Linköpings universitetSE–581 83 Linköping

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Linköping University | Department of Computer and Information ScienceBachelor’s thesis, 16 ECTS | Datateknik

2020 | LIU-IDA/LITH-EX-G--20/027--SE

What is draining the ba ery onthe PineTime smartwatch?– Final thesis

Vad tömmer ba eriet på PineTime-smartklockan?– Kandidatarbete

Lukas Berglund

Supervisor : John TinnerholmExaminer : Jonas Wallgren

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Abstract

Power management is an important part when developing a smartwatch to meet userexpectations of battery lifetime and functionality. To develop a good power managementstructure, measurements of the device is crucial to understand the behavior and powerconsumption of hardware and software.

In this thesis, measurements have been made to hardware connected to the PineTimesmartwatch to get knowledge of its behavior and power consumption. A structure forhardware and software was developed to enable the possibility to debug, flash, and performmeasurements. The purpose of the measurements is to find possible areas of improvementthat have the potential to lower the overall power consumption of the device. From themeasurements, the st7789V display driver and the hardware design was detected to havea high power consumption. Bluetooth was also found to have an odd behavior in termsof power consumption. From these findings, suggestions on how to lower the powerconsumption are presented.

Acknowledgments

I would like to show appreciation for the help I received during the work of this project.John Tinnerholm and Jonas Wallgren from Linköping University for being my supervisor andexaminer. From Endian Technologies AB, I would like to thank Tord Andersson who hasbeen my supervisor and Albin Söderqvist and Benjamin Lindqvist for guidance and help withdevelopment.

iv

Contents

Abstract iii

Acknowledgments iv

Contents v

List of Figures vi

List of Tables vii

1 Introduction 21.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Aim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.4 Research questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.5 Delimitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Theory 42.1 PineTime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.2 Zephyr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.3 Energy efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.4 Power management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.5 Power management in an Android smartwatch . . . . . . . . . . . . . . . . . . . . 52.6 Energy hungry components of a smartwatch . . . . . . . . . . . . . . . . . . . . . 62.7 Power saving strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Method 83.1 PineTime setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.2 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.3 Method of measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4 Results 124.1 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.2 Possible improvements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5 Discussion 185.1 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185.2 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195.3 The work in a wider context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

6 Conclusion 21

Bibliography 22

v

List of Figures

3.1 The four debug ports (red) and the charging ports (blue) on the PineTime moth-erboard. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2 The position of the charging ports used to measure with Otii. . . . . . . . . . . . . . 11

vi

List of Tables

4.1 Benchmark measurements of the Experimental environment and the PineTimeHypnos project. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.2 Power consumption when connecting hardware to the SOC. . . . . . . . . . . . . . . 134.3 Power consumption when configuring hardware in prj.conf. . . . . . . . . . . . . . . . 134.4 Power consumption when implementing hardware in main.c. . . . . . . . . . . . . . . 144.5 Power consumption when disabling hardware by software in main.c. . . . . . . . . . 144.6 Power consumption when enabling hardware by software in main.c. . . . . . . . . . 15

vii

Acronyms

BT Bluetooth. 7

CPU Central Processing Unit. 4, 6, 9, 10, 18

GND Ground. 9

IoT Internet of Things. 2

mA Milliampere. 12

OS Operating System. 4

RTOS Real time operating system. 4

RTT Real-time text. 11, 15

SOC System on a Chip. vii, 4, 5, 10, 11, 13, 14, 20

UI User interface. 6, 7

µA Microampere. 13–17, 19

1

1 Introduction

This work summarizes a candidate thesis from Linköpings University that has been carriedout at Endian Technologies AB. Endian Technologies is a company working with Internet ofThings (IoT) in Gothenburg.

1.1 Background

Endian Technologies AB is a company in Gothenburg that designs and develops products andservices for the connected world. They have recently started a proof of concept project todevelop software for the PineTime smartwatch using Zephyr as the operating system. Withthe capabilities of the smartwatch, there are great opportunities to develop custom systemsthat fit different markets. The smartwatch is said to have a battery life of 7 days [1], but fromexperiences in the PineTime Hypnos project [2] that is not the case. From measurements onthe current version [3], the battery life is experienced to be 22 hours instead of the claimed168 hours. This is not optimal considering the potentials of this device and system. To findthe source of the problem, measurements of the individual components and overall powerconsumption must be considered.

1.2 Motivation

For battery-driven devices in general, high power consumption has a bad impact on manythings. The most obvious is the risk of downtime, perhaps more importantly the additionalcosts and the bad effect on the environment. With a high power consumption, a device needsto be charged more often, this means a higher number of charging cycles. Each charging cycleshortens the expected lifetime of a battery as a component [4][5]. So, with a higher numberof charging cycles, the battery will wear out faster. The materials required in the making ofa battery have a bad effect on the environment, with a high contribution to greenhouse gases[5]. To by a new one will impose a cost on the user, and with the hardware design of thePineTime smartwatch, it is hard to change the battery. So, if the battery dies the smartwatchdies.

