AN1416, Low-Power Design Guide - Microchip Technologyww1.microchip.com/downloads/en/AppNotes/01416a.pdf · Low-Power Design Guide. AN1416 DS01416A-page 2 2011 Microchip Technology

AN1416Low-Power Design Guide

INTRODUCTION

Low-power applications represent a significant portionof the future market for embedded systems. Everyyear, more designers are required to make designsportable, wireless and energy efficient. This documentseeks to simplify the transition to low-power applica-tions by providing a single location for the foundationsof low-power design for embedded systems. Theexamples discussed in this document will focus onpower consumption from the viewpoint of the microcon-troller (MCU). As the brain of the application, the MCUtypically consumes the most power and has the mostcontrol over the system power consumption.

As with all designs, it is important for the designer of alow-power embedded system to consider trade-offsbetween power consumption, and other factors, such ascost, size and complexity. While some low-power tech-niques can be used with no cost to the system, othersmay require trade-offs. This guide will give examples ofthese trade-offs where applicable. However, it is notfeasible to discuss all possible trade-offs, so an embed-ded designer should keep in mind the possible systemlevel impacts of power-saving techniques.

This design guide will refer to Low-Power modesavailable on PIC® MCUs, but will not go into detailabout these features. For information about theLow-Power modes available on PIC MCU devices,refer to AN1267, “nanoWatt and nanoWatt XLP™Technologies: An Introduction to Microchip’sLow-Power Devices” (DS01267).

LOW-POWER BASICS

The definition of low power varies significantly fromapplication to application. In some systems, there isplenty of energy available to run from, but thelow-power designer is attempting to minimize operatingcosts or maximize efficiency. While in other applica-tions, there may be a limited power supply, such as acoin cell battery, which determines the power con-sumption requirements of the system. These systemsrequire different focuses to minimize power. It is impor-tant to consider and understand what causes powerconsumption and where to focus power minimizationefforts to create an effective low-power system.

Main Sources of Power Consumption

In CMOS devices, such as microcontrollers, the totalpower consumption can be broken down into two broadcategories: dynamic power and static power. Dynamicpower is the power consumed when the microcontrolleris running and performing its programmed tasks. Staticpower is the power consumed, when not running code,that occurs simply by applying voltage to a device.

DYNAMIC POWER

Dynamic power consumption is the current which isconsumed during the normal operation of an MCU. Itincludes the power lost in switching CMOS circuits andthe bias currents for the active analog circuits of thedevice, such as A/Ds or oscillators.

To understand where switching losses originate from,consider a CMOS inverter, as shown in Figure 1.

FIGURE 1: CMOS INVERTER DYNAMIC POWER CONSUMPTION PATHS

This inverter will consume little to no power when theinput is at VDD or VSS. However, when the signalswitches from VDD to VSS, there is a transition periodwhere the PMOS and NMOS will both be biased in thelinear region, allowing current to flow from VDD toground. Also note, in a real system there is someamount of load capacitance on the output bus. There isadditional current consumption associated with thecharging and discharging of this bus capacitance whenthe logic level changes.

Authors: Brant IveyMicrochip Technology Inc.

VDD

OutputInput

2011 Microchip Technology Inc. DS01416A-page 1

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The average power consumed by dynamic switchinglosses of a single gate can be defined by the followingequation:

EQUATION 1: DYNAMIC POWER CONSUMPTION

Where V is the system voltage, f is the switchingfrequency and C is the load capacitance.

Note that this equation is for a single CMOS device, notthe entire MCU. When considering the entire MCU, thisequation will be multiplied by a scaling factor (), whichvaries depending on the switching frequency of all ofthe gates in the device.

Equation 1 reveals a few important points to considerabout how to control dynamic power consumption. Thefirst point to consider is that voltage is the most significantfactor in dynamic power consumption because thevoltage term is squared. Reducing the system operatingvoltage will have a significant impact on power consump-tion. Another major consideration is which of thesecomponents can be modified in a system. Everyembedded system has different requirements whichwill limit the ability of a designer to adjust the voltage,frequency or load capacitance.

For example, the embedded system designer haslimited control over C, the internal load capacitance.The capacitance is a function of the internal MCU lay-out and design. It is up to the MCU manufacturer to limitthe switching of load capacitance by utilizing properlow-power IC design techniques, such as properly gat-ing clock signals. The only control the system designerhas over internal load capacitance is the ability toenable and disable MCU features individually. A savvylow-power designer should ensure that, at any point ina program, only the currently needed features of theMCU are enabled and all others are turned off.

A designer does have control over the external loadcapacitance of a signal that is routed to an I/O pin.These capacitances can be much larger than the inter-nal capacitance of the device and can cause significantlosses. For this reason, it is important for a designer toreview a design for stray capacitance on digital switch-ing. Refer to the “Hardware Design” section for moredetails on I/O low-power design techniques.

Operating voltage is primarily defined by the processtechnology used in the manufacture of the MCU. As pro-cess geometries shrink, the operating voltage decreasesand the device consumes lower dynamic power. Anembedded system designer can utilize this knowledge byselecting MCUs which are capable of operating at lowervoltages. However, if the minimum system voltage isdefined by some other component of the system, such asa sensor or communications interface, this will require acost versus power trade-off. This would require anadditional voltage regulator for the MCU power, which

would increase system cost. Interestingly, in somecases, it can be more power efficient to run at a highervoltage if it allows for the removal of a regulator. In thiscase, the power lost in the regulator is higher than theincrease in dynamic current from operating at a highervoltage.

Frequency is typically the most variable of the factorscontributing to dynamic power, and as such, is usuallythe component adjusted by embedded designers toactively control power consumption. The optimaloperating frequency for a system is determined by acombination of factors:

• Communications or sampling speed requirements

• Processing performance

• Maximum peak current allowed

As the power equation indicates, lower frequencies willresult in lower dynamic current. However, it is importantto keep in mind that execution speed is also a factor inpower consumption. In some cases, it may be optimal torun at a higher frequency and finish an operation morequickly to allow the system to return to Sleep for minimalpower use. Also, consider that at low frequencies,dynamic switching current may no longer dominatesystem power consumption. Instead, the static powerconsumption used in biasing the analog circuits on theMCU will dominate. This can limit the effectiveness ofreducing frequency as a power-saving technique. At thispoint, the designer should focus on techniques to reducestatic power.

STATIC POWER

Static power consumption encompasses all of thepower required to maintain proper system operationwhile code is not actively running. This typicallyincludes bias currents for analog circuits, low-powertimekeeping oscillators and leakage current. Staticpower is a major concern for battery-based systems,which spend significant portions of the applicationlifetime in Sleep mode.

Analog circuits, such as voltage regulators andBrown-out Resets (BOR), require a certain amount ofbias current in order to maintain acceptable accuracy astemperature and voltage vary. In order to offset the cur-rent consumption of many of these modules, the besttechniques are to utilize flexibility built into the MCU toenable and disable analog blocks, as necessary, or toutilize lower power and lower accuracy modes.

P = V 2 f C

DS01416A-page 2 2011 Microchip Technology Inc.

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Timekeepers, such as the Watchdog Timers (WDT) andReal-Time Clock/Calendars (RTCC), are also usuallyconsidered part of the system’s static power consump-tion. Even though these modules are actively switchingand could be thought of as dynamic because they arealways running at a constant frequency, and consumevery little power, it makes more sense to include them instatic power calculations. Nonetheless, as dynamiccircuits, they follow the same rules for power optimiza-tion, as mentioned in the “Dynamic Power” section andare primarily affected by voltage, frequency and capaci-tance. By nature, these oscillators are low-frequencyand low-power oscillators. For instance, 32 kHz crystaldrivers are usually designed to allow the crystal to oper-ate with as little peak-to-peak voltage as possible, whilemaintaining stable oscillation. Refer to the “Low-PowerCrystal Design” section for more information onreducing crystal oscillator power consumption.

