with a LiMnO2 chemistry, are viable energy stor-
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 How much juice available? mAh, mAh
Current Continuous current available mA, mA, A
Op Temp Range (C) deg C
Size (mm) Some prismatic, some cylindrical mm
Cycle Life (80% Depth of Discharge) # recharge cycles* 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 (%)
Rechargeablecycle
number/(cycles)
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.
Many chemistries perform poorly at low temperatures. Lithium Ion batteries are among the best in
this 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, a 1F SuperCap with a 2.5V initial charge can power an average load of 10µA for
about 19 hours before its voltage drops to 1.8V, which is the lowest operating voltage for many
semiconductor devices.
A SuperCap has a significant initial self-leakage, however. For a 1F SuperCap, that leakage is likely
to be about 100µA for the first few hours. This drastically shortens the time that the capacitor
could power the 10µA load, reducing it to between 1 and 2 hours. Accommodating this high leak-
age current mandates a sizeable energy harvester.
Figure 6 shows leakage current in a GZ115, a relatively small 0.15F SuperCap offered by Cap-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
can be greatly reduced by ensuring that the capacitor is fully charged for 3 to 4 days before it is
deployed. Proper conditioning causes the self-leakage to drop dramatically, enabling the SuperCap
to hold most of its charge over time.
0 20 40 60 80
Time (hrs)
LeakageCurrent(mA)
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
device has declined by 30%. For example, a 1F SuperCap charged to 2.5V and placed in a 50°C
environment loses 30% of its capacity after about 25,000 hours. That same SuperCap loses 30%
of its capacity after only 8,000 hours at 60°C, and after just 2,800 hours (17 weeks) at 70°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
increases by a factor of about 5x. As indicated, the lifetime at 70°C increases to about 90 weeks.
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-
ed in the graph by the alternating dots and dashes of the 1.8V line. It shows a lifetime about 1.8x
longer 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. This degraded performance is highlighted in Figure 8 by the lower curve derived from
the data obtained with a 35µA charge current. When SuperCaps are applied in energy-harvesting
applications, the manufacturer’s recommended charging procedure should be followed.
0 20 40 60 80
106
105
104
103
Temperature (ºC)
Life(Hours)
70503010
90 weeks
17 weeks
30% drop in
capacitance @
2.5V continuous
30% drop in
capacitance @
1.8V continuous
50% 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
very low leakage levels, about 4% per year. This is low enough to be ignored for all but the most
demanding 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:
• During the charging operation, Lithium ions are freed in the LiCoO2 cathode as electrons are
moved 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.
• When a solid-state battery is being discharged, Lithium atoms are ionized in the anode. Then
the 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
50µAh device can drive an average load of 1µA for 50 hours without being recharged.
Such solid-state batteries have a major advantage over conventional rechargeable batteries: they
deliver a higher cycle life. For instance, the CB050 achieves 1000 cycles at 25°C with a 50% 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
generate the charging current at least 6 hours per day, a solid-state EnerChip CBC050 battery has
to be able to provide power for the other 18 hours. During that time, the EnerChip could deliver
an average current of about 3µA before requiring a recharge. For many energy-harvesting based
applications, 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
batteries fit where existing coin cells or other rigid batteries cannot. They provide a 3V outputs at
capacities ranging from 10 to 25 mAh.
0 10 20 30 40 50
2.5
1
0.5
0
Time (hrs)
Voltage(V)
Supercapacitor Charging
500µA
200µA
100µA
50µA
35µA
1.5
2

Figure 9: Solid-State Batteries offered by Solicore, Inc.
Solicore’s materials expertise enables the extended-life (3-5 year) power sources now used in the
millions 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
chips like the Renesas RX111 MCU are enabling engineering teams to create extended-life remote
and 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, manufacturers of water faucets now are using small turbines to collect energy
from moving tap water and using it to power electrically operated valves.
Energy harvesting offers the following important advantages over alternative solutions:
• With EH technology, very-low-power products can be powered for their lifetimes, cutting
servicing and battery replacement costs.
• In commercial applications, energy harvesting does away with the need for periodic shut-
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.
• Energy harvesting is environmentally friendly because it reduces the flow of replacement
batteries into landfills, thereby decreasing the waste stream.

