Low Power Sensing

freescale.com
White Paper
Low-Power Sensing
Energy-efficient power solutions
Table of Contents
2
Impact of Power
Consumption on
Portable Products
2
Achieving the Lowest
Sensor Power
Consumption
3
Smart Sensing for Low
Power Consumption
5
Integrated MCU/Sensor
Operation
8
Tools Supporting Each
Low-Power Method
Abstract
Increasing system functionality in portable electronic systems
has traditionally occurred at the expense of higher power
consumption and shorter battery life. Sensors designed for
low-power consumption at both the device and system levels
can reverse this trend while providing additional functionalities
that delight users. With its micro-electromechanical systems
(MEMS) sensor, MCU and sophisticated power management
technologies, Freescale Semiconductor can help users address
the ongoing issue of increasing functionality without sacrificing
battery life. This white paper will describe three approaches
that benefit different applications.
10Conclusions
11References
Low Power Sensing
Impact of Power Consumption on Portable Products
The power consumption of a battery-powered portable product has a direct relationship to
feature content and battery size that allows suppliers of smartphones and other portable
devices to differentiate their products. For example, table 1 shows battery life versus battery
size and average current draw (power consumption) for four popular smartphones based
on recent testing [1, 2, 3]. The current consumption is strictly a calculation based on Iave =
battery size/battery life. Improved battery life occurs with either a larger battery, lower currentconsuming components and low-current strategy, or a combination. It is worth noting that the
smartphone with the largest battery did not have the highest current draw and, even with a
large feature set, it had the longest life. In contrast, the smartphone with the smallest battery
had the lowest battery life and the lowest current draw.
Average Current Consumption for Smartphones
Smartphone
Battery Life1 Hours
Battery Size (mAh)
Current (Ave) (mA)
Motorola DROID RAZR MAXX
14.88
3300
222
Motorola DROID RAZR HD
9.62
2530
263
Samsung Galaxy S III
9.40
2100
223
Motorola DROID 4
9.08
1785
197
®
1
According to CNET testing
Table 1: Battery life, battery size and current consumption comparison for four smartphones
One of the power-saving techniques enabled by sensors is placing the smartphone in a very
low-power mode based on user inactivity. The inactivity is detected through motion sensing
by the accelerometer that also provides the screen rotation function. In addition to this system
power-saving function, design of the sensor can contribute to reduced power consumption
at the sensor and system level. Freescale has established three different design approaches
to provide design flexibility for customers looking to achieve specific design goals for their
products.
Achieving the Lowest Sensor Power Consumption
The first, and perhaps most direct, approach to a low power consumption design targets
the sensor itself. The sensor can have one or more lower power states such as a shutdown
mode and an extremely low-power operating mode. In many instances, this type of sensor’s
operation is directly controlled by the system designer in the end application. As a result, it
provides many power consumption advantages.
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MMA865xFC Accelerometer Digital Logic
EN = Low
OFF* Mode
VDD = Off
EN = Don’t Care
VDD = On
EN = High
SHUTDOWN Mode
ACTIVE Mode
VDD = On
EN = Low
VDD = On
EN = High
One Sample
Is Acquired
STANDBY Mode
VDD = On
EN = High
*OFF mode can be entered from any state by removing the power
Figure 1: The operating modes of a sensor design with extremely low power consumption enable system designers to
achieve specific design goals
An example of this design approach is an ultra-low-power tilt sensor. As shown in figure 1,
the MMA8491Q accelerometer starts completely off (VDD = 0) in the OFF mode. Whenever
a measurement is required, the sensor is power cycled. When a reading is made, the
sensor can be turned on for a millisecond every minute and essentially consume no power.
The extreme low-power capabilities of the MMA8491Q reduce the low data rate current
consumption to less than 400 nA per Hz. To minimize the amount of time the sensor is on for
a reading, it has an ultra-fast data output time of about 700 μs.