FIGURE 2: NMOS TRANSISTOR LEAKAGE CURRENTS

Leakage current is caused by the non-ideal operation ofthe MOSFETs used in CMOS devices. As process tech-nologies shrink and transistors become smaller, thereare several aspects of the FET which no longer behaveas they would in an ideal system. Current begins to leakfrom the FET drain to the source, even when the gate isbelow the conducting threshold. This current is calledsub-threshold leakage. Sub-threshold leakage occursbecause the drain and source are physically closer in anarrower transistor. The narrower a transistor is, thelarger this leakage becomes. Additionally, leakage isaffected by temperature and voltage. For MCUs, usingsmall processes at high temperatures and maximumvoltage, leakage can amount to many A of current.

Similar to dynamic power, some of the aspects ofleakage power are outside of the control of the embed-ded system designer. Selecting an MCU with a largerprocess technology will reduce leakage, but with thetrade-off of having higher dynamic current consump-tion. Temperature is also out of the designer’s control,set by the requirements of the system. Generally, thebest option for reducing leakage is to reduce voltageand power off unneeded circuits. Some PIC® MCUsprovide features, such as Deep Sleep, which powerdown additional circuits in the device to reduce thepower consumption below typical Sleep levels.

PROCESS TECHNOLOGY TRADE-OFF CONSIDERATIONS

A critical trade-off becomes apparent when trying to opti-mize both dynamic and static power. Smaller processtechnologies have considerably lower dynamic power,but at the cost of much higher leakage current. Table 1shows a comparison of some commonly used processtechnologies, and the relative static and dynamic powerconsumption for each technology. In some cases, as aresult of this trade-off, it can be difficult for a designer todetermine which is more important to reducing powerconsumption for a system. In order to select the correctMCU, which will minimize power for a particular system,it is important for the designer to know whether dynamicor static power has a more significant impact on thesystem. This process is called ‘Power Budgeting’ and isdiscussed in detail in the “Measuring Power Con-sumption” section. This trade-off between dynamic andstatic power is what makes MCU design optimizationsmatter. MCU manufacturers are constantly searching forways to maintain low leakage as process technologiescontinue to shrink.

TABLE 1: COMPARISON OF PROCESS TECHNOLOGY POWER CONSUMPTION

Sub-ThresholdLeakage

Gate

Gate Leakage

Substrate

Source Drain

VDDVSS

VG

Process Technology

Leakage Power

(Normalized)

Dynamic Power

(Normalized)

Core Voltage

0.35 m 0.5 2.8 3.0V

0.25 m .75 2 2.5V

0.18 m 1 1 1.8V

130 nm 1.5 .75 1.5V

90 nm 2 0.44 1.2V


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WHAT IS LOW POWER?

Low power means different things for different systems.Some applications focus on dynamic power consumptionas they must remain running constantly, such as a powersupply. For these applications, the only concern is how toreduce dynamic power as much as possible to improveefficiency. On the other hand, a battery-powered applica-tion would typically be more concerned with the Sleepmode power consumption, as it tends to spend most ofits time in Sleep mode.

However, these Run and Sleep mode power consump-tions are not the only aspects that are important to powerconsumption. For example, wake-up time can have acrucial impact on systems with a low active duty cycle.Consider a system which only remains awake for 10 s

to read and store an input before returning to Sleep. Inthis example, it is not feasible to start up a crystal oscil-lator which could take milliseconds before it is stable.The MCU must be able to wake-up and operate in a fewmicroseconds to effectively manage the application.

To understand the importance of the various aspects ofthe system’s power consumption, the example applica-tion, shown in Figure 3, will be used to show variouslow-power design techniques and trade-offs. A datalogging sensor takes a sample every 100 mS and thenstores the data to EEPROM once a full page of data isavailable (32 samples). The system runs for 50 S totake a sample and for 5 mS to store the data toEEPROM. The sensor is using the PIC16LF1826, a 3VXLP PIC microcontroller.

FIGURE 3: LOW-POWER SYSTEM EXAMPLE

U1

8

7

6

51011

12

13

14

4

17

18

12

3

16

15

5

1

2

3

4

C2

0.1 F

67

8

910

k

10k

U2

32.768 kHz

Serial EEPROMY1

33 p

F

C4

33 p

F

C5PIC16LF1826

C3

22 p

F

R3

1k

100k

R2

R1100k

0.1 F

C13.3V

R5

R4

VDD

MCLR

RA0

RA1

RA2

RA3

RA4/T0CKI

OSC1/CLKIN

OSC2/CLKOUT

VSS

RB0/INT

RB1

RB2

RB3

RB4

RB6

RB7

RB5

VCC

WP

SCL

SDA

A0

A1

A2

GND


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Measuring Power Consumption

When measuring the overall power consumption of asystem, there are two values which are of primary con-cern: average power consumption and maximumpower consumption. Average power consumption isthe sum of the total energy consumed by the system inDynamic and Static Power modes, divided by the aver-age system loop time, as shown in Figure 4. Averagepower is important because it provides a single value,which can be used to accurately determine battery lifeor the total energy use of the system.

FIGURE 4: CALCULATING SYSTEM AVERAGE POWER CONSUMPTION

Maximum power consumption is the worst-case powerdraw required by the system. It is important to deter-mine maximum power consumption in order to properlydesign the system power supply. For example, manybatteries perform differently depending on the rate ofcurrent draw from the battery, and it is important toknow what current levels the system’s battery iscapable of handling.

While measuring power consumption may seemstraightforward, accurately capturing average powercan be complex for many systems. Most embeddedapplications do not spend enough time doing a specifictask for an ammeter to accurately measure and displaythe current without modifying the code to wait in a par-ticular state. An oscilloscope can be used to get an ideaof the current profile, but may not be able to accuratelycapture the power consumption for Low-Power modes.As a result, it is often necessary to combine multiplemeasurement methods in order to accurately model thepower consumption of a system in all modes.

MEASURING STATIC POWER

There are a few concerns to consider when measuringstatic power consumption. First, ensure that theammeter used for testing has a high enough resolutionto accurately measure the MCU’s power consumption.For many nanoWatt XLP MCUs, the static power can belower than 100 nA, which can be outside the maximumresolution for most handheld multimeters and somebench DMMs.

Another concern is the voltage drop out of the ammeter.When set to a low-current range for measuring staticpower, there will usually be a notable voltage drop acrossthe meter. This can interfere with voltage-sensitivecircuits, and often, a Brown-out Reset will occur in anMCU if a short, high-power pulse develops. This voltagedrop is frequently not specified in the product documen-tation and must usually be measured by using a secondmultimeter or oscilloscope.

Cu

rren

t

Powers Down

Operation Time

Static Power –Power-Down Mode

Dynamic Power –Active Mode withDevice Running

Wake-up Time

Average Current = Iactive x tactive + Ipowerdown x tpowerdown

tactive + tpowerdown


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Generally, it is not feasible to measure static power withan oscilloscope. For currents under 10 A, a shunt resis-tor of at least 5 kOhm must be used in order to create ameasurable voltage drop. Using this large of a resistor islikely to interfere with proper operation of the systemwhen not in the low-power state.

