Low Power Design Basics

Introduction

Low power means average power consumption in usage of the electronic product is at its minimal via low power design techniques in both hardware and software level respectively.

The benefit of low power can be broken down in two parts:

1. extended usage for battery powered devices

2. lowering power grid burden for plugged in devices

Lastly, the often overlooked area for low power design is carbon footprint due to carbon emissions. This reason is that carbon dioxide generation is directly proportional to energy consumption originated from power plants by burning fuel or coal to generate that electricity.

One might dismiss this statement initially and have a hunch that it is unrealistic for consumer electronics to have a such significant impact on carbon emissions, but what if I tell you that 250,000 equivalent of car emissions per year can be saved if we reduce idle screen on time from 4 hours to half of an hour for just one year of TV roughly in tens of millions range? You might be surprised to learn that, and so was I when I first worked on it collaboratively with a hardware sustainability team. We did a power consumption impact of TVs on carbon emission study. As a result, the lesson to learn here is that large scale and always powered-on consumption electronics can make a significant impact in sustainability in carbon footprints.

The true goal of this article is to educate system EE, system architects, and systems engineers to be aware of their carbon footprint impact and while providing technical guidance/ideas to how to tackle this problem.

Background

Power states: This denotes various power consumption levels: active, idle, suspend, and off,  that the device enters under different uses.

Connectivity: This is related to wireless and/or wired interoperability capability with other connected devices within the network.

System on Chip (SoC): This is a heterogeneous multi function chip that embeds a generous purpose processor with special purpose processor along with internal memories and peripheral controllers to simplify hardware design. 

Design PHILOSOPHY

At a higher level, low power design is achieved by specialization and division of computation tasks to dedicated hardware blocks, which is controlled by the power management unit (PMU) and resource scheduler of an operating system. 

This might sound confusing at first; however, if we take a step back and examine the underlying reasons that a factory is efficient at producing goods, one can see that specialization and division of labor improves efficiency in resource allocation and execution. Similarly, in electronics design, "specialization" is achieved by using special purpose processors or commonly system on chip (SoC) which consists of special purpose processor such as graphic processor unit (GPU), audio processor (aDSP), power management unit (PMU), sensor fusion hub, radio modem, etc. Lastly, division of tasks is achieved by offloading tasks to these special purpose processors by software level controls. As a result, efficient low power design is achieved.

Example

Software level: Optimization made using controls from software systems.

TV Low Power Design:

• There are 4 power states for smart TV namely:

  • Active: This state refers to the power state where the TV screen is on.

  • Idle: This state refers to the power state where the TV screen is off but the TV hardware is kept active.

  • Suspend: TV enters the lowest power state where only minimal hardware is kept active to receive wake up signals such as remote key press, WiFi updates, etc.

  • Off: TV is completely powered off

With power states in mind, the general power number for most common 4k 55-inch TV is 100W in active, 10W in idle, 0.3W in Suspend, and 0 W in off. This implies the most power saving technique is to minimize all possible active states and transition to idle or suspend power state whenever one can without impacting user experience.

Depending on the product requirements, a general approach is having a timer that turns the TV from active mode to idle mode after idling for 15 mins and turn TV from idle state to suspend after an additional 30 mins. If the TV requires voice activation, then special audio hardware such as a wake word digital signal processor is needed to keep voice detection active while keeping the rest of TV hardware off to achieve suspended power state. We call this technique offloading, and it's widely used.

Low Power Sensor Design:

Sensors provide constant streams of information updating the state of electrical devices with information like temperature, motion, object orientation, etc. These constant streams of data are fed into sensor fusion cores for data processing and event detection such as the step counter of a fitness watch. A general approach for low power design is using an always-on low-power microcontroller (We generally refer to this processor as sensor hub) to offload event detection from the main application processor.

In practice, this sensor hub processor is either embedded in the System on Chip or acts as a standalone microcontroller which supplements the main processor.