• EH-based sensor designs allow measuring devices to be fitted deep inside aircraft wings and
other 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) cells have an operating voltage between 0.5V and 0.7V. Individual cells
are 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.
• Vibration and motion harvesters come in two main types, as discussed in Section II: those
that 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.
Motion harvesters in switches and other actuators contain an internal coil and magnetic core po-
sitioned such that operator actions induce a current to flow through the coil that can be captured.
Miniature versions (MEMS devices) typically have a mass secured to the end of a beam. That
assembly 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.
• Thermoelectric generators (TEGs) use the previously mentioned Seebeck effect. A small piece
of 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.
• RF sources can be harvested, but doing so produces such small amounts of energy that they
are 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
they are placed within the electromagnetic field produced by a 13.56MHz coil—in a point-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
µA or less, and the voltage might drop below 4V. Under such a condition, the cell would have dif-
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
system may consume only 3 or 4mA.
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
The microcontroller is the digital brain of any embedded system. Thus, choosing the right MCU for
an EH-based sensor product is a critically important design decision. Ideally, the optimum MCU for
battery and remote applications such as security sensors will offer the following features, among
others:
• A very-low-power architecture providing multiple power-down modes for maximizing
battery life
• Good performance for fast, efficient processing
• Very fast wake-up times from power-down modes to ensure that the system spends the
greatest possible amount of time in a low-power state, yet responds quickly to deliver
essential system operational capabilities.
• A hardware Digital Signal Processor (DSP) for rapidly and conditioning raw signals, filtering
sensor outputs, determining a signal’s spectral content, and eliminating false signals.
Peak
Power
ow Power Zone
10nW
1µW
RFID Tag
Sensors/
Remotes
Hearing Aid/
Wireless
Sensor
Bluetooth
Transceiver
100mW
1W
Laptop
Computer
10W
100nW
Real Time
Clock (RTC)
Watch/
Calculator
10µW
100µW
1mW
10mW
GPS
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
The 32-bit Renesas RX111 MCU is an ideal design choice for sensor products that apply energy-
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.
The RX111’s three power-controlled Run modes (High-Speed, Middle-Speed, and Low-Speed) and
three 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).
Renesas manufactures this chip with the same 130nm low-power, low-leakage process technology
used successfully in the popular RL78 series of MCUs.
Design Features of the RX111 MCU
Key features and characteristics of the RX111 MCU include the following:
• Exceptional Run-mode power efficiency: 100μA/MHz
• Sleep-mode power consumption as low as 310nA
• Ultra-fast wake-up time: 4.8μs
• Superior architecture: 3.08 CoreMarks/MHz performance
• Six operating modes, plus numerous other design options for saving power
• Standard and advanced on-chip peripherals: ADC, LVD, RTC, USB, and more.
Power-Controlled Operating Modes
The RX111 MCU lets system engineers tailor the available processing capability and the chip’s
power 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
The MCU’s supply voltage requirements aren’t affected by the power-controlled Run modes.
Operation is always allowed over the device’s full 1.8V to 3.6V range. However, the clock
frequencies usable in the High-, Middle-, and Low-Speed modes do depend on the supply
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
3.6 to 2.7V Up to 32MHz Up to 32MHz Up to 32MHz Up to 32MHz
Middle-Speed 3.6 to 1.8V Up to 8MHz Up to 8MHz Up to 8MHz Up to 8MHz
Low-Speed 3.6 to 1.8V
Up to
32.768kHz
Up to
32.768kHz
Up to
32.768kHz
Up to
32.768kHz
Details of RX111’s Low-Power Operating Modes
In the MCU’s low-power Sleep, Deep Sleep and Software Standby modes, different on-chip func-
tions are stopped or powered down, saving various amounts of current. Here are the details:
• Sleep mode—The CPU is stopped with data retained. This reduces the CPU’s dynamic current
consumption, which is a significant contributor to the MCU’s overall operating current. The
CPU wakes up from Sleep mode into the Run mode in only 0.21μs at 32MHz.
• Deep Sleep mode—The CPU, RAM and Flash memory are stopped, with data retained. At
32MHz with multiple peripherals active, the typical operating current is only 4.6mA. It takes
just 2.24μs for the CPU to wake up from Deep Sleep mode and enter Run mode.
• Software Standby mode—The PLL and all the oscillators except the sub-clock and IWDT
are stopped. Almost all of the RX111’s modules—CPU, SRAM, Flash, DTC and peripheral
blocks—are stopped, with data retained. The Power-on-Reset (POR) circuit remains opera-
tional, however. Also, if necessary the IWDT, RTC, and LVD modules can be operated. Current
consumption in this mode is from 350nA to 790nA, depending on whether or not the LVD and
RTC functions are used. When waking up in the 4MHz Run mode, CPU operation begins after
a 4.8μs delay. When waking up in the fast 32MHz Run mode, the wait time extends to 40μs.