The current version of the PineTime Hypnos project does not meet the expectations interms of battery life. So, to have the possibility to continue the work, the problem with the

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1.3. Aim

high power consumption needs to be identified and resolved. For that reason, measurementshave to be made on both hardware and software to locate the source of the problem. Themeasurements will also be useful in the further development of the smartwatch.

1.3 Aim

This thesis aims to give an understanding of the power management structure in the PineTimesmartwatch, show how hardware can be implemented and how it affects power consumption.This is done with the help of measurements on the overall and individual power consumption forhardware on the device. The purpose of the measurements is to show how power is distributedbetween components and based on these measurements find a set of possible improvements.The measurements will be performed using an Otii [6] (measurement unit) with code from atest environment that isolates individual components.

1.4 Research questions

The research questions for this thesis are based on performing measurements on the powerconsumption of the PineTime smartwatch, and to find areas that can be improved to lowerthe overall power consumption. The goal is to show how components affect the overall powerconsumption in different use cases.

The questions this thesis will answer are:

1. How can a good test-bed for hardware and software on the PineTime smartwatch bedeveloped to enable the possibility to do power measurements?

2. What is the overall and individual power consumption by hardware on the PineTimesmartwatch in different use cases?

3. Based on measurements, what areas of improvements can be found that have the poten-tial to lower the overall power consumption?

1.5 Delimitations

For this thesis, some delimitations were made to be able to focus on the research questions.

• Measurements will only be made to usable hardware on the PineTime smart-watch. The heart rate sensor can only take part in the hardware-related measurementsdue to a proprietary driver, witch is not supported by Hypnos.

• Five different test cases with a sufficient number of measurements will bemade to each hardware. This is to limit the number of measurements and to getdata that is relevant to each other.

• Prioritize measurements, defer improvements. The main purpose of this work isto do measurements and to locate possible areas of improvement. Some improvementsmay be made, but considering the complexity of the system most of them will be left asfuture work.

3

2 Theory

To perform power consumption measurements on a device, it is necessary to have basicknowledge of how it works and why it is important. This chapter presents the concepts that

are related and important to this work.

2.1 PineTime

PineTime is an open source development kit smartwatch that is capable of running self madeand custom built operating systems. The devices have a System on a Chip (SOC) design wheremost of the components can be found on the same board using the same Central ProcessingUnit (CPU). The SOC is an nRF52832 general purpose multiprotocol design from NordicSemiconductor with high functionality and low power consumption. The CPU belongs to theARM Coretex-M family and is developed to be efficient with energy and resources. It has anCPU speed of 64MHz, access to 64 KB RAM, and 512 KB flash.

2.2 Zephyr

The task of an Operating System (OS) is to manage a system combined with hardware andsoftware. Operating systems can be found in many forms depending on the task and thepurpose of the system. Linux, Windows and macOS are operating systems that often can befound in computers for daily use. The PineTime smartwatch on the other hand, can only runa lightweight operating system due to the limitations on memory, CPU speed and amountof power. Zephyr is a Real time operating system (RTOS) that can be applied to resource-constrained and embedded devices, including the PineTime smartwatch.

Power management on Zephyr is designed to be independent of the knowledge of whichSOC it runs on. This is done with the use of a smart device tree that describes the hardware,for pin position and connection of any usable device, instead of having a description for alldevices. This simplifies the implementation process of a component.

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2.3. Energy efficiency

2.3 Energy efficiency

Energy efficiency is one of the main parts of a good power management structure [7]. Thepower management structure has the purpose to manage power across a device, intending tobe energy efficient and to use as little energy as possible for a task [8]. To achieve this goal,all waste of energy must be found with the help of measurements and eliminated by modifyinghardware and software. Waste of energy can be described as the extra amount of energyconsumed by a system when performing a non-optimal task.

In battery driven devices, higher power consumption means more charging cycles. A charg-ing cycle is the process of charging a battery when required. Each charging cycle affects thehealth of a battery, so a large quantity of charging cycles have a bad impact on the lifetimeof a battery [4][5]. Replacing the battery on devices is in some cases possible, but in othernot. On the PineTime smartwatch it is close to impossible to change the battery withouttechnical knowledge. The materials required in the making of a battery have a bad effect onthe environment, with a high contribution to greenhouse gases [5].

Work has been done on optimizing the energy use in a system. Ikeyama et al. [9] havedone a work to try and lower the energy use in food plants, Liu et al. [10, 11] have successfullydesigned a structure that will save energy in an Android smartwatch by modifying the behaviorof functionality.

2.4 Power management

Power management is one of the most important aspects in the development of a device[12][13][11][14]. The structure is part of the operating systems with the purpose to allow usersand programmers, to control how power is distributed across the system, without affectingthe performance [15]. Controlling power means to control when a function is enabled anddisabled based on if the functionality is performing a task or not. This also applies to hardwareconnected to the system if it has access to the same power resource. For example, a componentthat is connected to a system is disabled when there is no work to be performed, and enabledwhen there is work to do.