The advantage of this design approach is readily apparent in environments with very low
duty cycle requirements and a long time between samples. In appropriate situations, it is the
option with lowest power consumption on the market. Applications where this strategy applies
include industrial and supply chain monitoring where the sensor only needs to be powered on
when the product is moving and not when it is in the warehouse.
Smart Sensing for Low Power Consumption
A second method for achieving low power uses integrated digital logic in the sensor design.
Frequently called “smart sensing,” the added digital capability can be used to enable the
sensor to perform its own internal power management. This approach has been used in
accelerometers, pressure sensors and magnetometers.
An accelerometer with integrated digital logic is an example of how lower power
considerations can be taken to the next level. With added logic, the sensor performs its own
internal power management and data can be sampled as required. Beyond simple shaking
of the device to power it on or off, more complex tasks such as double taps or a rotational
shake with the direction of rotation initiating one command versus another allow the sensor
to initiate a variety of useful functions.
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MMA865xFC Accelerometer Four Operating Modes
BYP
VDD
VDDIO
Voltage
Regulator
Clock
GEN
Internal
OSC
INT1
INT2
GND
Y-Axis
Transducer
MUX
X-Axis
Transducer
C-to-V
Converter
AAF
ADC
Embedded
Functions
I2C
Interface
SDA
SCL
Anti-Aliasing
Filter
Z-Axis
Transducer
32 Data Point
Configurable
FIFO Buffer
with Watermark
Gain
Freefall
and Motion
Detection
Transient
Detection
(i.e., fast motion,
jolt)
Enhanced
Orientation with
Hysteresis
and Z-Lockout
Single, Double
and Directional
Tap Detection
Auto-WAKE/Auto-SLEEP configurable with debounce counter and multiple motion interrupts for control
MODE Options
Low Power
Low Noise + Power
High Resolution
Normal
ACTIVE Mode
WAKE
Auto-WAKE/SLEEP
ACTIVE Mode
SLEEP
MODE Options
Low Power
Low Noise + Power
High Resolution
Normal
Figure 2: MMA865xFC accelerometer digital logic (Figure 1) and four operating modes (above)
The main applications for this low-power sensing approach are end products where portrait/
landscape detection is desired, such as cell phones or tablet computers. An accelerometer
with this capability can provide the desired functionality as well as sophisticated power
management for minimum power consumption. One example is the MMA865xFC
accelerometer that consumes only 6 µA in the lowest power mode, as shown in figure 2.
In this case, the sensor itself draws the power dictated by its design. The sensor’s embedded
logic detects events and notifies an external MPU over interrupt lines using an I2C interface.
Unlike the first low-power method that has no power consumption at its lowest level, this
sensor has a baseline power consumption of 6 µA, so it can automatically power itself on.
With a higher frequency of use, a device with more internal intelligence eventually becomes
more efficient.
Another example of integrated technology for reducing power consumption is the
accelerometer’s smart first in, first out (FIFO) buffer. An 8-bit or 12-bit configurable 32-sample
FIFO allows buffering of the data so a host system can power on the sensor and read the
data at a slower rate. As a result, the device can be kept in a lower power state more of
the time, achieving lower power consumption for itself and lower duty cycle operation for
the primary system whose power consumption can be hundreds or even a thousand times
higher than the accelerometer’s power consumption. By reducing the primary system’s power
on time by a mere 0.5 %, the system consumes less power even though the accelerometer
consumes more power. The system-level advantages more than compensate for the slightly
higher power consumption in the sensor.
The smart sensing for low power methodology has also been employed in pressure sensor
design. For example, the MPL3115A2 pressure sensor, a featured Freescale
Energy-Efficient Product Solution, has very low power consumption and smart features, and
requires zero external data processing since it converts the data to the format required by the
MCU. Processing the sensor data locally reduces communications with the host processor
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Low Power Sensing
and minimizes MCU usage. Consuming only 8 µA in its highest speed mode (oversample =
1), the sensor has two interrupts for auto-wake, minimum/maximum threshold detection and
autonomous data acquisition. In addition to performing internal computations, the pressure
sensor has a programmable output rate, a FIFO buffer and internal settings for low power
similar to the MMA865xFC accelerometer. These capabilities are very useful in mobile devices,
medical and security applications.