In the event that there is no ammeter available that iscapable of accurately measuring a very low-power appli-cation, an alternate method can be used, utilizing acapacitor. When operating in a Low-Power mode, anMCU will act as a constant-current sink. Using the equa-tion for constant-current discharge from a capacitor, it isrelatively easy to measure the power consumption of alow-power system. Connect a capacitor, such as a lowleakage 10 F ceramic capacitor, to the application, asshown in Figure 5, with switches to disconnect thecapacitor from the voltage supply and the application.

FIGURE 5: LOW-POWER MEASUREMENT, CAPACITOR METHOD

To measure low power in the capacitor method:

1. Connect both switches to run the system andallow the capacitor to charge to VDD from thevoltage source.

2. Disconnect the voltage source (and any voltagemeters) and allow the application to run from thecapacitor for a set amount of time. This timeshould be long enough to allow the capacitor todischarge by about 100 mV. Make sure to dis-connect the capacitor before the voltage hasdropped below the normal operating range ofthe application.

3. Disconnect the capacitor from the applicationand connect a voltage meter to measure theremaining voltage on the capacitor.

4. Using Equation 2, calculate the current for acapacitor under constant-current discharge,based on the delta voltage and discharge time.

For example, if a system is run from a 10 F capacitorfor 10 seconds, and experiences a voltage drop of100 mV, using Equation 2 indicates that the systemconsumes an average of 100 nA over that 10 seconds.

EQUATION 2: CONSTANT-CURRENT DISCHARGE FROM A CAPACITOR

MEASURING DYNAMIC POWER

Using an ammeter to measure dynamic power undernormal system operating conditions is usually not veryuseful. This is because the sampling speed of mostammeters is not fast enough to accurately measure thereal-time power consumption of the system, as it isexecuting code and changing states. To accuratelymeasure dynamic power with an ammeter, it is neces-sary to modify the system code to hold it in a particularstate in order to measure the power. This providesaccurate data for the current consumption of each state,but doesn’t provide the execution time informationneeded to calculate the average power consumption ofthe system.

An effective way to measure dynamic power consump-tion is to use an oscilloscope to measure voltageacross a shunt resistor. The oscilloscope will allow adesigner to determine the changes in power consump-tion as the system steps through various states duringoperation, as well as measure the time spent in eachstate. This facilitates the creation of a complete profileof the application’s power consumption. It is importantto size the shunt resistor appropriately. It should belarge enough to provide measurable resolution on thescope, but small enough not to cause a systembrown-out in high-power states. Usually, a 10-100 ohmresistor is appropriate for dynamic power measurements.

CREATING A POWER PROFILE

Once data is collected for both static and dynamicpower, the data can be used to create a power profilefor the system. The purpose of a power profile is toprovide the designer with a clear image of where theprimary sources of power use are in the system so thatit can be optimized. To create a power profile:

1. Break down the application into states based onvarying power consumption.

2. Measure the power and execution time of eachstate.

3. Determine the total energy consumed in eachstate by multiplying power and time.

Note: Some capacitor types have significantleakage current which could cause error inthis method. To account for the leakagecurrent, repeat the experiment for thesame time period with no load on thecapacitor to determine the amount ofvoltage change from leakage.

C

System

ChargeSwitch

Discharge Switch

Source

I = CVt


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TABLE 2: EXAMPLE POWER PROFILE CALCULATION

Once the profile is complete, the task of optimizing theapplication is much simpler. The profile clearly indi-cates which states of the system consume the mostpower so that the designer can focus his efforts onreducing the power of these states. The power profilealso simplifies the calculation of the system averagepower and the maximum power. Consider the profile inTable 2, which is based on the example applicationfrom Figure 3. For each Microcontroller mode, the exe-cution time is calculated and current consumption isbroken down by device. Note that in this profile, theworst power consumers are the Storing and Sleepmodes. A designer, using this system, should focuspower reduction efforts on these two modes first, asthey have the highest impact on the system.

Performance vs. Power

There are always trade-offs between performance andpower consumption in an embedded system. The key tolow-power design is creating a system which utilizes thestrengths and features of the controller to get the mostperformance within the power budget. Some criticalaspects to consider when designing for performance are:

• Wake-up Time

• Oscillator Speed

• Instruction Set Architecture (ISA)

• Peripheral Features

• Executing from Flash vs. RAM

ModeMode Time for

32 Samples (ms)

Current (A) Charge (A ×ms)By Device Mode Total

SleepMCU SleepSensor Off

EEPROM Off

32000.8

00

0.8 2560.0

InitializeMCU Wake-up

Sensor OffEEPROM Off

0.320.8

00

0.8000 0.26

Sample SensorMCU Run

Sensor OnEEPROM Off

1.2815016.5

0

166.5 213.12

ScalingMCU Run

Sensor OffEEPROM Off

0.321300

00

1300 416.00

StoringMCU Run

Sensor OffEEPROM On

5150

01000

1150 5750.00

TOTAL 3206.92 — — 8939.38

Average Current (A), Total Charge/Total Time

2.788

Peak Current (A) 1300


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Wake-up Time

Wake-up time is critical for systems with short activetimes, as the wake-up process often consumes asimilar amount of current as normal operation.

The primary component of wake-up time is the OscillatorStart-up Time (OST). Typical start-up times for variouscommon clock sources on PIC MCUs are shown inTable 3. Note that most high-accuracy and high-speedsources, such as crystals and Phase-Locked Loop (PLL)

oscillators, have long start-up times. One method, whichallows a system to use a slow start-up source with a fastwake-up time, is the Two-Speed Start-up feature onmany PIC MCUs. Two-Speed Start-up initially wakes upfrom the internal RC oscillator, which typically starts upin a few s. The system runs from this oscillator until theprimary clock source stabilizes, reducing total operatingtime by running any code that is not timing critical, asshown in Figure 6.

FIGURE 6: TWO-SPEED SCOPE

TABLE 3: TYPICAL WAKE-UP TIMES FOR VARIOUS OSCILLATORS

Clock Speeds and Power Efficiency

One of the most common questions about low-powerdesign asks: What is the best frequency to run at in orderto minimize power consumption? Is it better to run at thelowest speed possible or to run at high speed and thenSleep afterward? While this will vary by MCU, usuallyhigher speeds are more efficient than low speeds. Thisoccurs primarily because of the fixed current of initiallypowering the oscillator and the MCU. At low frequencies,this fixed bias current represents a significant portion ofthe total power consumption. However, at higherfrequencies, it becomes negligible. The result is that thehigher frequencies typically have lower A/MHz, allow-ing for more efficient operation. Refer to Figure 7 to seehow higher speeds can be more efficient.

Oscillator FrequencyStart-up Time

(ms)

Internal Fast RC Oscillator

8 MHz .001-.010

Low-Power RC Oscillator

31 kHz 0.3

Primary Crystal 8 MHz 0.5-1.0

Internal RC + PLL 32 MHz (8 MHz x 4)

1.0

Primary Crystal + PLL 32 MHz (8 MHz x 4)

1.5-2.0

Secondary Crystal 32.768 kHz 100-1000


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FIGURE 7: DYNAMIC CURRENT vs. OSCILLATOR SPEED, DEMONSTRATING EFFICIENCY OF HIGH SPEEDS

There are a couple of caveats to this rule to consider. Ifthe power source is low impedance and capable ofhandling high currents without an issue, it remains true.However, for higher impedance sources, such as coincell batteries, it no longer holds. With lower currentpower sources, the internal impedance of the powersupply will reduce the effective power output to theMCU when high current is consumed. This can result inthe reduction of the battery life below the rated capac-ity. It can also cause unreliable system operation due tounexpected Brown-out Resets when the battery’soutput voltage drops, due to the high-current load.