Zephyrs power management subsystem provides the possibility to take full advantage ofthe power saving strategy on the SOC, with the use of APIs. The structure is divided intothree parts [16].

1. Tickless Idle. This is an event-based power saving strategy that is divided into twomodes. When at least one thread is active the system runs on timer intervals, but whenno work is scheduled and no threads are active, the timer disabled to save power.

2. System Power Management. This part decides how power management is handledwhen the kernel has no work to perform and has entered the idle state. In this case,sleep states are useful to lower the power usage by the system. Two kinds of sleep statescan be selected with the difference of allowing the system to lose execution context, ornot, during the sleep period.

3. Device Power Management. This part decides how power management handle de-vices on the system. This is done by assigning devices to different power managementaliases depending on what state they are in and the purpose of the work.

2.5 Power management in an Android smartwatch

Liu and Qian [10] have in their research measured and optimized the energy use of an Androidsmartwatch. The data for the tests and development was collected from 30 smartwatches ondaily use to get a real understanding of the power consumption. Before any modifications of

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2.6. Energy hungry components of a smartwatch

the smartwatch, they observed three things standing out when it comes to power management.The two first observations are related to the display and illustrate how it makes up the largestpart of the power consumption when the screen is active and inactive. The third representsa comparison between the smartwatch and a smartphone, where the observations are thatthe radio, Bluetooth and WiFi, for the smartwatch has a much smaller effect on the powerconsumption compared to the smartphone. The authors propose some improvements to makethe smartwatch more energy efficient. Considering that the screen stands for the largest portionof the power consumption for the smartwatch, an improvement is needed. The first suggestionis to use and inform about a power friendly design for the User interface (UI). For an o-leddisplay, as in this case, a darker UI is more power friendly than a bright one. The secondsuggestion is to optimize and lower the time the smartwatch display is enabled by changingthe behavior for the display. The last suggestion is to send notifications to the smartwatch ina chunk, to reduce the total power consumption for managing notifications.

2.6 Energy hungry components of a smartwatch

A smartwatch is known for having a display, fast CPU, and a small battery. Compared toa smartphone, the battery of a smartwatch is usually more than ten times smaller, makingthe total amount of available energy limited. Still, smartwatches are expected to have longbattery life.

Liu et al. [10] have done a study to show the usage of a smartwatch in the wild. The energyhungry components for a smartwatch are mostly the same as for a smartphone, because of thesimilarity of components and usage. In the study, the authors discover that the screen standsfor the largest impact on power usage. This is the same discovery as Liu and Qian’s [11] madein their work of an Android smartwatch. The reason why the display uses a lot of energy isrelated to the behavior and the size of the component. The display of an Andriod device isenabled in two out of three states, not including the charging state, which is the awake anddozing state. The awake state is when the device is fully awake and can interact with the user.In the dozing state, all applications are suspended but the user can still see the time with asimplified timer interface on the display. The dozing state itself stands for 50.6% of the totalpower use making it the most power consuming state, while the awake state stands for 27%power use with only 2% awake time. The overall power consumed by the display is 30% overall states.

The second most energy hungry component is the CPU [10]. In the dozing or sleep statethe CPU has a low power consumption, though in the active state the power consumption ishigher. The overall consumption of the CPU is 29% with most of the power used in the activestate.

Compared to smartphones, the radio of a smartwatch does not have a big impact on theoverall power consumption. This is mostly because of two reasons. The first reason is thatthe amount of data transferred is a lot less and in smaller volumes. The second reason is thatmost of the smartwatches are not equipped with a cellular interface, which is more energyhungry than other radios. Smartwatches are instead using short-range radio to communicate,such as Bluetooth or WiFi. The radio on a smartwatch stands for 3.4% of the overall powerconsumption.

The components identified with high power consumption are Display, CPU, and Radio(Bluetooth/WiFi).

2.7 Power saving strategies

Liu et al. [10, 11] propose some techniques on how to save energy in a smartwatch based onmeasurements of the most energy hungry components. The proposed improvements are listedbelow:

6

2.7. Power saving strategies

1. Reduce the awake time. By default, the awake time for the Android smartwatch is5 seconds. This can be reduced by lowering the default value to x < 5 seconds or bydeveloping a smart sensor function that, based on the movement of the watch, can turnit on or off. Using a timer from 1 to 4 seconds the total saving is between 4-9%, whichin time equals 1.7 to 3.8 hours depending on the usage.

2. Improve the sleeping mechanism. The smartwatch is consuming the least amountof energy when in sleep mode. It is therefore important to use sleep mode as frequentlyas possible. By default an Android smartwatch enters sleep mode after an idle periodof 35 min. This can easily be modified to a timeout time of, for example, 5 to 10 minand will give a total power saving of 9-13%, which equals 3.8 to 5.4 hours in total timesavings.