One final example of the smart sensor approach to minimizing power consumption is a
magnetometer. The MAG3110 magnetometer for E-compass applications has an I²C serial
interface and is capable of measuring local magnetic fields up to 10 Gauss with output data
rates (ODR) up to 80 Hz. These output data rates correspond to sample intervals from 12
ms to several seconds. I²C communications can occur in both standby and active operating
modes. In standby mode, the typical current consumption is only 2 µA. Power consumption
at the maximum ODR of 80 Hz is typically 900 µA but this can be reduced to 17.2 μA at an
ODR of 1.25 Hz and even further to 8.6 µA at a sub 1 Hz ODRs. With its integrated signal
processing and control capabilities, the magnetometer can offload the computations from
an external MCU and minimize the communications as well to provide further system
power reduction.
Integrated MCU/Sensor Operation
In the third design approach for low-power sensing, the sensor utilizes the local computing
capability of an MCU. This functionality can be achieved in the same package or simply by
taking into account the joint/system-level operations of these two components.
Termed “local compute capability,” this system-level methodology can offload sensor-based
(and other) computations from the system application processor even further than the second
low-power approach. For example, for a primary system that uses 100 to 1000 times as
much power, a local MCU that can perform the required computations (and allow the other
processor to go into a lower power mode) delivers a significant power savings at the system
level, resulting in savings that justify adding the smaller MCU. Unlike the second case that
provided interrupt capability for basic functions such as portrait/landscape transition, a FIFO
buffer or similar functions, in this last case, all of the computations can be performed internally
providing the full capability of a sensor hub.
This design methodology can implement very complicated filtering systems such as Kalman
filters where significant computing capability and power consumption would normally be
required from the host processor. Examples include the MMA9550L sensing platform, a
featured Freescale Energy-Efficient Product Solution, and other platforms in this family. In
addition to the integrated MCU, sensing software also provides additional capabilities to reduce
power consumption. (Note: The MMA9550L has insufficient resources for Kalman filtering.)
A typical application or use case is a pedometer that must always be on to obtain the
required amount of data to calculate the number of steps, distance and other values on
a continuous basis. This type of capability cannot be integrated with a cell phone without
significantly decreasing battery life since the phone would essentially always have to be on.
In contrast, the local compute method has much lower power consumption than the primary
computing system in the phone. This makes the pedometer a viable application and avoids
battery life as a limiting factor. A free or low-cost downloaded app that counts steps using the
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Low Power Sensing
phone’s integrated accelerometer costs battery life for the cell phone. This same functionality
built into the phone using an integrated sensor hub could provide the pedometer capability
while the phone’s primary processor and display are powered off.
The MMA9550L consumes only 2 µA in STOPNC mode with internal clocks disabled. Table 2
shows the current consumption for this and the two other operating modes.
MMA955xL Accelerometer Current Consumption
Characteristic
Supply current in STOPNC mode
Supply current in STOPSC mode
Supply current in RUN mode2
1
2
Symbol
Condition(s)
Min
Typ
Max
Unit
IDD-SNC
Internal clocks disabled
–
2
–
µA
IDD-SSC
Internal clock in
slow-speed mode
–
15
–
µA
IDD-R
Internal clock in fast mode
–
3.1
–
mA
All conditions at nominal supply: VDD = VDDA = 1.8 V
Total current with the analog section active, 16 bits ADC resolution selected, MAC unit used and all peripheral clocks enabled.
Table 2: Supply current characteristics1 of the MMA955xL accelerometer in three operating modes
A tire pressure monitoring system (TPMS), such as the MPXY8xxx, is a pressure sensor
example of the third case. With its integrated pressure sensor and MCU as well as an
accelerometer and radio frequency (RF) transmitter, the computing is performed locally to save
power by minimizing communications with and computations by the primary system. The data
has to be transmitted wirelessly using minimal power for the sensing node to have sufficient
battery life without increasing the battery size and, as a result, the total system cost. Tables
3, 4 and 5 show the impact of power consumption in different modes for the MCU during
measurements and RF transmissions.