Second, at higher speeds, the system voltage oftenbecomes important. Many MCUs are not capable ofoperating at full speed through the entire voltage rangeof the device. Therefore, when running at high speedfrom a battery-powered application, it is necessary tomonitor VDD to determine if the battery voltage isdropping close to the minimum level required forfull-speed operation. If VDD drops below this voltage, itcan cause code mis-execution. Most MCUs provide aLow-Voltage Detect (LVD) feature, which will interruptthe device if VDD drops close to this range. This willallow the firmware to reduce the operating frequency atlow voltage, allowing the system to extend its lifetime.

Instruction Set Architecture (ISA)

The primary dynamic power specification, advertisedby MCU manufacturers, is A/MHz or A/MIPS. Unfor-tunately, as many designers have discovered, neitherof these specifications are very accurate as they don’ttake into account the MCU’s CPU architecture. To trulycompare power consumption, it is necessary to con-sider how much actual work is performed per unit ofenergy consumed. Different architectures will performvarying amounts of work in a single clock cycle. Thismakes the Instruction Set Architecture of an MCU animportant component of power consumption. Clockingscheme, cycles per instruction and available instruc-tions all have a major impact on the performance of thedevice, which directly affects the amount of work donein one cycle, and therefore, the power consumption.

The first major considerations are the MCU clock ratioand cycles per instruction. The clock ratio is the ratio ofthe input system clock frequency (FOSC) to the internalinstruction clock frequency (FCY or SYSCLK). Eight-bitPIC MCUs have a divide-by-4 architecture, while 16-bitPIC MCUs have a divide-by-2 architecture. PIC32devices have a 1-to-1 clock ratio. It is important to con-sider the clock ratio when checking the specificationsfor a device. In some cases, devices specify powerconsumption based on FOSC and others based on FCY.Devices which specify FOSC will refer to the speed inMHz (e.g., 8 MHz), while a device specifying FCY willuse MIPS (e.g., 8 MIPS).

IDD at 3.3V, 25°C

0

2

4

6

8

10

12

14

16

18

0 5 10 15 20 25 30 35

OSC Frequency (MHz)

I (m

A)

Low Speed500 μA/MHz

High Speed300 μA/MHz


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However, clock ratio cannot be considered alone whencomparing power consumption. The other importantcomponent to the equation is cycles per instruction. AllPIC MCUs are RISC architectures, with most of theinstructions taking one cycle to execute. Even so, manyother MCUs (even some which advertise as RISC)require multiple cycles per instruction. This differencecan make it very difficult to directly compare power con-sumption between different architectures. In somecases, if the majority of the instructions on an MCUrequire a given cycle count, an average cycle/instructionvalue can be calculated and used for power compari-sons; others are complex enough that this is not possible.If this is the case, it is generally only possible to comparepower consumption by using a benchmarking test.

Benchmarking is the only complete method to comparedevices which take Instruction Set Architecture intoaccount. It is also the most time-intensive method tocompare devices. For a simple benchmark, there aresome open source benchmark standards, which can beused to do get an initial comparison. By combiningbenchmark completion times with data sheet powerconsumption specifications, it is often possible to get areasonable comparison of device power consumption.Even so, these benchmarks usually utilize only the CPUof a device and do not exercise device peripherals oradvanced features. An ideal comparison is to use asimplified application, which performs the major time or

power consuming portions of a system, and performs apower budget analysis using this simplified system as amore accurate benchmark.

Properly Utilizing Peripherals

Peripheral features on a microcontroller can help sub-stantially reduce power consumption. However, often itcan be deceiving which peripherals consume highpower and which are low power. For example, whencomparing a UART to an SPI, many designers wouldchoose the UART as the lower power module. Becauseit requires fewer toggling I/Os and runs at a lowerspeed, this limited speed actually makes the UARTworse for power consumption, as an SPI is usually ableto complete a transaction much more quickly than aUART. While the SPI may consume more power duringthe transaction, afterward the device can go to Sleepand eliminate the highest source of power consump-tion, the CPU. Table 4 provides a list of some of thecommon features on MCUs and gives a rough baselinefor their estimated power consumption. Of course, thiswill vary by vendor, peripheral settings and by applica-tion, but it should provide a basis for determining whichmodule to use when multiple choices are available.

The following sections provide some further tips onhow to effectively use various peripherals to reduce thepower consumption of a system.

TABLE 4: POWER CONSUMPTION OF COMMON PERIPHERALS

Serial CommunicationsCurrent

(A)Time to Send 10 Bytes (ms)

Total Charge (A ms)

UART (57.6k) 200 1.74 347.22

I2C™ (400 kHz) 1000 0.25 250.00

SPI (4 MHz) 700 0.02 14.00

Analog ModulesCurrent

(A)Time to Convert 10 Samples (ms)

Total Charge (A ms)

Low-Speed A/D (100 ksps maximum) 250 0.1 25

High-Speed A/D (500 ksps maximum) 1000 0.02 20

Low-Speed Comparator 10

High-Speed Comparator 100

DAC/CVREF 5

Low-Power ModulesCurrent

(A)

Brown-out Reset (BOR) 5

Watchdog Timer (WDT) 0.5-1.0

Real-Time Clock and Calendar (RTCC)

0.5-1.0

Timer (31 kHz) 0.5-1.0


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ANALOG-TO-DIGITAL CONVERTERS (A/D)

The Analog-to-Digital Converter (A/D) is primarily ananalog module and many of the circuits used to per-form conversions do not vary significantly with speed,such as reference voltage circuits, signal amplifiers,signal buffers, etc. Because these analog circuits makeup the bulk of power consumption for the A/D, it isusually beneficial to run an A/D at a higher speed thannecessary for the application, disabling it in-betweensamples. The A/D should be set to use the fastest con-version clock possible and reduce the sampling time tothe minimum necessary to maintain measurementaccuracy.

DMA AND FIFO BUFFERS

A Direct Memory Access (DMA) controller is a powerfultool to reduce power consumption. The DMA improvesperformance by off-loading data transfer tasks from theCPU. Any features which can reduce CPU run time canhelp to drastically reduce power consumption, as clock-ing the CPU is the most power-intensive task in anMCU.

Many peripherals, capable of operating in Sleep, havebuilt-in FIFO buffers which reduce power by enablinglonger Sleep times. The FIFOs will store received orsampled data and will interrupt once the buffer is full.This allows the device to only wake up once to processthe data. This consumes much less power than fullywaking up a device to allow the CPU to handle eachindividual transfer.

BROWN-OUT RESET (BOR)

Brown-out Reset (BOR) protection circuits are adouble-edged sword for low-power applications. On onehand, they protect the application from mis-execution asbatteries die, or when high transient currents cause dipson the voltage supply. On the other hand, they tend toconsume high power while a device is in Low-Powermodes. An average BOR, in order to maintain accuracy,typically has a bias current of about 5 A.

It is important to utilize flexible features on a BOR inorder to have the best power performance. Someuseful power reducing BOR features are:

• Automatically disabled BOR in Sleep mode. Many PIC MCUs have the ability to automatically disable the BOR when the device enters Low-Power mode. Because the primary purpose of a BOR is to protect the device from mis-execution of code at low voltage, it is often no longer needed when the CPU is not running. Therefore, it is valuable to have a module that is automatically disabled in Sleep and automatically re-enabled when waking up the CPU.