3. Energy efficient User interface (UI). Considering that the display stands for 30%of the total power consumption, there is room for improvements. This can be doneby finding a combination of display brightness and UI colors that will lower the totalpower consumption. For the Android smartwatch a color transformation to black andred results in reducing the wake-up display energy by 41.4%.

4. Delay-tolerant push notification. Notifications sent from the smartphone can bebundled together and delayed to lower the awake time of the radio (Bluetooth (BT)),with the effect of a lowered power consumption.

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3 Method

This chapter describes the method used to answer the research questions. First the structurefor the measurement method on hardware and software is described. Then a description of

how the measurement methods are performed is presented.

3.1 PineTime setup

Figure 3.1: The four debug ports (red) and the charging ports (blue) on the PineTime moth-erboard.

When working with hardware, it is essential to have a good test-bed that is user-friendlyand durable. To achieve this test-bed with the ability to flash, debug, and do measurements,

8

3.2. Experimental setup

some hardware modification was made on the PineTime smartwatch. The smartwatch isequipped with 4 debug ports, in the order SWDIO, SWDCLK, 3.3V, and GND, that can befound under the battery. A cable was attached from the debug ports to the correspondingoutputs on a J-Link EDU mini [17] to be able to flash and debug. To make the structureeven stronger, some holes were drilled in the chassis for the cables to run through. It is nowpossible to connect the smartwatch to a computer with the ability to flash and debug, butin order to do measurements some further modifications were needed. To get the possibilityof doing measurements, two cables, 5V and Ground (GND) were connected to the chargingports, found on the right side of Figure 3.1. These were later connected to the Otti Arc [6] forperforming measurements.

3.2 Experimental setup

The purpose of the environment is to do measurements of hardware on the PineTime smart-watch, where each hardware can be implemented separately. The environment itself is themost power-efficient implementations for the PineTime smartwatch, making it the benchmarkfor the lowest possible power consumption. At startup, the CPU enters idle mode to be powerefficient.

Listing 3.1: Code from the experimental setup.

#include <zephyr . h>

void main ( void ){

//// Add code here f o r implementat ion and enab led / d i s a b l e hardware// be f o r e the CPU en te r s i d l e mode .//

while ( t rue ){k_cpu_idle ( ) ;

}}

9

3.3. Method of measurement

In the experimental environment Listing 3.1, software for the specific measurement is imple-mented before the CPU enters idle mode. This ensures that the individual power consumptionfrom a measurement is the difference between the benchmark and current consumption, withno effect of other functionalities. The measurement parts are divided into different use casesin order to simulate all possible scenarios and to make it easier to find areas that can beimproved. The experimental environment is the foundation for each measurement to buildupon.

1. Connected. Some of the hardware has a physical connection to the SOC that thatcan be easily detached. In this test, hardware will individually be disconnected andconnected again to see the effect on power consumption.

2. Configuration, no implementation. When building a Zephyr application, the com-piler needs to know which hardware and functionality the program is expected to useto get access to them. This is done with the use of a provided configuration file calledprj.conf, in the Zephyr environment. The measurement in this part is supposed to showwhether or not the included driver is optimal, by measuring the hardware before andafter it is configured in the configuration file.

3. Configuration and implementation. This part use the configuration from the previ-ous part and combines it with implementing hardware in the software with the providedfunctionality from the driver. The purpose of this test is to see if implementing but notenabling hardware will affect power consumption.

4. Hardware disabled by software. The purpose of this test is to see how disablinghardware by software will affect power consumption. This is done using the disablingfunctionality from the provided driver. This part is important for developing an idlemode with lower power consumption.

5. Hardware enabled by software. When hardware is enabled by software, it is activeand ready to be used. This is the most power-consuming state and can be seen as theawake mode. Measurements will be made on the hardware when it is enabled by softwareand active.

The division of the measurements is designed to make it easier to understand the behaviorof power consumption for each hardware and to gain knowledge on how possible improvementscan be made to lower the power consumption. If possible, improvements will be made on theimplementation of the hardware. Otherwise the knowledge will be used to create improvementproposals for future work.

3.3 Method of measurement

The measurements are performed using an Otii Arc [6] as the measurement tool. Otii Arc isa small device, connected to a computer, that can supply power for a device and measure itspower use. The measurement of the PineTime is performed on the 5V charging port, with thebattery disconnected, to receive a true power use for the whole device.

10

3.3. Method of measurement

Figure 3.2: The position of the charging ports used to measure with Otii.

3.3.1 Hardware to measureIn the design of the PineTime smartwatch, some components have an external connection tothe SOC while the other has a permanent connection. The measurements will be performedon the hardware and functionalities listed below.

• Display (LCD, st7789V)

• Backlight (st7789V)

• Touch (CST816S)

• Bluetooth (bluetooth 5 LE)

• Logging (Real-time text (RTT))

• Heart rate sensor (HALS3300)

Note: The heart rate sensor can only take part in the hardware-related measurementsand not in software-related ones due to a proprietary driver. The backlight and display arefrom the same component but have there own connections to the SOC and driver. Hence themeasurements are separated from each other.