Bluetooth Connectivity Low Power Design:

• Bluetooth devices exchange information such as data and command based on agreed timed connection intervals where connected devices can exchange information reliability; hence devices would not miss each other’s transmission and acknowledgement because an agreement of connection interval is established. However, this brings up the question: What is the relationship between frequency of intervals and power consumption?

• That is the question that low power design needs to address. The reason is that all the connections need to be kept active while minimizing power consumption. This allows battery powered Bluetooth devices such as a remote control to have extended battery life.

• In order to find the right balance between frequency of connection and average power consumption, low-energy Bluetooth protocol is chosen instead of classic Bluetooth protocol. The main difference between them is that low-energy protocol implements lower data throughput and flexible connection intervals, which allow radio transmission duration to be minimal and frequency to be less often, hence less energy is used.

• Basic connection configurations for Bluetooth low-energy is to reduce connection interval frequency by increasing slave latency parameters (i.e this parameter allows specified skipping of peripheral Bluetooth reconnection requests from the Host bluetooth device) while keeping the connectivity active and user-experience response latency at minimal. This balance/trade-off is a challenge for all product manufacturers. One thing to note that protocols have limitations and are constantly evolving to support lower power state requirements.

Hardware Level: Optimization made using controls from the hardware system.

Power Electronics Design:

• Generally, power electronics must be selected appropriately for each respective load. For high-power rails, switch mode power supplies are used. For low-power loads that require precise and clean output, linear regulators are used. To achieve overall high efficiency design, each power rail and the sharing of rails are optimized to minimize power conversion loss in each stage of power transmission. 

• One technique to further save power is to use power gating where a subsystem can be completely powered off and turn off the power supply rails to that chip by gating the power rails. This generally is achieved for turning off the power supply for that chip by either using a load switch or for turning off the regulator completely.

  • However, this technique must be well thought out to make sure the device switches off and on without risking or degrading the other functions of the device.

Clock Source:

• Use slow clocks for low power hardware blocks and fast clocks for active and fast hardware blocks. Hardware blocks are often implemented by the IC manufacturers as part of the IC, so it is the responsibility for the system EE to find the appropriate clock source to feed into the clock input ports.

• One can use a standalone crystal or a temperature control crystal (TCO) as a low-frequency deviation/error clock source or having the clock come from a host device such as application processor or a power management IC, which is often kept active and can generate a clock signal in all power states of the electronic products. 

• Configuration of digital PLL to lower precious ones reduce power consumptions. For instance, if fractional PLL is not needed, switch to a normal one can reduce PLL consumptions. In the end, for low power electronics such as mobile computer or wearables every microwatt counts.

• Example Design for Bluetooth:

• Basic hardware design is to use different clocks, fast and slow clocks, for bluetooth chips. What I mean by clocks is that the bluetooth radio transmission intervals timing is derived from a clock source by the Bluetooth chip. Generally, a slow 32 kHz clock  (e.g., a crystal is used) is perfect for clock generation for precision connection intervals calculation when bluetooth chips enter a low power state. In addition to that, a slow clock keeps other hardware blocks active during the low-power state of the Bluetooth chip which functions as a monitor for interrupt and/or wake-up signals in order to instantaneously bring the Bluetooth chip to active mode for on timed data transmission or reception. The fast clock, generally 10s of MHz, is used to activate the radio transceiver hardware and network processors.

Summary & Conclusion:

• Low power design is aiming to reduce overall average power consumption of the product in usage which with scale has a significant impact on the carbon footprint.

• Four general power states of consumer electronics are: active, idle, suspend, and off.

• Transition from high power states to lower states is the goal of software level low-power design

• Increasing overall power delivery efficiency is the goal of hardware level power design.

• For connectivity devices, minimized re-connection frequency is the practical design technique to reduce power consumption of the radio hardware.

• For multi-sensor device, connect all sensors to a low power sensor hub.

Low power design is very crucial to meet today's consumer demand due to miniaturization and portability of hardware products. The main challenge is for the device to stay connected while maintaining an extended period of usage time. As a system level engineer, both hardware and software level designs must be considered to meet the user's wants and needs.