• Accuracy and power setting controls. Some PIC MCUs have a BOR with programmable cur-rent consumption. This allows a designer to choose the current range, which makes the most sense for the system, based on the accuracy and voltage levels required.

• Low-Power BOR or “Deep Sleep” BOR mode. Sometimes an application doesn’t require protec-tion from mis-execution in the form of a constantly active BOR. However, completely disabling the BOR circuit can leave an MCU vulnerable to lock-up if the supply voltage drops close to the transistor threshold voltage, without dropping all the way to ground. As a result, many newer PIC MCUs have a Low-Power BOR (LPBOR), which provides downside protection to ensure the MCU will always have a Power-on Reset (POR). This mode typically consumes about 50 nA to enable, considerably less than a full-fledged BOR.


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WAKE-UP SOURCES

The proper selection of wake-up sources is often anoverlooked MCU feature, which can help reduce powerconsumption. Modern MCUs have a wide variety ofwake-up sources and can wake up from almost anyperipheral module on the device. While many applica-tions will traditionally wake up, based on a timer tocheck for events, power can be reduced by enabling aperipheral, which is specifically capable of capturing aparticular event. For example, I2C™ and SPI modulescan receive data while asleep, then wake up the deviceafterward. This can save power over having to wakethe device every few milliseconds to check for incomingtransmissions.

Another example is the value of a Real-Time Clock(RTC) versus using a timer module for wake-up. Keep-ing time with a timer module usually requires waking adevice, once every second, to update the clock andcheck for an alarm condition. However, a Real-TimeClock and Calendar (RTCC) will allow the device tokeep time and remain asleep until the alarm occurs,reducing power.

Executing from Flash vs. RAM

Powering and reading the Flash memory of an MCU isone of the most significant contributions to powerconsumption. Reading from RAM consumes less powerthan Flash. To take advantage of this difference, manyhigh-end MCUs, such as the PIC32, provide an option toexecute code from RAM to help reduce dynamic powerconsumption. However, there are trade-offs and over-head to consider in order to determine if it is applicableto a particular application.

In order to run from RAM, the code must be copied fromFlash to RAM. This task consumes significant time andpower. In order to be worthwhile, the functions run fromRAM, which generally should be limited in size orshould be computationally-intensive functions.

A potential trade-off to consider is that for some devicearchitectures, running from RAM can reduce perfor-mance due to the limitations of using a single memorybus for execution and operation. For example, theHarvard architecture used in PIC32 devices, normallyuses one memory bus to read from Flash and anotherto read from RAM. This allows the device to run at highspeed without stalling the processor. However, whenoperating from RAM, only a single memory bus is used,which can force the processor to stall during someinstructions which write to RAM. Make note of thesepotential differences on a device when calculating thevalue of executing from RAM.


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SOFTWARE DESIGNSoftware design has a critical impact on low-powersystem design. Selecting the best low-power devices, andoptimizing hardware for low power, can easily be wastedif the proper techniques are not utilized when writing sys-tem software. The following sections provide somegeneral tips for reducing power consumption in software.

Conditional Code Execution

One software technique, which can be effectively usedin many applications, is to utilize a conditional wake-upand code execution structure. For example, Figure 8shows a graph of the power consumption of the sensorpresented in Figure 3. The sensor system takes32 samples before writing to the EEPROM. This allowsthe system to minimize power by running for a very

short time on most wake-up events and ensuring thatthe high-current operation of writing to Flash is done asinfrequently as possible. If each sample was writtenindividually, the system would consume significantlymore current.

Additionally, this system uses a varying system clock tofurther improve power. This system uses a lower speedclock source, during the sampling and storingprocesses, and only uses the high-speed clock for thescaling process. This is because the sampling and stor-ing times are fixed by the A/D and EEPROM which areused in the system. Reducing the frequency during thistime, reduces current consumption without increasingrun time. The scaling time depends only on the perfor-mance of the MCU and varies with operating frequency.Therefore, during this time, it is optimal to run quicklyallowing the system to return to Sleep afterward.

FIGURE 8: SENSOR POWER CONSUMPTION

Store

Scale

Sample

Repeat for 32 Samples, then Store Data

Low-Speed System Clock

High-Speed System Clock

Time

Cu

rre

nt


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Idle and Doze

Generally, it is easy to tell when Low-Power modes,such as Sleep, should be used in an application, asthere is an obvious down time when the system canenter Sleep mode. Dynamic power-saving modes, Idleand Doze, tend to be less obvious. For example, somecases when a system should use Idle or Doze mode are:

• Replacing Wait loops. Low-power applications should avoid loops where the CPU is performing time-wasting actions, such as polling a bit. System designers should replace these loops with a call to Idle mode, such as follows:

Replace: while(!ADCInterruptFlag); With: while(!ADCInterruptFlag){Idle(); //wake on ADC interrupt

}This will have the same effect in the code, but at amuch lower power cost, as the CPU will not beclocked until the A/D interrupt wakes up the device.Note that this will not impact other functionality of thedevice, as other interrupts can wake up the deviceand be handled properly, with the device returning toIdle afterwards. In cases where the CPU needs toperform some processing while waiting on the flag,consider using Doze mode rather than Idle to run theCPU at a slower speed while waiting.

• Running high-speed peripherals. Consider the scenario where a system’s primary job is to main-tain a high-resolution PWM output. The high-resolution requirement will force the system to maintain a high-speed clock. In this case, Sleep mode cannot be used to reduce power consump-tion due to the need to maintain the PWM. Similarly, Idle mode may not be feasible because the CPU will need to monitor the system state to update the PWM, as necessary. Even so, it is unlikely that the CPU will need to run at the same speed as the PWM in order to perform this job. In this case, Doze mode is an ideal option, allowing the CPU to perform the necessary tasks at low speed.

Interrupts vs. Polling

In the effort to wipe out power draining Wait loops fromthe system, a low-power system designer shouldconsider switching from a polling-based system to aninterrupt-based system. Utilizing interrupts allows the

system to use power-saving modes, such as Sleep andIdle, much more frequently in place of waiting for anevent with polling. Consider the sensor presented inFigure 3 and detailed in Table 2. If this applicationrequired a significant amount of processing time foreach sample, it would not make sense to wait in Idlemode, polling the A/D flag while taking the sample.Instead, an interrupt-based method could be used sothat the system could process the previous samplewhile taking the current one. If the system is simple anddoesn’t require much processing, or have backgroundtasks which need to run frequently, polling can be thebetter option as it allows simpler, more compact codewithout any additional current consumption.

Power Optimization and ‘C’

‘C’ is a great tool for writing embedded programs, butcompared to Assembly, C takes away much of thedesigner’s control over the code. As a result, some engi-neers are hesitant to use C in applications which requireminimal power consumption, preferring to use Assemblyto manually optimize the code.

While it is true that code written by highly experiencedengineers using Assembly will almost always be moreefficient than C, current compilers have optimizationsthat minimize the differences. As a result, it is oftenworthwhile to use C for the code portability and mainte-nance improvements it offers over Assembly. In fact, asthe complexity of a project increases, the likelihood thatthe C compiler will be more efficient than handwrittenAssembly increases as well. Assembly has the mostvalue when used for very small programs or small sec-tions of larger programs, which are processing-intensiveand can be more easily hand optimized.