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4 Results

This chapter presents possible improvements and outlines the measurements described in themethod chapter.

4.1 Measurements

Before the individual measurements of hardware, two benchmark measurements of the Pine-Time smartwatch are made. The first is to show the lowest possible power consumption, withdisconnected hardware running the software from the experimental setup. The second bench-mark measurement is the actual power consumption of the PineTime Hypnos project as itstands right now with functionality for the display, touch, battery, button, and Bluetooth.

Benchmark measurementsMeasurement Idle mode (mA) Awake mode (mA)

Test environment 0.899 0.899PineTime Hypnos project [2] 8.57 16.30

Table 4.1: Benchmark measurements of the Experimental environment and the PineTimeHypnos project.

4.1.1 ConnectingIn this test, the software used is the benchmark described in section 3.2. When measuring theindividual power consumption each hardware is separately connected and disconnected fromthe smartwatch. By using the benchmark measurement and the new measurement, the actualpower consumption of each hardware is calculated.

Not all components can be physically disconnected from the device. The accelerometersensor, vibrator actuator, and Bluetooth cannot be disconnected. They are therefore attachedto the device for all measurements, including the benchmark.

The overall power consumption is the total amount of power used in the whole device, andindividual consumption is the calculated power consumption for specific hardware in relationto the benchmark Table 4.1.

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4.1. Measurements

Measurements of connecting hardwareMeasurement Overall consumption (µA) Individual consumption (µA)Benchmark 899 899Backlight 914 15

Display and Touch 907 8Heart rate sensor 1010 111

Everything connected 1040 141

Table 4.2: Power consumption when connecting hardware to the SOC.

The test shows that the hardware design of the PineTime smartwatch has room for im-provement, Table 4.2. Each component has direct access to the power source, regardless of theCPU:s awareness. An experimental measurement was made on a Nordic development boardusing the same SOC, nRF52832, but with no connected components. The purpose of thisexperiment is to see the power consumption from the SOC alone running the same code. Thismeasurement showed a power consumption of 242 µA.

4.1.2 ConfigurationIn this test, measurements are made on the selected hardware when it is configured in theZephyr configuration file. The configuration file proj.conf is used by the compiler to knowwhich dependencies and drivers the system is requiring. It is important to have a correctconfiguration for the system to work.

The purpose of this test is to show if the included drivers, for each hardware, are optimaland have a suitable implementation. A non-optimal driver can affect the functionality andpower consumption of hardware.

From now on, all components are connected to the device to give a true and realisticpower consumption. So the benchmark for the remaining parts represents the lowest powerconsumption when everything is connected to the device (subsection 4.1.1).

Listing 4.1: Example from prj.conf in Zephyr.## Example c o n f i g u r a t i o n in p r j . conf#CONFIG_SPI=yCONFIG_DISPLAY=yCONFIG_ST7789V=yCONFIG_ST7789V_RGB565=y

Listing 4.1 is an example of how configuration is made in prj.conf using Zephyr. Theconfiguration in this example enables the use of the display and the driver st7789V.

Measurements of configuring hardwareMeasurement Overall consumption (µA) Individual consumption (µA)Benchmark 1040 1040

Display without st7789V 1040 0Display with st7789V 4030 2990

Backlight 1040 0Touch 1050 10

Bluetooth 1040 0Logging 1040 0

Table 4.3: Power consumption when configuring hardware in prj.conf.

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4.1. Measurements

Power consumption should not increase when components are configured. However, if thatis the case, something is wrong with the included driver. Most measurements in Table 4.3show no effect on the power consumption of the device, only the measurement of Display withst7789V show a significant increase.

The display attached to the PineTime smartwatch is an st7789V, which has its driverseparate from the general implementation of a display in Zephyr. For that reason two mea-surements are made to the configuration of the display, one with the driver for st7789V andone without, to show the behavior of both drivers.

4.1.3 ImplementationIn this test, measurements have been performed on hardware when they are connected to theSOC, configured, and implemented in the software. Implemented, in this case, means that thehardware is initialized and ready to be used but has not been enabled or disabled by software.Each measurement is separated from each other and only the required configurations andimplementations are included.

Measurements of implementing hardwareMeasurement Overall consumption (µA) Individual consumption (µA)Benchmark 1040 1040

Display without st7789V 1040 0Display with st7789V 6510 5470

Backlight 7290 6250Touch 1040 0

Bluetooth 1180 140Logging 1040 0

Table 4.4: Power consumption when implementing hardware in main.c.

In this test (Table 4.4), the Display with st7789V and Backlight stands out with high powerconsumption. It seems like the backlight is enabled when implemented because of the screenlighting up. The display, using st7789V, is not enabled by default.

4.1.4 Disabled by softwareThe hardware is ready to be used by the software when connected and configured. To lowerthe overall power consumption, components, and functionalities that are not in use should bedisabled by software and enabled again when expected to be used. This is a good practice forminimizing energy wasted in a system.