Table 3 shows the power consumption levels and wake-up times of the RX111 MCU.
Table 3: Power Consumption and Wake-Up Times of the Renesas RX111 MCU.
Additional RX111 Power-Saving Capabilities
Although the Sleep, Deep Sleep and Software Standby modes of the RX111 MCU are very help-
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
software to individually control the MCU’s on-chip functions to obtain further reductions in dy-
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-Voltage Lockout—This protection circuit turns off power to the load and conserves the
charge 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-Voltage Protection—This function monitors the charge voltage. If the voltage rises
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
additional charge from being pushed into the battery. Whichever method is used, the battery
is 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 OCO
32KHz
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
Any
Interrupt,
LVD, POR,
Ext Reset
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
Any
Interrupt,
LVD, POR,
Ext Reset
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)
ON
Power
OFF
Clock
OFF
OFF
Power
ON
Clock
OFF
Power
On
Clock
OFF
ON
Retain Retain
Flash
(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: Peripheral clocks all stop, CPU NOOP-Loop, Flash access 25%, Peripheral modules all stop
Typ: Peripheral clocks all running no divider, CPU all command operation, Peripherals modules on – DTC/RSPI 1 channel, MTU 1 channel, CMT 1 channel

• Over-Current Protection—This protection circuit performs a function similar to a circuit
breaker in a house. When an excessive amount of current is delivered to the load, the over-
current circuit isolates the load from the battery, ensuring that heavy loads don’t pull the
battery voltage down too low. When this function activates, there usually is a fault condition
in the system that must be corrected before normal operation can resume.
The MAX17710 PMIC
Maxim’s MAX17710—shown schematically in Figure 13—is a good example of the latest genera-
tion of highly integrated Power Management ICs (PMICs). It is designed to operate with Thin Film
Batteries (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.
Several innovative features are incorporated into the MAX17710. As previously discussed, one
characteristic 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
MAX17710 PMIC can charge an energy-storage capacitor from the TFB.
If for any reason the thin-film battery cannot deliver the full current required by the load, the PMIC
discharges 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.
When sensor products incorporate a wireless connectivity module, that radio device draws a
significant amount of current when it transmits data. The EM9301 that is discussed in the next
section, for example, requires 12mA on Tx and 13mA on Rx. Fortunately, most messages can be
limited to less than 5ms. The MAX17710 PMIC could switch on a 32µF storage capacitor that could
supply that current for the full 5ms, discharging from 3.8V down to 1.8V in the process. The PMIC
would 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
300µA to a load. This would recharge the 32µF capacitor in less than one second, quickly making
the 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
SEL1
3.3/ 2.3 /1.8V
Select
0.1µF
1µF
MAX17710
BATT
CHG
0.1uF
GND
LX
BATT
Boost Reg
PGND
FB
REF
SEL2
CPCKP
PATENT PENDING
Output
Linear Reg
Linear Charge &
Ideal Diode Control
Shunt Protection to
Reject Overcharge
Disable
LCE
µP
AE Event
Detector
*
State
Machine
Overdischarge and
UnderVoltage protectionThin 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.)

The MAX17710 provides a special shutdown mode if the battery voltage drops to 3V. When that
happens, the PMIC disconnects the battery from the load, reducing the drain on the battery to less
than 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.
A boost function is another feature provided by the MAX17710 PMIC. It enables thermoelectric
generators 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. Figure 14 shows the company’s LTC4071 shunt battery charger.
Figure 14: Linear Technology’s LTC4071 Shunt Battery Charger.
New power-management solutions are being introduced by semiconductor companies besides
Maxim and LTC. Specifically, Intersil and Texas Instruments are among the others that now have
offerings in this area. Again, comprehensive product searches are advised before making design
choices.
Wireless Connectivity Modules
Most wireless chips and modules operate in the 2.4GHz ISM band using ZigBee, Z-Wave, or Blue-
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-STATE
DETECT
OSC
CLK
ADJ
0.9sec – 7sec
PULSED
DUTY CYCLE = 0.003%
30µs – 200µs
NTCBIAS
NTC
RNOM
10k
10kT
–
+
–
+
–
+HBO
LBSEL LBSEL MUST BE TIED TO V CC OR GND
VCC
BAT
GND
EA
ADC
LTC4071
1.2V
1.2V
MP2
MP1
BODY
DIODE
Li-Ion
BATTERY
+
VIN
RIN
SYSTEM
LOAD