There are optimizations which can be done by theprogrammer, in C or Assembly, which cannot beperformed by a compiler optimizer. One example of anoptimization, which can drastically improve code, is tosimplify program flow for common cases. Consider thecase where an A/D sample has been completed and it isidentical to a previous sample. Optimized code wouldcompare the current sample to the previous sample, aswell as other common values prior to performingtime-intensive processing steps. If the new samplematches one of these values, the code can use a precal-culated result and avoid significant processing time. Insome cases, this modification to the program architec-ture can drastically reduce total execution time. Mostapplications include some cases that appear muchmore frequently than others in many of the application’sfunctions, which can take advantage of this method.


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HARDWARE DESIGN

Board Level Considerations

Designing low-power hardware follows a few, verystraightforward requirements:

• Maximize impedance in current paths

• Minimize impedance in high-speed switching paths

• Minimize leakage currents

• Minimize operating duty cycles

The following recommendations utilize these rules, invarious circuits of a system, to minimize the powerimpact of those components.

PUSH BUTTONS

The primary problem with push button power consump-tion is that buttons typically interact in the very slowhuman time domain, rather than the high-speedembedded system domain. A very quick button presswill still take hundreds of milliseconds. With a 10 kOhmpull-up on a button, this means hundreds of milli-seconds of 300 A of power, which is considerable in alow-power system. Increasing the value of the pull-upcan help to reduce power, but at high resistances, mayreduce noise immunity. Additionally, increasing the sizeof the pull-up increases the time constant of the signal,which increases the rise time before high voltage isdetected.

A better method is to use the MCU’s internal pull-upresistors as the voltage source for a push button.Because the internal pull-up can be enabled and dis-abled, dynamically in code, as soon as the button pressis detected, the MCU can disable the pull-up, removingthe current path to ground. This improves the push but-ton circuit in other ways as well, as it provides a simpledebouncing and reduces the external componentcount. For a button which needs to be able to detect apush-and-hold state, the pull-up can be periodicallyre-enabled to determine if the button is being held.

LEDs

LEDs are a power consumption problem for similarreasons as buttons. They consume high power to run(2 mA-50 mA) and must be run for long periods of timein order to be useful to a user. The following are two ofthe main techniques for reducing LED powerconsumption:

• Drive the LED at a lower duty cycle using a PWM. This gives the code some degree of control over the LED power consumption by being able to dynamically reduce the brightness, if necessary.

• Drive the LED at lower current levels by increas-ing the size of the current-limiting resistor. This is an especially useful technique with newer, high brightness and high-efficiency LEDs. These LEDs can be blindingly bright when driven at the rated current levels. However, for a system which doesn’t require a bright LED, they can be driven at much lower currents and still remain highly visible. While a 2 mA LED might not be very visible below this rating, a high brightness LED, rated for 20 mA, can often still be highly visible when driven under 1 mA.

CONNECTING AND CONTROLLING I/O PINS

In many applications, incorrect use of the I/O pins is acommon source of unexpected, high-power consump-tion. There are a few rules for using a bidirectional I/Opin on a PIC microcontroller to minimize powerconsumption.

Unused Port Pins

By default, PIC microcontroller I/O pins power up asinputs. A digital input pin consumes the least amount ofpower when the input voltage is near VDD or VSS. If theinput is at some voltage between VDD and VSS, thetransistors inside the digital input buffer are biased inthe linear region and they will consume a significantamount of current. On an unused I/O pin, which is leftfloating, the voltage on the pin can drift to VDD/2 oroscillate, causing the unused I/O pin to consume signif-icant power. The I/O pin can use as much as 100 A ifa switching signal is coupled onto the pin.

An unused I/O pin should be left unconnected, but con-figured as an output pin, driving to either state (high orlow), or configured as an input with an external resistor(about 10 k) pulling it to VDD or VSS. If such a pin canbe configured as an analog input, the digital input bufferis turned off, preventing the excess current consump-tion caused by a floating signal. Any of these methodswill prevent the floating node case, minimizing power.


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Analog Inputs

Analog inputs have a very high impedance so they con-sume very little current. They will consume less currentthan a digital input if the applied voltage would normallybe centered between VDD and VSS. Sometimes, it isappropriate and possible to configure digital inputs asanalog inputs when the digital input must go to alow-power state.

Digital Inputs and Outputs

As long as a digital input is pulled to VDD or VSS, onlythe pin input leakage current will be consumed.

There is no additional current consumed by a digitaloutput pin, other than the current going through the pinto power the external circuit. Close attention should bepaid to the external circuits to ensure that the output isbeing driven to the state which causes the lowestpower consumption. If an external circuit is powereddown, make sure that any I/O pin connected to it isdriven low to prevent sinking current through thedisabled circuit.

For digital inputs and outputs with high switching fre-quencies, make sure that there is no stray capacitanceon the bus by minimizing trace length and eliminatingunnecessary components.

SIZING PULL-UP RESISTORS

I/O lines with pull-up or pull-down resistors can be ahigh source of power drain in an application and areoften targeted as a source of power optimization. Asnoted before, push buttons and LEDs have power con-sumption set by the size of the resistor used. For alow-power application, the general rule is to usepull-ups that are as large as possible. However, adesigner should take note of the effect this will have onthe circuit. For example, on an I2C module, the pull-upsize determines the maximum speed of the bus. Usingpull-ups that are too large will increase the time con-stant of the bus and slow down communications. Thiscould result in a net increase in current consumption.

CAPACITOR LEAKAGE CURRENT

All capacitors have a small amount of charge lossthrough the dielectric, even after fully charging. Thisloss is typically referred to as the ‘capacitor’s leakagecurrent’. The amount of leakage current varies bycapacitor size and type. Typically, tantalum and electro-lytic caps are high leakage, and ceramic and filmcapacitors are low leakage.

TABLE 5: LEAKAGE CURRENTS FOR COMMON CAPACITORS

Leakage specifications vary for different capacitors. Insome cases, leakage current is specified directly. Inother cases, it is determined by the capacitor’s InsulationResistance (IR). The Insulation Resistance is typicallyspecified in either Megaohms or Megaohms Mega-farads. To determine the leakage current in the lattercase, use the following formula:

EQUATION 3: CAPACITOR LEAKAGE CURRENT CALCULATION

Note that the initial leakage current for some capacitorscan be higher as the chemicals in the dielectric are polar-ized. The current will start high and drop to the ratedleakage value as the capacitor is held at full charge.When selecting capacitors for a low-power system,wherever possible, it is best to select capacitors with lowleakage current.

Powering External Circuits

The MCU is only a single component in a complete sys-tem and all devices in the system will contribute to thesystem’s total power consumption. While many modernICs are designed with Low-Power modes, other com-ponents have different standards for what low powermeans. While an MCU might have a Sleep mode withpower consumption near 100 nA, often devices, suchas external transceivers, consume a few microamps inStandby mode. As a result, it is often desirable to allowthe MCU to control power to external circuits in a sys-tem in order to minimize total power consumption.Some external circuits, which an MCU can easilycontrol that consume high power, are analog voltagedividers, LCD backlights, sensors and transceivers(e.g., RF and RS-232). Consider the system shown inFigure 9. By modifying the system so that the voltagedivider (R1) and serial EEPROM (U2) are controlled bythe MCU’s I/O (RA0 and RB0, respectively), the systemleakage current can be substantially reduced. Forhigh-power devices, which cannot be driven by theMCU’s I/O directly, use the PIC device to drive aMOSFET and power the circuit. Making these modifica-tions causes the design to resemble the optimizedlow-power system from Figure 3.