In this test, measurements have been made on hardware when these are disabled by softwareto see whether or not the power consumption is lowered as expected.

Measurements of disabling hardware by softwareMeasurement Overall consumption (µA) Individual consumption (µA)Benchmark 1040 1040

Display without st7789V 1120 80Display with st7789V 6500 5460

Backlight 1040 0Touch 1050 10

Bluetooth 1180 140Logging 1040 0

Table 4.5: Power consumption when disabling hardware by software in main.c.

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4.1. Measurements

With the hardware disabled by software, the backlight and touch functionalities are dis-abled as expected, resulting in almost no individual power consumption (Table 4.5). However,the driver for the display once again has a high power consumption. This time with powerconsumption that is five times higher than the combined consumption for all other measure-ments.

4.1.5 Enabled by softwareThe earlier measurements are important for developing a low idle mode power consumption.The opposite of idle mode is when the device is fully awake and ready to be used, this is theawake mode and it stands for 2% of the usage period [11].

In this test, measurements have been performed on hardware to see the power consumptionwhen awake and used by a user. Each measurement is separate from each other and thehardware is configured, implemented, and enabled by software. This test is showing thehighest amount of energy consumed by hardware.

Measurements of enabling hardware by softwareMeasurement Overall consumption (µA) Individual consumption (µA)Benchmark 1040 1040

Display without st7789V 1120 80Display with st7789V 7150 6110

Backlight 7290 6250Touch 3630 2590

Bluetooth 1130 90Logging 4350 3310

Table 4.6: Power consumption when enabling hardware by software in main.c.

The measurement Display without st7789V is not important in this test and may give afaulty reading considering that the display needs the st7789V driver to work.

How the measurements were made in Table 4.6:

• Display with st7789V: This measurement shows the power consumption of the dis-play, using the driver for st7789V when it is enabled by software. Before entering sleepmode in main.c the display was configured, implemented, and enabled using the includedfunctionalities from the driver. This made the power consumption constant making iteasy to get a good reading.

• Backlight: The backlight can be described as an illumination behind the displayto light up the content printed by the display functionality. Without the backlight itis impossible to see anything on the display. The measurement of the backlight wasperformed in the same way as with the display.

• Touch: When pressing on the display an interrupt will activate the touch functionalityfor around 2 seconds. To measure the maximum power consumption of the touch func-tionality, reading is performed on the 2-second interval. Five measurements were madeto get the average reading from the intervals.

• Bluetooth: In the Bluetooth measurement, the smartwatch is connected and pairedto a smartphone. The reading is the power consumption from when the smartwatch andsmartphone is paired together.

• Logging: Logging is necessary when developing software for the smartwatch, but it isimportant to disable when not needed, to save power. In this project an Real-time text(RTT) J-link EDU mini, connected to the smartwatch debug ports section 3.1, was used

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4.2. Possible improvements

to flash and debug. For this measurement the smartwatch is printing log messages frommain.c to a Linux command line RTTLogger using the J-Link described in section 3.1.

4.2 Possible improvements

With input from the measurements some problems in the power management structure havebeen identified.

4.2.1 Display driverThe largest problem for not reaching a low power consumption on the PineTime smartwatchis caused by the driver for the display (st7789V) not being optimal. The driver cannot fullydisable the display by software, which alone makes the overall power consumption high (sub-section 4.1.4). To find a solution to this problem, an experiment was performed on modifyingthe driver. The goal was to find a way to implement the ability to fully disable the display.

In the driver for the st7789V display, two functions are responsible for enabling anddisabling the display st7789v_blanking_on disables the display and st7789v_blanking_offenables it. By default, these functions are using ST7789V_CMD_DISP_ON andST7789V_CMD_DISP_OFF as aliases to turn the display on and off. Consideringthat the measurements show no difference in the power consumption when using theST7789V_CMD_DISP_OFF alias and the fact that the datasheets [18], for this dis-play, refers to it as a power saving strategy something must be wrong. An experimentwas performed to see the effect of using sleep alias instead of the default display aliases.The alias ST7789V_CMD_SLEEP_IN was used to let the display into sleep mode andST7789V_CMD_SLEEP_OUT together with ST7789V_CMD_DISP_ON to exit sleepmode. The experiment was successful, showing the potential of lowering the idle mode powerconsumption of the PineTime smartwatch. With this approach, the overall idle mode powerconsumption is lowered from 7150 µA to 1040 µA for the st7789V display without sacrific-ing functionality. To convert the experiment into a correct implementation for the st7789Vdriver, a more advanced approach is needed. This experiment was performed together withAlbin Söderqvist [19][20].

Helped by the measurements in this work, Albin Söderqvist was able to resolve the problemwith the excessive power consumption of the display by patching the driver. His patch is nowincluded in upstream Zephyr and can thus be used by any Zephyr project. [16][19].