MCU Power Consumption in TPMS
Characteristic
Symbol
Min
Typ
Max
Unit
Standby Supply Current (VDD = 3 V, TA = 25 ˚C)
Stop1 mode, LFR, LVD and TR all off
ISTDBY
–
0.36
0.9
µA
Adder for LFR (continuous ON, any mode)
ISTDBY
–
93
112
µA
Adder for temperature restart (TR)
ISTDBY
–
10
20
µA
Stop4 mode, LFR, LVD and TR all off
ISTDBY
–
73
95
µA
Standby Supply Current (VDD = 3 V, TA = 125 ˚C)
Stop1 mode, LFR, LVD and TR all off
ISTDBY
–
9
19.5
µA
Adder for LFR (continuous ON, any mode)
ISTDBY
–
80
112
µA
Adder for temperature restart (TR)
ISTDBY
–
10
20
µA
Stop4 mode, LFR, LVD and TR all off
ISTDBY
–
95
140
µA
MCU Operating Current (VDD = 3 V, TA = 25 ˚C)
0.5 MHz fBUS, BUSCLKS1 = 1, BUSCLKS0 = 1
IDD
–
0.68
0.8
mA
1 MHz fBUS, BUSCLKS1 = 1, BUSCLKS0 = 0
IDD
–
0.94
1.1
mA
2 MHz fBUS, BUSCLKS1 = 0, BUSCLKS0 = 1
IDD
–
1.46
1.7
mA
4 MHz fBUS, BUSCLKS1 = 0, BUSCLKS0 = 0
IDD
–
2.50
2.9
mA
VDD ≤ 3.6, TA = 0 ˚C to 70 ˚C unless otherwise specified.
Table 3: Power consumption of the MCU in the TPMS
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Power Consumption of Measurements in TPMS
V
Characteristic
Symbol
Min
Pressure and Temperature Measurement
Sensor measurement time2
tPM
–
Peak current (VDD = 3.3 V)3
IP
–
Total power consumption
QP
–
Acceleration Measurement (X- or Z-Axis)4
Sensor measurement time (LP Filter ON)2
tAM
–
Peak current (VDD = 3.3 V)3
IA
–
Total power consumption (LP Filter ON)
QA
–
Temperature Measurement
Sensor measurement time2
tTM
–
Peak current (VDD = 3.3 V)3
IT
–
Total power consumption
QT
–
Voltage Measurement
tVM
–
Sensor measurement time2
IV
–
Peak current (VDD = 3.3 V)3
Total power consumption
QV
–
Typ
Max
Unit
3.3
4.0
6.28
–
–
7.3
mSec
mA
µA-sec
3.82
3.4
3.77
–
–
4.5
mSec
mA
µA-sec
1.13
3.4
1.17
–
–
1.4
mSec
mA
µA-sec
0.26
3.4
0.35
–
–
0.8
mSec
mA
µA-sec
1
C
C
C
C
C
C
C
C
C
C
C
C
2.3 ≤ VDD ≤ 3.3, TA = –40 ˚C to +125 ˚C unless otherwise specified.
NOTES:
1
Fully compensated pressure and temperature reading using the PCOMP routine with an average of eight readings. Power consumption
can be reduced if readings are uncompensated.
2
Measurement times dependent on clock tolerances.
3
Peak currents measured using external network shown in Figure 17-5 (in the MPXY8xxx data sheet) with RBATT equal to zero ohms.
4
Fully compensated acceleration reading using the ACOMP firmware routine with single reading and the 500 Hz low-pass filter active.
Power consumption can be reduced if readings are uncompensated.