Capacitor Leakage (10 F)

Electrolytic 5 A

Tantalum 1 A

Ceramic 20 nA

Film 5 nA

V CIR (M MF)

I (nA) =


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FIGURE 9: NON-OPTIMIZED LOW-POWER SENSOR SYSTEM

There are a few concerns to be aware of when utilizingthis technique. First, ensure that all I/O pins, connectedto the disabled external circuits, are in the proper statesbefore powering components on or off. Ordinarily, thebest procedure for handling the I/O is the following:

• Powering on a device:

1. Tri-state all connected I/O.

2. Power on the external device and wait for thedevice to start up.

3. Configure the I/O to the operational state.

• Powering off a device:

1. Tri-state all connected I/O.

2. Power off the external device.

3. Configure the I/O to drive low as outputs.

The I/O pins are tri-stated, prior to changing the state ofthe device, to ensure that there are no bus conflictswhile the system is powering up or down. When anexternal circuit is disabled, it is good practice to drivethe connected I/O pins low as outputs to ensure thatthose lines are not allowed to float.

When using this technique, consider the cost of repow-ering a device versus the static power consumption.For an external device with high inrush current, largecapacitors to charge, or long power-on times, there is ahigh-current consumption associated with power-on.The charge lost having to repower these componentscan cause more current consumption than is saved bypowering it down. Make sure to perform a power bud-get on the full system to determine which state is lowerpower.

Power Supply Design

The selection and design of a power supply for alow-power system can have major impacts on the powerconsumption of the system. Linear regulators, switchingregulators and other PMICs all require some amount ofpower to operate, and many are tuned for systems withhigh-power requirements. For battery-powered systems,it may not be feasible or desirable to use a regulator atall in the power supply.

Switching regulators usually provide greater efficiencyfor applications, operating at higher power levels. Thismakes them ideal for line powered, low-power systems,which focus on increasing efficiency and reducingdynamic power consumption. Even so, they tend to havevery low efficiency at low supply currents. A boost regu-lator, that can be 95% efficient at 50 mA, is likely to haveefficiency around 20% or lower at a load current of10 A. Keep this in mind when designing a low-powersystem, which may spend most of its time asleep, as itcan result in a linear regulator being a more efficientchoice overall.

In some cases, it can be ideal to run a battery-poweredsystem without a regulator. In particular: LiFeS2(Lithium AA Batteries), LiMnO2 (Coin Cells) and Alka-line batteries all operate within the 1.8-3.6V operatingrange of most low-power microcontrollers. Refer to thebattery section of this document for more details onthese battery chemistries. By designing a systemcapable of operating directly from the battery,significant gains in system lifetime are possible.

U1

8

7

6

510

11

12

13

144

17

18

12

3

16

15

5

1

2

3

4

C2

0.1 F

67

8

9

10k

10k

U2

32.768 kHz

Serial EEPROMY1

33 p

F

C4

33 p

F

C5PIC16LF1826

C3

22 p

F

R3

1k10

0k

R2

R1100k

0.1 F

C13.3V

R5

R4

VDD

MCLR

RA0

RA1

RA2

RA3

RA4/T0CKI

OSC1/CLKIN

OSC2/CLKOUT

VSS

RB0/INT

RB1

RB2

RB3

RB4

RB6

RB7

RB5

VCC

WP

SCL

SDA

A0

A1

A2

GND


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Consider a low-power system designed to run from twoalkaline AA batteries. These batteries have a linear dis-charge curve and will discharge from 3.6V to 2.7V overthe first 50% of the battery life. Using a low dropout 2.5Vregulator, only about half of the battery capacity will beused before the regulator is below the minimum operatingvoltage. The charge remaining in the batteries from 2.7Vdown to 1.8V cannot be used, wasting half of the batterycapacity. Additionally, there will be some amount of quies-cent current consumption by the regulator duringoperation, further reducing the effective battery life. Thereare two major trade-offs of operating without a regulator.One is the loss of a consistent voltage reference for theapplication. However, most modern MCUs have an inter-nal reference voltage that can be used in place of theregulator to determine the current VDD voltage and per-form analog conversions. The other trade-off is that all ofthe components of the application will need to support awider voltage range, which can increase costs.

Low-Power Crystal Design

One of the major impacts of the attempts to reduce MCUpower consumption has been in the drivers forlow-power 32.768 kHz crystals. In order to minimize cur-rent consumption, the drive strengths for a typical crystaldriver have become very low, so much so, that now acrystal can effectively be run at current levels below1 A. The result of this, is that the crystal circuit designmust be done very carefully, and be well tested to ensurethat it will start up correctly and run accurately.

The basics of low-power crystal design are to place thecrystal as close as possible to the MCU and if possible,surround the crystal driver circuit with a ground ring toprevent coupled noise.

Additionally, it is very important to ensure that the tankcapacitors are properly sized for both the crystal and thedriver. Note that this is a major change from many oldercrystal designs. The average load capacitance for a32 kHz crystal is typically 12.5 pF, which results in tankcapacitors of about 22 pF. However, as the crystal drivercurrents have dropped, they may no longer effectivelystart a crystal with this high of a load capacitance.Low-power crystals with ideal load capacitances, from3.7 pF to 6 pF, should be used in these systems to getmore reliable operation at low power.

For a more complete analysis of how to designlow-power crystal driver circuits, refer to AN1288,“Design Practices for Low-Power External Oscillators”,AN849, “Basic PICmicro® Oscillator Design” andAN949, “Making Your Oscillator Work”.

BATTERY CONSIDERATIONSA large number of low-power systems are mobile orremote devices running from battery power. The follow-ing sections will provide an introduction to designing abattery-based application, focusing on some of the keydesign decisions when using a battery.

Battery Chemistries

Battery chemistry has a major impact on the specifica-tions and performance of a battery. Each chemistry hasa specific voltage discharge profile, internal impedanceand self-discharge current. Some of the most commonprimary battery (non-rechargeable) chemistries areAlkaline (standard AA/AAA), lithium-manganese diox-ide (coin cells) and lithium-iron disulfide (lithiumAA/AAA). The most common rechargeable chemistriesare nickel-metal hydride (rechargeable AA/AAA) andLithium Ion. Table 6 shows a summary of thecharacteristics of the various battery chemistries.

The voltage profile describes how the output voltage ofthe battery changes as it is discharged. Most batterieswill provide graphs in the data sheet, showing the voltageprofile at specific discharge currents. The voltage profileaffects the regulation and operating voltage requirementsof a system. For example, alkaline batteries have a slop-ing discharge curve which will drop linearly as the batteryis used. Applications which require a steady voltage willneed a regulator to ensure a consistent operating volt-age. By contrast, most lithium chemistries have very flatdischarge curves. They will output a constant voltageuntil the end of the battery life when the voltage output willrapidly drop off. The flat discharge curve lithium chemis-tries are harder to fuel gauge since it is no longer feasibleto measure battery voltage to determine remainingcharge. However, a system will be able to use a higherpercentage of the energy in the battery, before thevoltage drops too low, compared with alkaline batteries.

The internal impedance of a battery is a result of thechemical nature of batteries. Because they require achemical reaction to release energy, batteries cannotreact instantaneously to the electrical needs of asystem. As a result, a short pulse of high current on abattery will often result in a sharp drop in the outputvoltage, which a designer must consider when creatinga battery-based system. This is particularly true of lith-ium coin cell batteries, which often have current limitsaround 10 mA, due to the high internal resistance.Operating above this limit will waste a large amount ofthe capacity of the battery as voltage drops across theinternal resistance.