The EM9301 Bluetooth Smart module
One solution for adding Bluetooth Smart capabilities to an embedded system is the small, low-en-
ergy EM9301 radio module offered by Energy Microelectronics. It’s ideal for implementing Blue-
tooth Smart wireless communication in portable devices and very-low-power applications.
Figure 15: Simplified Block Diagram of the EM9301 Bluetooth Smart Module.
The EM9301 is optimized for ultra-low power wireless sensing, remote control, and monitoring
applications and operates on as little as 0.8V. It can be powered from a wide range of single-cell
batteries or energy harvesters.
The device combines the Physical layer, Link layer and Host Controller Interface (HCI) layer func-
tions. The flexible chip is fully Bluetooth Smart qualified for single-mode Master and Slave ap-
plications.
In Receive mode, the EM9301 consumes only 13mA, much less than competing products, which
require up to 23mA. An integrated DC/DC converter powers the EM9301 RF circuitry while concur-
rently delivering up to 100mA to an external device (e.g., sensors, system MCU, LED indicators,
or displays and driver circuits). Its RF output power is programmable from +4dBm to –20dBm in 6
steps.
Putting the EM9301 in the Sleep or Idle modes, in which it consumes less than 1µA, saves system
power. The radio’s overall implementation bill of material (BOM) and the its size can be reduced
significantly 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
AVSS_PLL2
AVDD_PA
AVSS_PA
AVSS_PLL1
AVSS_RF
VSS BIAS_RVDD
Power Management
Digital LDO
Bandgaps
RF core
LDOs
Central Biasing
SEL
VBAT Battery
Level
Detector
VCC1
SW_DCDCVSS_DCDC
DCDC
Converter
Xtal
Osc
XTAL1
XTAL2
Control Logic
IRQ
WU/CSN
SPI_SCK
UART_RX/
SPI_MOSI
UART_TX/
SPI_MISO
Host
Controller
Interface
Bluetooth
Low
Energy
Controller
Frequency
Synthesizer PA
LNA
IF 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 Sensor
To 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.
The hypothetical system design is shown below in Figure 16. It uses of two sensors: one for wak-
ing up the system MCU and another for measuring the vibrations from the windowpane. The
microcontroller 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 CellMic
Front Side, Sticks to Window Glass
Double Sided Tape
MCU EM9301
Antenna
Batteries
Chip Cap
Mic

MCU, a chip that offers power-down modes drawing as little as 350nA . In operation, the sensor
wakes 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
generate a voltage on one of the MCU’s one of its interrupt lines, quickly putting the CPU into Run
mode. As noted above, the Renesas RX111 wake ups within a few microseconds.
After it’s in Run mode, the MCU captures the signal from the wider bandwidth, more accurate
‘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
due strong winds or things bumping into the window. The MCU ignores such events because they
contain less high-frequency content.
Windowpane breakage sensors
As indicated above, the example design gets from two separate sensors: One that wakes up the
MCU when an event happens, and a second for accurately measuring the spectrum of the vibra-
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,
ranging from about 20kHz to 1MHz. Breakage is a catastrophic event that produces a distinct spec-
tral signature that can be readily separated from the spectra of casual events.
A piezoelectric transducer such as Mide’s V20W with zero-gram tip mass is used for the ‘Alert’
sensor. It generates an open-circuit voltage of 4V with as little as 0.25g of acceleration. It’s a good
choice for waking up the MCU when suspicious vibrations are detected.
Because the V20W piezoelectric sensor generates both positive and negative voltages, its signal
goes to a full-wave Schottky-diode bridge rectifier to ensure that the MCU gets a positive signal re-
gardless of transducer polarity. Specifically, the output of that rectifier goes through a 1kΩ resister
to a 3.6V zener-diode shunt across the IRQ pin on the RX111 MCU. The zener diode protects the
IRQ pin against damaging high-voltage spikes.
To help eliminate false sensing from non-breakage events, a low-pass R-C filter can be inserted
before the signal enters the IRQ pin.
The second transducer, the ‘Data’ sensor, is the microphone that accurately measures the vibration
signature. A tiny MEMS device is a good choice here. The device operates from 1.8 to 3.6V while
consuming 0.65mA, a current low enough to be driven from one or two of the RX111’s GPIO pins.
Those pins are rated to source 4mA, so putting two pins in parallel ensures plenty of current drive.
That MEMS sensor takes 10ms to wake up. Some applications might require faster turn-on perfor-
mance. Experimentation with the complete system helps to fine-tune such development issues.
Because the frequency response of the MEMS microphone rolls-off significantly beyond 100kHz,
no sampling beyond 200 kHz is necessary, even though the window glass vibrates at up to 1MHz
when it breaks.
Signal acquisition
The RX111 MCU, the heart of this system design, draws less than 1µA in a power-down mode and
has an under-5µs wake-up time after an interrupt. Also, the chip incorporates an ADC for digitiz-
ing the vibration signature and provides a built-in low-power hardware DSP circuitry that quickly
performs spectral-analysis calculations.