Table 4: Power consumption of measurements in the TPMS
RF Transmission in TPMS
V
Characteristic
Symbol
Min
Typ
Max
Unit
mA
mA
RF Transmission Supply Current, TA = 25 ˚C
VDD = 2.1 V, PWR4:0 = 01110
C
Data 1, FSK or ASK
IDD
–
7.82
8.45
C
Data 0, ASK, Xtal oscillator, VCO, PLL only
IDD
–
TBD
TBD
C
Data 1, FSK or ASK
IDD
–
8.25
8.90
C
Data 0, ASK, Xtal oscillator, VCO, PLL only
IDD
–
TBD
TBD
P
Data 1, FSK or ASK
IDD
–
8.53
9.20
C
Data 0, ASK, Xtal oscillator, VCO, PLL only
IDD
–
TBD
TBD
mA
mA
VDD = 2.5 V, PWR4:0 = 01110
mA
mA
VDD = 3.0 V, PWR4:0 = 01011
VDD = 3.3 V, PWR4:0 = 01011
C
Data 1, FSK or ASK
IDD
–
8.80
9.50
mA
C
Data 0, ASK, Xtal oscillator, VCO, PLL only
IDD
–
TBD
TBD
mA
2.1 ≤ VDD ≤ 3.6, TA = –40 ˚C to +125 ˚C unless otherwise specified.
Target power output 5 dBm using dynamic RF power correction firmware routine.
Table 5: Power consumption of the RF transmission (at 315 MHz carrier frequency) in the TPMS
With a typical lowest power consumption of 0.36 µA in Stop1 mode to a maximum of 9.5 mA
during RF transmission, the tables illustrate the importance of minimizing the use of higher
power consumption modes to obtain maximum battery life of the sensors mounted at each
tire in the TPMS.
Based on the level of digital control, the three low-power management strategies provide
users alternatives for minimizing power consumption and optimizing battery life.
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Table 6 shows a comparison of the techniques using accelerometer, pressure sensor and
magnetometer values from each product’s data sheet. When sensor measurements require
more complex computing, offloading the main processor that methods 2 and 3 enable
provides substantially greater system-level power consumption savings.
Accelerometer, Pressure Sensor and Magnetometer
Type
1
2
3
2
2
3
Power Reduction
Method
Lowest sensor power
consumption
Smart sensing with
integrated digital logic
Integrated MCU/
sensor operation
Smart Sensing with
integrated digital logic
Smart Sensing with
integrated digital logic
Integrated MCU/
sensor operation
Sensor Type
Lowest Current
Consumption
Level of
Digital
Control
MMA8491Q
Accelerometer
400 nA at 1 Hz sample rate
Low
MMA865xFC
Accelerometer
6 µa in lowest power mode
Medium
MMA9550
Accelerometer
MPL3115A2
Pressure
MAG3110
Magnetometer
MPXY8xxx
Pressure
Example
Sensor
Product
2 µa in STOPnc mode with
internal clocks disabled
8 µa in highest speed mode
(oversample = 1)
8.6 µa in active/2 µa in
standby
Medium
0.36 µa in Stop1 mode
High
High
Medium
Table 6: Comparison of three power-saving approaches/methodologies
Tools Supporting Each Low-Power Method
In addition to reducing users’ development effort and time to market as well as system
power consumption, Freescale development kits show how to take advantage of Freescaledeveloped software. They allow users to more readily develop their own code to achieve the
lowest power for a specific application.
Low-Power Method 1
For this customer-implemented and managed power consumption technique, there are
several tools (as well as data sheets, application notes and more) to help achieve low power
consumption. Specific reference designs include:
• Electronic tamper detection smart meter reference design
• Kinetis KM3x MCU single-phase metering reference design
• Kinetis KM3x MCU two-phase metering reference design
All reference designs use the MMA8491Q tilt sensor.
The DEMOMMA8491 standalone demo is an easy-to-use tool that contains two boards. The
breakout board has a sample device and the demo board can be positioned for tilt detection.
The DEMOMMA8491 accelerometer evaluation kit application note (AN4292) explains how to
use this kit.