Self-discharge current is caused by the ongoing chem-ical reaction inside the battery. Even when no current isbeing drawn from a battery, the internal chemical reac-tion is taking place. This means that some of the batterycapacity is lost over time. Alkaline batteries and mostsecondary batteries have high self-discharge currents,which can make them unsuitable for very long lifetimeapplications. Lithium primary batteries have very lowself-discharge, typically less than 1% of battery life peryear. Battery manufacturers usually specifyself-discharge as a component of shelf life, indicatingthe number of years a battery can remain unused andretain most of its capacity (typically 80%).


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TABLE 6: ELECTRICAL CHARACTERISTICS OF COMMON BATTERY CHEMISTRIES

Estimating Battery Life

Using the average current from the calculated powerbudget, it is possible to determine how long a batterywill be able to power the application. Table 7 shows life-

times for typical battery types using the average powerconsumption of the system, described in the powerbudgeting example from Table 4. The equation forcalculating battery life is given in Equation 4:

EQUATION 4: BATTERY OPERATING LIFETIME CALCULATIONS

TABLE 7: BATTERY LIFE ESTIMATIONS FOR LOW-POWER SENSOR

Note that Equation 4 is a simple tool meant for gener-ating an initial estimation. Reviewing the results of theequation in Table 4 for the AAA sized batteries showsthat it predicts a lifetime that exceeds the typical shelflives for these battery types. It is possible that the bat-tery will physically fail via other means, such as thepackaging leaking, before it is fully discharged. Adesigner should be aware of all of the limitations of abattery when calculating its design lifetime.

In order to simplify the process of power budgeting andbattery life estimation, Microchip provides a tool, theXLP Battery Life Estimator, which can be used to get abasic calculation of battery life based on the batterybeing used and the system power profile. This tool sup-ports most nanoWatt XLP PIC MCU devices and can beconfigured for a wide range of battery types. The toolcan be downloaded from www.microchip.com/ble.

Chemistry TypeTypical Form

FactorNominal Voltage

Voltage Profile

Self-Discharge (%/mo)

Nominal Internal Resistance

Alkaline Primary AA/AAA 1.5V Sloped 0.08% 150-300 m

Li/MnO2 Primary Coin Cell 3.0V Flat 0.05% 10k-40k m

Li/FeS2 Primary AA/AAA 1.5V Flat 0.30% 90-150 m

Lithium-ion Secondary Varies 3.6V Flat 20% 30-40 m

Ni/MH Secondary AA/AAA 1.2V Sloped 30% 30-40 m

Battery Capacity (mAh) Self-Discharge (%/yr) Battery Life (years)

CR2032 220 1% 8.3

Lithium AAA 1150 1% 36.7(1)

Alkaline AAA 1150 4% 17.5(1)

Li-ion(2) 850 20% 4.4

Note 1: This time exceeds the typical shelf life of the battery, indicating the battery may physically fail before fully discharging.

2: Capacity varies by size; value used is typical.

Life (hours) =Capacity (mAh)

System Current + Battery Self-Discharge Current (mA)


http://www.microchip.com

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CONCLUSION

Low-power design is a complex topic, far beyond whatcan be covered in a single document. This applicationnote provides an introduction to many of the topicswhich a low-power system designer would encounterduring the design process. A designer must be awareof the sources of power consumption in a system, keypower related specifications of components, hardwareand software power optimization techniques, and therelationship between power and performance. Withknowledge of the low-power design aspects, which arecritical for their applications, designers can focus ondiving deeper into these topics in order to create a trulypower optimized system.

REFERENCES

Microchip Low Power Design Center, www.microchip.com/xlp

Energizer Product Data Sheets, Energizer Inc., http://data.energizer.com/.

B. Ivey, AN1267, “nanoWatt and nanoWatt XLP Technologies: An Introduction to Microchip’s Low-Power Devices” (DS01267), Microchip Technology Inc. 2009.

Microchip Applications Staff, “Combined Tips ‘n Tricks Guide” (DS01146), Microchip Technology Inc., 2009


Note the following details of the code protection feature on Microchip devices:

• Microchip products meet the specification contained in their particular Microchip Data Sheet.

• Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the intended manner and under normal conditions.

• There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data Sheets. Most likely, the person doing so is engaged in theft of intellectual property.

• Microchip is willing to work with the customer who is concerned about the integrity of their code.

• Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not mean that we are guaranteeing the product as “unbreakable.”

Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of ourproducts. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such actsallow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.

Information contained in this publication regarding deviceapplications and the like is provided only for your convenienceand may be superseded by updates. It is your responsibility toensure that your application meets with your specifications.MICROCHIP MAKES NO REPRESENTATIONS ORWARRANTIES OF ANY KIND WHETHER EXPRESS ORIMPLIED, WRITTEN OR ORAL, STATUTORY OROTHERWISE, RELATED TO THE INFORMATION,INCLUDING BUT NOT LIMITED TO ITS CONDITION,QUALITY, PERFORMANCE, MERCHANTABILITY ORFITNESS FOR PURPOSE. Microchip disclaims all liabilityarising from this information and its use. Use of Microchipdevices in life support and/or safety applications is entirely atthe buyer’s risk, and the buyer agrees to defend, indemnify andhold harmless Microchip from any and all damages, claims,suits, or expenses resulting from such use. No licenses areconveyed, implicitly or otherwise, under any Microchipintellectual property rights.

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Trademarks

The Microchip name and logo, the Microchip logo, dsPIC, KEELOQ, KEELOQ logo, MPLAB, PIC, PICmicro, PICSTART, PIC32 logo, rfPIC and UNI/O are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries.

FilterLab, Hampshire, HI-TECH C, Linear Active Thermistor, MXDEV, MXLAB, SEEVAL and The Embedded Control Solutions Company are registered trademarks of Microchip Technology Incorporated in the U.S.A.

Analog-for-the-Digital Age, Application Maestro, chipKIT, chipKIT logo, CodeGuard, dsPICDEM, dsPICDEM.net, dsPICworks, dsSPEAK, ECAN, ECONOMONITOR, FanSense, HI-TIDE, In-Circuit Serial Programming, ICSP, Mindi, MiWi, MPASM, MPLAB Certified logo, MPLIB, MPLINK, mTouch, Omniscient Code Generation, PICC, PICC-18, PICDEM, PICDEM.net, PICkit, PICtail, REAL ICE, rfLAB, Select Mode, Total Endurance, TSHARC, UniWinDriver, WiperLock and ZENA are trademarks of Microchip Technology Incorporated in the U.S.A. and other countries.

SQTP is a service mark of Microchip Technology Incorporated in the U.S.A.

All other trademarks mentioned herein are property of their respective companies.

© 2011, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved.

Printed on recycled paper.

ISBN: 978-1-61341-832-1

DS01416A-page 21

Microchip received ISO/TS-16949:2009 certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona; Gresham, Oregon and design centers in California and India. The Company’s quality system processes and procedures are for its PIC® MCUs and dsPIC® DSCs, KEELOQ® code hopping devices, Serial EEPROMs, microperipherals, nonvolatile memory and analog products. In addition, Microchip’s quality system for the design and manufacture of development systems is ISO 9001:2000 certified.


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08/02/11

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Documents

AN1416, Low-Power Design Guide - Microchip Technologyww1.microchip.com/downloads/en/AppNotes/01416a.pdf · Low-Power Design Guide. AN1416 DS01416A-page 2 2011 Microchip Technology