When the windowpane experiences a breakage or is otherwise disturbed, causing the RX111 to
receive 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
can be discerned. This requires the MCU’s clock to run at about 6MHz. The design has a VCC of
3.6V, which supports the fastest conversion speed.
It should be sufficient to take only 0.1 seconds of samples (200,000 x 0.1 = 20k samples) after a
suspicious event. However, experimentation may be necessary to determine exactly how many
samples are required and at what frequency they must be acquired.
In Run mode the RX111’s ADC consumes 1mA and its CPU consumes 0.6mA, for a total of 1.6mA
during the sample acquisition process. Assuming that this takes 0.1 second, the total Ah con-
sumed from the battery is 0.1s x 1.6mA = 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-
formed on the 20,000 data points produced by the ADC. The calculations involve about 80,000
multiplies, 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
with MCUs that lack such built-in hardware and thus must perform them in software. The extra
speed decreases power consumption, besides improving the system’s response time.
In the example design, if the MCU clock is boosted to 32MHz for the DSP computations, the
spectral-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
completing the calculation) is 3.2mA x 10s / 3600 (seconds/hour) = 9µAh. Of course, this is a rare
activity, one that’s only activated when there is a disturbance on the windowpane.
The MCU is put into Deep Sleep mode when it isn’t analyzing an event or communicating with the
remotely located main security system controller; i.e., nearly all the time. In that mode with the
real-time clock (RTC) running, the RX111 draws 650nA. In 24 hours, this amounts to 15.6µAh. So
this is the largest contribution to the average power consumed by the MCU.
The reality is that for many types of monitoring equipment, the quiescent power draw dominates
the overall power budget. Quiescent current is consumed 24/7/365, whereas the activities being
monitored 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
commercial buildings, the ZigBee or Z-Wave protocols might be better choices.
For this example design, the EM Micro EM9301 Bluetooth Smart device provides wireless connec-
tivity. The design calculations that follow are based on the emBeacon example found on the EM
Microelectronics website. The energy profile from that example is shown in Figure 17.

Figure 17: Power Consumed by the EM9301 Bluetooth Smart During a Transmission.
Although EM Micro’s reference design configures the EM9301 in beacon mode (Tx only), the radio
module 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
that the total current used by the EM9301 for transmitting this information is 92µAs from startup to
shutdown. Since transmissions normally occur once every 5 minutes, the total power consumed
by the Bluetooth Smart module’s data transmissions in one hour is
92 x 60 (minutes/hour) / 5 (events/minute) = 1,104 mAs
= 1,104 µAs / 3600 (seconds/hour) = 0.3µAh
However, to complete the sensor’s handshake with the central controller, the power consumed
by the EM9301’s receiver must also be calculated. The radio draws 13mA in reception mode. This
example 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
interval the EM9301 consumes a total of 13mA x 20ms = 260µAs