The LFSTBEB8491 Sensor Toolbox accelerometer development board contains an
MMA8491Q accelerometer daughter card. The LFSTBEB8491 quick start guide provides
additional support for this tool.
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Low-Power Method 2
The MMA865xFC Sensor Toolbox user guide for the MMA865xFC Sensor Toolbox
accelerometer kit provides users the hardware and software for the demonstration and
evaluation of the MMA865xFC accelerometers.
LFSTBEB865X Sensor Toolbox boards for MMA865xFC accelerometers contain MMA8652FC
and MMA8653FC accelerometer daughter cards and the sensor interface board.
The DEMOSTBMPL3115A2 Sensor Toolbox MPL3115A2 development kit includes the
USB communication board, the interface board and the pressure sensor evaluation board for
the MPL3115A2 pressure sensor. An example of the breadth in design support tools is shown
in figure 3.
Sensor Toolbox Development Kit Boards
Figure 3: USB communication board, interface board and pressure sensor evaluation board for the DEMOSTBMPL3115A2
Sensor Toolbox kit
The LFSTBEB3110 development kit enables users to develop both accelerometer and
MAG3110 magnetometer designs.
The RD4247MAG3110 is a complete kit containing three PCBs: MAG3110 magnetometer
and MMA8451 accelerometer daughter cards, sensor interface board and LFSTBUSB
communication board for running the Freescale Sensor Toolbox PC software.
Low-Power Method 3
KITMMA955xLEVM Sensor Toolbox for smart sensing platform is a complete kit (including
software) with no additional boards or components required. KITMMA9550LEVM and
KITMMA9551LEVM evaluation kits for the MMA955xL devices demonstrate the common
accelerometer user cases and provide access to device-specific features. The MMA9550L
and MMA9551L Sensor Toolbox user guide explains how to use the evaluation kits.
The MPXY8xxx design reference manual provides complete information including firmware
function examples and application source code for the MPXY8xxx TPMS solution.
The RDFXWIN8USB is a 12-axis sensor reference platform for Windows® 8 that greatly
simplifies the implementation of local compute capability and the resulting power consumption
savings for applications such as sensor fusion for motion control.
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Conclusions
To satisfy the requirements of a broad range of customers with varied use cases, Freescale
has established three different methodologies for low-power sensing. These approaches were
developed through the combined efforts of application experts in our sensor organization and
other divisions. With these three distinct approaches, engineers can think differently about
low-power operation and achieve their system design goals in an expedient manner based on
having Freescale as a technology partner and utilizing the tools Freescale has developed to
support the three low-power strategies.
A few of the ideas that have already been developed and implemented were presented.
However, there are several other concepts that Freescale experts are working on for future
implementation. These ideas address the requirements that customers have defined and
some that we have discovered based on our extensive capabilities in sensors, MCUs, power
management and RF technologies. Contact us for more details.
Energy-Efficient Solutions Program
The Energy-Efficient Solutions program and mark highlight Freescale products that excel
in effective implementation of energy-efficient technologies or deliver market-leading
performance in the application spaces they are designed to address. Freescale’s energyefficient product solutions include microcontrollers, processors, sensors,
digital signal controllers and system basis chips optimized for high
performance within the constrained energy budgets of their target
applications. Our solutions enable automotive, industrial, consumer and
networking applications and are truly energy efficient by design. For
more information, visit freescale.com/energyefficiency.
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References
1. Smartphones with long battery life (roundup)
2. Cell phone battery life charts
3. How we test: Cell phones and smartphones
For more information about Freescale low-power sensors,
visit freescale.com/sensors
Freescale, the Freescale logo, the Energy Efficient Solutions logo and Kinetis are trademarks of Freescale Semiconductor,
Inc., Reg. U.S. Pat. & Tm. Off. All other product or service names are the property of their respective owners.
© 2015 Freescale Semiconductor, Inc.
Document Number: LOWPOWERSENSWP REV 2