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Squeezing the Most Out of Battery Life using ARM®

Cortex®-M Series Processors Jacob Beningo, CSDP

Beningo Engineering

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Lecturer

Contact Info

Jacob Beningo Principal Consultant

P.O. Box 400

Linden, Michigan 48451

: info@beningo.com

: 248-719-6850

: Jacob_Beningo

: Beningo Engineering

: JacobBeningo

: Embedded Basics

Session Overview

Introduction to Low Power Systems

Understanding Energy, Current and Power

Fundamentals of Low Power Systems

Example Application w/ Baseline

Minimizing Energy

– Low Power Modes

– Software Architecture Design

– Tips for Energy Conservation

– Practical Considerations

Introduction

Proliferation of Low-Power Devices

ARM® Cortex®-M Series – Upwards Upgradeable

– M0, M0+, M3, M4

– 32 bit Architecture

– Low Cost

– Connectivity

– Code Reuse

– Energy Efficient

Introduction

http://www.arm.com/products/processors/cortex-m/index.php

STM Family of Microcontrollers

Introduction

So what is the problem?

• Demand for longer battery lives

• Devices are used more often for longer periods of time

• Sensors are being deployed with no end in sight

• Batteries cost $$$

• Requires time and effort to swap them out

Understanding Energy and Current

3.7 V

Understanding Energy and Current

Understanding Energy and Current

Energy Profile Key Points

• Power profile can be calculated from A x V

• Energy Profile can be approximated by current only!

• Integrating the energy profile will provide mAh used

• Derivative of the energy profile provides instantaneous

power

• Can be used to identify heavy energy usage periods

• Can be used to estimate battery discharge time

Energy Profiling Technique

Using External Tools and Software

– IAR I-Scope probe (Real-time profiling)

– 200 uA Resolution

– 200 KHz sample rate

Calculating System Average Power Consumption

Fundamentals of Low Power Systems

Operating Time

Curr

ent

Dynamic Power

Power Down Wake-Up

Static Power Avg Current

Introduction

Function MCU

Mode

Execute

Period

Execute

Time

Current

Duty

Cycle

Average

Current

Send Data Run,

48 MHz

1 s 0.005 s 15.2 mA 0.5 % 0.076 mA

Receive

Data

Run,

32 MHz

1 s 0.005 s 10.5 mA 0.5 % 0.0525 mA

Drive LED Run,

1 MHz

1 s 0.009 s 5 mA 0.9 % 0.045 mA

Battery

Monitor

Run,

1 MHz

1 s 0.001 s 2.1 mA 0.1% 0.0021 mA

Sleep Standby 1 s 0.98 s 0.5 mA 98 % 0.49 mA

Total 0.6656 mA

Expected Battery Life = Battery mAh Rating / Avg Current

1000 Hours = 680 mAh / 0.6656 mA

Computing Average Current

ST Microelectronics STM32CubeMx MCU Battery Calculator

Fundamentals of Low Power Systems

Example Application Problem

– Mobile Medical Sensor Device

– Read Sensor Data

– Analog Sensors

– Blink System Status

– Blink blue when reading data

– Blink red when battery low

– Send data over UART once per second

– Simulates radio connectivity

Example Application Problem

Freescale Kinetis-L Freedom Board Overview

Example Application Problem

Expansion Header

Expansion Header

Kinetis-L

Capacitive

Touch

Tri-Color

LED

Accelerome

ter

Reset

Open

SDA

USB

SWD

Baseline Energy Profile

– No sleeping

– Peripherals on

Example Application Problem

System Duty Cycle

– MCU awake for short period of time

– Maximize MCU sleep time

Design Goals to Minimize Energy Profile

Power Modes

Run Mode

– CPU clocks enabled

– All peripherals enabled

– Code executing out of flash

– Internal references on

– Voltage Throttling STM32L (3 modes)

– Typical datasheet current at 3 volts is 6.4 mA

Power Modes

Wait Mode

– CPU clocks disabled

– All peripherals clocks disabled

– Code executing out of flash

– Flash doze mode enabled

– Typical datasheet current at 3 volts is 3.7 to 5.0 mA

Power Modes

Entering Wait Power Mode

Power Modes Example

Entering Deep Sleep Power Mode

Power Modes Example

Stop Mode

– CPU clocks disabled

– All peripherals clocks disabled

– Code executing out of flash

– Core and system disabled

– Typical datasheet current at 3 volts is 2.5 to 3.7 mA

Power Modes

Entering Stop Power Mode

Power Modes Example

VLPW Mode

– CPU clocks disabled

– All peripherals clocks disabled

– Low power wait mode for flash

– Flash doze enabled

– Typical datasheet current at 3 volts is 135 to 496 uA

Power Modes

Entering VLLS1 Power Mode

Power Modes Example

Don’t forget to initialize!

Power Modes Example

Power Modes Example

Power Modes Example

Power Modes Example

What can be saved by using Power Modes?