4.2.2 Hardware designThe second problem is caused by the hardware design of the PineTime smartwatch not beingoptimal. When a component is connected to the device, it has direct access to the powerresource, which implies that components are using power regardless of the CPU:s awareness.This is obvious in the test where hardware is connected to the device (subsection 4.1.1), andfor the fact that the power consumption on the developing board is 4.3 times lower runningthe same software. With this hardware design, it is difficult to reach a really low powerconsumption because no matter how optimal the software is, the hardware is always going toconsume the same amount of energy. Power domains could be the solution to this problem. Apower domain divides a system into different areas to get the possibility to manage the powersupply for these areas. The control of power can be done by adding digital switches betweenthe hardware and power resource.

4.2.3 Bluetooth behaviorThe Bluetooth functionality is energy efficient through all tests. But still, an odd behaviorhas been found using the software from the PineTime Hypnos project [2]. The Bluetooth

16

4.2. Possible improvements

functionality is supposed to use less power when disabled by software, but this is not the case.It is using more power when supposed to be power efficient. A solution to this could be toalways have the Bluetooth functionality enabled, but this feels like the wrong solution. Giventhe small change in power consumption with this odd behavior, no further investigation hasbeen made.

4.2.4 Heart rate sensorHaving components connected to the smartwatch that cannot be used is pointless. The heartrate sensor has a driver that is proprietary, which means that it cannot be used in this project.In the connecting measurement (subsection 4.1.1), the heart rate sensor was found to have thehighest individual power consumption of all components. This means that it will use powerwhen connected, even if it is implemented and used in the software or not. So with that inmind, a smart way to save 111 µA of power is to disconnect the heart rate sensor from thesmartwatch.

4.2.5 BacklightThe backlight shows no odd behavior in the measurements but has a high power consumptionwhen activated. The high power consumption is probably related to the brightness level ofthe backlight. From the software in the PineTime Hypnos project, only one brightness levelis considered. In the documentation for the PineTime smartwatch [21], three brightness levelsshould be possible to use. So, to solve the problem with the high power consumption of thebacklight when activated, the three brightness levels should be considered. To see the effect onthe power consumption measurements and experiments must be performed on the brightnesslevels. This part is left for future work.

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5 Discussion

This chapter discusses the method and results of this work and how it can be put in a widercontext.

5.1 Results

The purpose of this thesis is to get an understanding of the power consumption for hardwareon the PineTime smartwatch and use the knowledge to develop power friendly software. Themeasurements are separately made in an enclosed environment to minimize the risk of error.This is the first work to show the power consumption of hardware connected to the PineTimesmartwatch, so before the measurements were made it was hard to know what to expect.Instead, with the knowledge from previous work [10] [11], the expectations were that thedisplay and radio would have high power consumption, but that was not all true. Fromthe measurement of the PineTime Hypnos project, it was obvious that something was wrongconsidering the significant difference in the power consumption compared to the benchmarkmeasurement.

5.1.1 MeasurementsAmong other things, it was found that the hardware design has a flaw that allows componentsto have direct access to the power resource. From a power management perspective, this isnot an optimal design considering that the CPU cannot control where energy is spent and bywhich component. The solution to this could be to redesign the hardware or add switchesbetween components and the power resource that the CPU can control.

From the measurements of the software (subsection 4.1.2 - subsection 4.1.5), the driverfor the display st7789V is the component with the highest power consumption. It seemslike the functionality, in the driver, that is supposed to disable the display is incomplete andsub-optimal. This is obvious in the measurement of disabling hardware by software (subsec-tion 4.1.4) where all components except the display were able to provide a low power con-sumption. An experiment was made to find the problem, and if possible correct it. Findingthe source of the problem was considerably easy. It turned out to be caused by a sleep aliasthat did not work as intended. When changing this alias to another sleep alias, the powerconsumption dropped. The problem was located and a possible improvement was found.

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5.2. Method

The measurements show that the power consumption for the radio is low. Usually, theradio stands for a large part of the power consumption in a device, considering the energyneeded to turn the radio module on [11]. But in this case, the radio (Bluetooth) was one ofthe most power efficient components through all measurements, even in awake mode. This isprobably favored by this smartwatch being equipped with a low energy Bluetooth [21]. Fromthe measurements an odd behavior in the power consumption was detected for the Bluetoothfunctionality. Strangely enough, Bluetooth uses more power when implemented or disabled bysoftware than when enabled by software. As the Bluetooth functionality was deployed in thePineTime Hypnos project [2], it is using less power when advertising than not. But consideringthe small effect on power consumption, no further investigation was made.

Without the help of measurements the problem with the Bluetooth, display, and hardwaredesign would never have been found.

5.1.2 Possible improvementsGiven that the battery life is 22 hours for the PineTime Hypnos project, it was essential to findareas that can be improved to lower the high power consumption. The possible improvementsobserved with the help of measurements should all be able to lower the individual and overallpower consumption. The software related improvements should be considerably easy to fulfill,but the hardware related is more complicated. Modifying hardware is more complex thanmodifying and testing software. To solve the problem with components having direct accessto the power resource it is necessary to redesign the hardware structure. For that reason, itis easier to solve the software related problems. With a power consumption of 1040 µA whenall components are connected to the PineTime smartwatch, it is possible to reach a batterylife of around 170 hours. This is calculated by dividing the power usage (1040µA) with thebattery capacity (180 mAh). By solving the software related problems and disconnecting theheart rate sensor, it should be possible to get close to the claimed battery life of 168 hours.