Because recptions also occur 12 times per hour, the total current consumed in Receiver mode in
one hour is
260µAs x 12 = 3.1mAs
=3.1mAs / 3600 (seconds/hour) = 0.9µAh
In total, it takes only 1.2 seconds every hour for the EM9301 to perform Bluetooth transmissions
and receptions. During that time the EM9301 drains the sensor system’s battery by 1.2µAh. When
the sensor system isn’t communicating, the MCU puts the radio module into Deep Sleep mode,
where it consumes 9µA. Thus, in one hour the Bluetooth Smart module consumes 9.0µAh.
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, total power consumed per hour by the EM9302 is
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
positive power rail and the EM9301 and turned on by the MCU only when a wireless transmission
is required. The FET can be controlled from a GPIO pin on the MCU, assuming that the microcon-
troller’s quiescent power is sufficiently low, as it is with the RX111.
When the RX111 wakes up, it turns on the FET, applying power to the Bluetooth Smart module, as
described above. Eliminating the EM9301’s Deep Sleep quiescent current makes the sensor system
much more power efficient.
Energy storage and power management
In the example design, the Renesas RX111 MCU keeps the average current consumption below
1µA. The EM9301 wireless module powered via the FET switch consumes another 1µA. There-
fore, the sensor system’s total average consumption is only 2µA. That being the case, a venerable
CR2032 lithium battery could power the example sensor for a period of about 5 years, as Figure 3
on 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
storage element that can supply the average current for one day—36µAh if the energy harvester
can charge that battery for at least 6 hours per day.
The storage element, in conjunction with the power management chip, must be able to supply
the sensor’s peak current requirements. The combination of Cymbet’s 50µAh CBC050 solid-state
battery, Maxim‘s MAX17710 power management IC and a storage capacitor can deliver the 20mA
peak currents for the short periods (~20ms) required for Bluetooth Smart handshake transmis-
sions.
The MAX17710 uses a clever technique of charging a capacitor from the solid-state battery, and it’s
the capacitor that delivers the requisite peak current needed by the EM9301 radio chip. The capaci-
tor can be recharged in several seconds from the CBC050 after being discharged during wireless
transmissions.
A design challenge related to the CBC050 battery is that if it discharges 36µAh during the night,
this is about 70% of its capacity. That deep discharge will reduce the battery’s lifetime to less than

1000 cycles (3 years). One solution would be to connect four CBC050 batteries in parallel. This
reduces the daily depth of discharge per battery to about 10µAh, or a 20% Depth of Discharge, and
that reduction extends the battery lifetime to beyond of 10 years.
Energy harvester
Windows in structures are almost always receive light for some portion of a day. Therefore, a
solar 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
both indoor and outdoor lighting. Web searches will reveal other suppliers that offer small photo-
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
Renesas makes it easy to for system engineers to create new designs that utilize the RX MCUs
and 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
generate code optimized for an RX100 MCU. It functions through a simple GUI windows
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: Renesas offers complete support for a wide-range of 3rd party
development options including full support for the IAR Embedded Workbench and the
KPIT GNU RX Compiler. RTOS and USB support options are available from Micrium, CMX
Systems, RoweBots, Segger and Express Logic.

V. Summary
This white paper has presented an overview and background material on the design of very-low-
power embedded systems, emphasizing energy-harvesting technology and its application. Many
important 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. Appendix
Companies Mentioned
Cap-XX – SuperCaps
www.cap-xx.com
Cymbet – Solid-state Batteries
www.cymbet.com
Decawave – Advanced RTLS Solutions
www.decawave.com
Energy Micro – Bluetooth Smart Modules
www.emmicroelectronic.com
EnOcean – Wireless Building Management
Solutions Using Energy Harvesting
www.enocean.com
Leviton – Licensee of EnOcean Technology,
Electrical OEM
www.leviton.com
Linear Technology Semiconductor
Manufacturer (Power Management Chips)
www.linear.com
Maxell – Component Supplier (Supercaps)
www.maxell-usa.com
Maxim – Semiconductor Manufacturer
(Power Management Chips)
www.maxim-ic.com
MicroStrain – Sensor Systems
www.microstrain.com
Mide – Piezo Transducers
www.mide.com
Perpetuum – Vibration Harvester Powered
Wireless Sensor Systems
www.perpetuum.com
PowerCast – RF-Based Energy Harvesting
Solutions
www.powercastco.com
Renesas – Semiconductor Manufacturer (MCUs)
www.renesas.com
Sanyo Amorton – Solar Cells
www.panasonic.net/energy/amorton/
en/products/
Solicore – Thin Batteries
www.Solicore.com
Tadiran – Battery Manufacturer (Lithium Thionyl
Chloride Batteries)
www.tadiran.com
Varta – Battery Manufacturer
www.varta.com

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