Power Modes Example

Power Mode Current Draw (mA) % Savings Runtime (hrs)*

Run 18.0 - 37.8

Wait Mode 15.9 11.6% 42.8

Deep Sleep 14.8 17.8% 46

Software Architecture Effects

Energy Savings Start with your Architecture!

Software Architecture RTOS

OSTick Suppression

Software Architecture Effects (ISR’s)

Wait for Interrupt (WFI) instruction

Software Architecture Effects (ISR’s)

Sleep on Exit

Software Architecture Results

Sleep on Exit

Tips for Energy Conservation

Tip #1 – Turn Off Unused Module and Clocks

– If it is not being used then don’t let it consume energy!

– Clock gating stops the peripheral from causing logic to switch which uses energy

Tips for Energy Conservation

Power = Energy / Time = A x V / t

A x V / t = C x F x V2 + IL * V

A / t = C x F x V + IL

Dynamic Current ~ C x F x V

Tip #2 – Lower the MCU Voltage

Tips for Energy Conservation

Tip #3 – Configure Unused I/O

– Put them in their lowest energy state

– Unused pins as inputs and pulled high or low?

– Drive unused pins high or low?

– The Cortex M0+ recommends

– Float the physical pin

– Drive low

– Or pull low with 10k pull-down

Tips for Energy Conservation

Tips # 4 – Throttle the System Clock (48 -> 24 MHz)

– Do you really need to execute all those cycles?

Tips for Energy Conservation

Stop

Peripherals

Clocks

Updated

CPU +

Peripherals

Resume

Considerations for Selecting an Optimal Frequency

– Communication Device Requirements

– Processing Performance

– Maximum peak current allow by the system

Steps for frequency throttling

Tips for Energy Conservation

Tip #5 – Use Autonomous Intelligent Peripherals

– ATMEL sleepwalking

– Setup a peripheral to monitor an event

– Periodic ISR not necessary

– Energy Micro Reflex System

– Peripherals monitor event

– Peripherals directly communicate with each other

– Can trigger other peripherals

Tips For Energy Conservation

Tip #6 – Execute the Program from RAM

– Copy Flash -> RAM on start-up

– Functions should be small

– Computationally intensive

– Beware of bus contention!

Tip #7 – Beware of Communication Channels

– Can be power hungry peripherals

– May require periodic waking

– Use DMA to lighten the Load

– Can keep the CPU asleep

– Transfer data from peripheral to memory

– Uart Serial Wake-Up

– Peripheral Sleeps until data is received

– Wakes up and then performs functions

Tips for Energy Conservation

Tips for Energy Conservation

Tip #8 – Use a Low Power Timer or RTCC

– Lower Energy Usage due to slower OSC

– Used to interrupt system and wake up from extremely

low power state

Tip #9 – Use Compiler Optimizations

Tips for Energy Conservation

Tips for Energy Conservation

Tip #10 – Use Energy Profiling Tools

Conclusions

Power Mode Current Draw (mA) % Savings Runtime (hrs)*

Run 18.0 - 37.8

Wait Mode 15.9 11.6% 42.8

Deep Sleep 14.8 17.8% 46

Deep Sleep + Peripherals Off 14.15 21.4% 48

Deep Sleep + Peripherals off +

24 MHz

13 27.8% 52.3

Deep Sleep + Peripherals off +

24 MHz + Speed Optimized

12.8 28.9% 53.2

Deep Sleep + Peripherals off +

24 MHz + Speed Optimized +

Sleep on Exit

12.8 28.9% 53.2

Optimization Table Results

Conclusions

Power Mode Current Draw (mA) % Savings Runtime (hrs)*

Run 18.0 - 37.8

Wait Mode 15.9 11.6% 42.8

Deep Sleep 14.8 17.8% 46

Deep Sleep + Sleep on Exit 13.45 25.3% 50.5

Deep Sleep + Sleep on Exit +

Peripherals off

13.40 25.5% 50.7

Deep Sleep + Sleep on Exit +

Peripherals off + 24 MHz

12.8 28.9% 53.2

Deep Sleep + Sleep on Exit +

Peripherals off + 24 MHz +

Speed Optimized

12.8 28.9% 53.2

Optimization Results Take 2

Conclusions

How much energy was able to be saved by using these techniques?

Going Further

Free 52 Low Power Design Techniques Newsletter – Weekly Session

– Tips and Tricks

– Hands-on Experiments

– Audio and Video Commentary

– Sign-up at http://eepurl.com/2xJFr

Do It Yourself

– Duplicate the Results

– Perform Experiments

– Self-Paced Workshops

– Webinars

Contact Information

Contact Info

Jacob Beningo Principal Consultant

P.O. Box 400

Linden, Michigan 48451

: info@beningo.com

: 248-719-6850

: Jacob_Beningo

: Beningo Engineering

: JacobBeningo

: Embedded Basics