Reducing Noise and System Costs by Managing Switching Power Supplies as Real-Time ProcessesSubash Sachidananda & Dr. Alex DeanDept. of ECE - NC State University

EMI- and Energy-Aware Scheduling of Switching Power Supplies in Hard Real-Time Embedded SystemsSubash Sachidananda & Dr. Alex DeanDept. of ECE - NC State UniversityRTAS 2011

Overview

0Goal0Switch mode power

supplies (SMPS)0Real-time scheduling0Our work

Energy Optimization

Switching Power

Supplies

Embedded Systems

Real-Time Systems

Energy Optimization

Application Domain - Embedded Systems

Goal

0 Reduce power and energy used by embedded computing systems in a cost-effective way

0 Basics0 Two parts: Static and dynamic0 P = SPVCC

2 + CPVCC2fClock

0 Power a V2

0 Energy is power * time0 Competing pressures for energy optimization

0 Shut off unused subsystems0 When running, run as fast as possible to minimize static power (must

raise supply voltage to speed up clock)0 When running, use minimum voltage which supports logic’s clock

frequency

Switching Power Converters

0Function0 Efficient conversion of voltage up (boost) or down

(buck), or both (buck-boost)0Benefits

0 Can run circuitry at lowest feasible voltage 0 Can scale voltage dynamically as needed to support

changing clock speed0 Battery voltage variations across discharge curve do not

affect operating point of circuit

SMPS Extends Battery Life

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battery Linear SMPS

Hours

Pow

er (

mW

)

Boost Converter Operation

0Switch (transistor) S turns on

0Current starts flowing through inductor L and S

0Switch S turns off0Current flowing through

L now goes through diode to charge C and power load

Switching Converter Challenges

0 Low-frequency “noise” at switching frequency. 0 Easy to remove with capacitors

0 High-frequency “noise” at harmonics of switching frequency. 0 Reaches circuit in three ways: conducted, reflected, radiated0 Very sensitive to PCB layout: trace length, capacitor placement0 Can be 100 mV

More on Harmonics

0 “Unconscionable amounts of bypass capacitors, ferrite beads, shields, Mu-metal and aspirin have been expended in attempts to ameliorate noise-induced effects.” [Jim Williams, Linear Technology Application Note 70]

Reducing Harmonics

0Methods0 Redesign PCB and test. Repeat until acceptable0 Use high-quality (expensive) capacitors0 Limit slew rate of switches, use sinusoidal drive0 Change to balanced topology0 Insert inverse of harmonic

0Drawbacks0 More complex hardware design raises cost, size, mass0 Some methods reduce efficiency

System Schedule and Noise

Real-Time System Analysis

0Problem statement0 We have a system of periodic software tasks running on

a processor0 How do we make sure all tasks meet their deadlines (are

schedulable)?0Approaches

0 Use response-time analysis0When does the last task finish in the worst case?

0 Use a utilization-based test0How much of the processor’s time could we use?

Real-Time System Model0Assumptions

0 Single CPU0 TContextSwitch = 0

0 tasks are periodic with period ti

0 Deadline Di = periodti

0 No data dependencies between tasks

0 Constant process execution time Ti

Burns & Welling

Scheduling Approaches

0 Dimensions to task scheduling0 Static vs. dynamic task ordering0 Preemptive vs. non-preemptive0 Prioritized vs. non-prioritized

0 Fixed vs. dynamic priority

0 Common scheduling approaches for real-time systems0 Dynamic task ordering0 Preemption among tasks0 Priority assigned based on

0Task frequency (“rate monotonic”, RM), or0Deadline frequency (“deadline monotonic” DMS), or0 Earliest deadline first (EDF)

Utilization-Based Schedulability Tests

0Utilization: Fraction of time processor is busy

0Easy for EDF: Schedulable if U < 100%0Harder for RMS/DMS

0 Schedulable if utilization U < Umax

12 /1 mMax mU

m

i i

iTU1

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0 10 20 30 40

Number of Tasks

Max

imu

m

Uti

lizat

ion

Our Contributions

0Goal0 Ensure that noisy SMPS will not switch while a noise-

sensitive task is running0Make-and-Take Approach

0 Put the SMPS under control of the task scheduler0 Enhance the real-time scheduling model math to include

SMPS activity

Task Scheduler Controls SMPSGet ready task Ti

Stop SMPS, measure Vsupply

Restart SMPS

Is task Ti rivalrous with SMPS?

Run task Ti

Is Vsupply > VThreshold,i?

Wait until Vsupply > VThreshold,i

Stop SMPS

YesYes

No

No

Real-time Model Updated

0Start with execution time Ti for each task i

0Add in time if needed to run power supply to charge capacitor0 For non-rivalrous tasks, Ti* = Ti

0 For rivalrous tasks, Ti* = Ti + TSMPS,i

0Simple yet remarkably powerful

Experimental Evaluation

0Build a system and see …

DOES IT WORK?

Hardware0QSK62P MCU board

0 16-bit, 24 MHz, 32K SRAM, 64K ROM

0 3-5V operation0Boost converter

0 Dirt-Cheap Value-engineered

0 450 kHz switching freq.0 3.7 V input (Lithium cell)

0Spec: 3.8 V to 4.8 V Vsupply

Application Software0Tasks t1-t3 sample analog values (pressure,

temperature, audio) and are sensitive to SMPS noise0 Add in corresponding SMPS active time requirement

0Task t4 transmits data out UART, is not sensitive

Schedulability Analysis

0Use rate-monotonic priority ordering, preemptive fixed-priority scheduling

0Utilization test0 Initial task set: U = 0.0500 After adding SMPS: U* = 0.1280 Utilization bound for RMS = 0.766

0So system is schedulable and will never miss a deadline

Mutual Exclusion Enforced

0 T1* ready to run after T3*, but Vsupply is too low

0 So SMPS runs first, then T1 runs0 Vsupply is low, so scheduler turns it on and can also run T4

Task Released While SMPS On

0T2* ready to run, but SMPS is running0Scheduler measures Vsupply decides it is high enough to

shut off SMPS and run T2* to completion

SMPS Idle When Not Needed

0 Scheduler runs when task T3* is released0 Determines Vsupply high enough to run T3* without SMPS

Future Work

0Tighten up utilization bound0Enable more overlap of SMPS operation with noise-

insensitive tasks0 Enhance scheduler0 Tighten up schedulability model

0Support buck conversion0Support multiple voltage domains

Conclusions

0Practical to use real-time scheduling to enable use of noisy power converters in noise-sensitive applications without adding hardware

Thank you!

0alex_dean@ncsu.edu0http://www.cesr.ncsu.edu/agdean

Appendix

1 - Task Scheduler Controls SMPS

Flowchart of task scheduler

SMPS Allows Low-Voltage Operation

0Different Minimum Voltages

Supporting Preemptive vs. Non-preemptive