5.2 Method

The method part described how to perform measurements on the PineTime smartwatch toanswer the research questions.

5.2.1 Test-bedThe modifications to the hardware worked out well. The test-bed was strong, practical, andstill allowed you to wear the smartwatch on your wrist. The button, that is connected to thebattery, turned out to be crucial for software development as the system sometimes needed ahard reset to work. The principle of the software environment is to use a minimal amount ofcode on the same foundation for each measurement. This worked well when doing experiments,but it was sometimes hard to minimize the amount of software needed. In some cases, it wasdifficult to know exactly what code was necessary for a specific test, so there is a small risk thatmore software than needed was included. Regardless of this risk, the internal validity shouldbe rather high because measurements have shown that software not related to hardware isnot affecting the power consumption. Before measurements, the implemented code had to beverified to ensure that everything was working as intended. This was done using the logger andother hardware to test the implementation. The extra software needed for testing was laterremoved before the measurement was made. For example, the backlight had to be enabled totest that the implementation of the display was working.

The software and hardware modifications described in this work should allow other peopleto receive the same result from measurements, as long as the same hardware and software areused. The PineTime Hypnos project is an ongoing project. In order to receive the same result,it is crucial to use software from the same git commit [3].

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5.3. The work in a wider context

5.2.2 Division of measurementsThe division of the measurements made it easier to find odd behaviors and areas that canbe improved. Having measurements related to hardware and software was the right thing todo, considering that problems were located in the hardware and software design. The selectedmeasurements in this work are designed to match the research questions and the PineTimesmartwatch. This means that the division of measurements are unique to this work and maynot be suitable for other devices.

The measurements for enabling and disabling hardware by software are also essential inthe development of an idle and awake mode in future work. These measurements show thebehavior, in power consumption, of hardware when they are expected to be enabled or disabled.

5.2.3 Hardware to measureMeasurements were performed on all essential components and functionalities of the PineTimesmartwatch. The selection of hardware was straightforward. All components except the vibra-tor were selected because, as it stands right now, it is not included in the PineTime Hypnosproject. But, it would still be helpful to know the behavior of the vibrator for the futuredevelopment of the smartwatch. So, the principal made in this work can be used to performthe same measurements on the vibrator.

5.3 The work in a wider context

Power management is an important part when developing devices today, so a lot of the knowl-edge from this work can be put in a wider context. Not only the measurements and thetest-bed but the knowledge of how to do measurements and why it is important. Withoutmeasurements, it is impossible to know and understand how power is distributed betweenprocesses and components. This was the case before the measurements were made on thePineTime smartwatch, nobody knew of how power was distributed and the effect of the powermanagement structure.

With the help of measurements, possible improvements have been proposed with the pos-sibility to lower the overall power consumption. One can learn from someone else’s mistakes.The possible improvements in this work can be taken into consideration when developing othersimilar systems to reduce the risk of making the same mistake.

Components on the PineTime smartwatch, including the nRF52832 SOC, can be foundin other devices and systems. The measurements in this work show the actual behavior andpower consumption for individual components and drivers. This means that the measurementsare not unique to the PineTime smartwatch. The component will have similar behavior andpower consumption in other systems when using the same driver.

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6 Conclusion

The purpose of this work was to use measurements to show how different components impactthe overall and individual power consumption on the PineTime smartwatch, and based on theresult find areas of improvement. With this in mind, it is time to conclude this work based onthe research questions.

1. How can a good test-bed for hardware and software on the PineTime smartwatch bedeveloped to enable the possibility to do power measurements?This question is answered in section 3.1. Modifications to the hardware that enables thepossibility to flash, debug, and do measurements were successfully and deployed.

2. What is the overall and individual power consumption by hardware on the PineTimesmartwatch in different use cases?Measurements on the overall and individual power consumption can be found in sec-tion 4.1. Measurements were made on Bluetooth, Display, Logging, Backlight, andTouch in five different use cases that simulate different behaviors in the functionality.The five measurements were Connecting, Configuration, implementing, disabled by soft-ware, and enabled by software and were performed a sufficient number of times to receivea credible value.

3. Based on measurements, what areas of improvements can be found that have the potentialto lower the overall power consumption?An investigation to answer this question have been done in section 4.2. The measure-ments showed that the hardware design, Display driver, and Bluetooth functionalityhave the potential to be improved because of high individual power consumption andodd behaviors.

For future work, more experiments can be performed on the identified problems from thiswork. A successful experiment was performed on the problem related to the display driver. Butto reach a really low power consumption, the hardware must be modified. This can probablybe made in the ways described in subsection 4.2.2.

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