Slide 1Slide 2OverviewApplication Domain - Embedded SystemsGoalSwitching Power ConvertersSMPS Extends Battery LifeBoost Converter OperationSwitching Converter ChallengesMore on HarmonicsReducing HarmonicsSystem Schedule and NoiseReal-Time System AnalysisReal-Time System ModelScheduling ApproachesUtilization-Based Schedulability TestsOur ContributionsTask Scheduler Controls SMPSReal-time Model UpdatedExperimental EvaluationHardwareApplication SoftwareSchedulability AnalysisMutual Exclusion EnforcedTask Released While SMPS OnSMPS Idle When Not NeededFuture WorkConclusionsThank you!Appendix1 - Task Scheduler Controls SMPSFlowchart of task schedulerSMPS Allows Low-Voltage OperationSupporting Preemptive vs. Non-preemptiveReducing Noise and System Costs by Managing Switching Power Supplies as Real-Time ProcessesSubash Sachidananda & Dr. Alex DeanDept. of ECE - NC State UniversityEMI- and Energy-Aware Scheduling of Switching Power Supplies in Hard Real-Time Embedded SystemsSubash Sachidananda & Dr. Alex DeanDept. of ECE - NC State UniversityRTAS 2011Overview0Goal0Switch mode power supplies (SMPS)0Real-time scheduling0Our workEnergy OptimizationSwitching Power SuppliesEmbedded SystemsReal-Time SystemsEnergy OptimizationApplication Domain - Embedded SystemsGoal0Reduce power and energy used by embedded computing systems in a cost-effective way0Basics0Two parts: Static and dynamic0P = SPVCC2 + CPVCC2fClock0Power a V20Energy is power * time0Competing pressures for energy optimization0Shut off unused subsystems0When running, run as fast as possible to minimize static power (must raise supply voltage to speed up clock)0When running, use minimum voltage which supports logic’s clock frequencySwitching Power Converters0Function0Efficient conversion of voltage up (boost) or down (buck), or both (buck-boost)0Benefits0Can run circuitry at lowest feasible voltage 0Can scale voltage dynamically as needed to support changing clock speed0Battery voltage variations across discharge curve do not affect operating point of circuitSMPS Extends Battery Life0408012016020024028032036040044048052056060064068072076080084088092096010001040108011204.004.505.005.506.006.507.007.508.008.509.00battery Linear SMPSHoursPow er (mW)Boost Converter Operation0Switch (transistor) S turns on0Current starts flowing through inductor L and S0Switch S turns off0Current flowing through L now goes through diode to charge C and power loadSwitching Converter Challenges0Low-frequency “noise” at switching frequency. 0Easy to remove with capacitors0High-frequency “noise” at harmonics of switching frequency. 0Reaches circuit in three ways: conducted, reflected, radiated0Very sensitive to PCB layout: trace length, capacitor placement0Can be 100 mVMore on Harmonics0“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 Harmonics0Methods0Redesign PCB and test. Repeat until acceptable0Use high-quality (expensive) capacitors0Limit slew rate of switches, use sinusoidal drive0Change to balanced topology0Insert inverse of harmonic0Drawbacks0More complex hardware design raises cost, size, mass0Some methods reduce efficiencySystem Schedule and NoiseReal-Time System Analysis0Problem statement0We have a system of periodic software tasks running on a processor0How do we make sure all tasks meet their deadlines (are schedulable)?0Approaches0Use response-time analysis0When does the last task finish in the worst case?0Use a utilization-based test0How much of the processor’s time could we use?Real-Time System Model0Assumptions0Single CPU0TContextSwitch = 00tasks are periodic with period ti0Deadline Di = periodti0No data dependencies between tasks0Constant process execution time TiBurns & WellingScheduling Approaches0Dimensions to task scheduling0Static vs. dynamic task ordering0Preemptive vs. non-preemptive0Prioritized vs. non-prioritized0Fixed vs. dynamic priority0Common scheduling approaches for real-time systems0Dynamic task ordering0Preemption among tasks0Priority assigned based on 0Task frequency (“rate monotonic”, RM), or0Deadline frequency (“deadline monotonic” DMS), or0Earliest deadline first (EDF)Utilization-Based Schedulability Tests0Utilization: Fraction of time processor is busy0Easy for EDF: Schedulable if U < 100%0Harder for RMS/DMS0Schedulable if utilization U < Umax00.20.40.60.810 10 20 30 40Num ber of TasksMaximum Utilization 12/1mMaxmUmiiiTU1Our Contributions 0Goal0Ensure that noisy SMPS will not switch while a noise-sensitive task is running0Make-and-Take Approach0Put the SMPS under control of the task scheduler0Enhance the real-time scheduling model math to include SMPS activityTask Scheduler Controls SMPSGet ready task TiStop SMPS, measure VsupplyRestart SMPSIs task Ti rivalrous with SMPS?Run task TiIs Vsupply > VThreshold,i?Wait until Vsupply > VThreshold,i Stop SMPSYesYesNoNoReal-time Model Updated0Start with execution time Ti for each task i0Add in time if needed to run power supply to charge capacitor0For non-rivalrous tasks, Ti* = Ti 0For rivalrous tasks, Ti* = Ti + TSMPS,i0Simple yet remarkably powerfulExperimental Evaluation0Build a system and see …DOES IT WORK?Hardware0QSK62P MCU board016-bit, 24 MHz, 32K SRAM, 64K ROM03-5V operation0Boost converter0Dirt-Cheap Value-engineered0450 kHz switching freq.03.7 V input (Lithium cell)0Spec: 3.8 V to 4.8 V VsupplyApplication Software0Tasks t1-t3 sample analog values (pressure, temperature, audio) and are sensitive to SMPS noise0Add in corresponding SMPS active time requirement0Task t4 transmits data out UART, is not sensitiveSchedulability Analysis0Use rate-monotonic priority ordering, preemptive fixed-priority scheduling0Utilization test0Initial task set: U = 0.0500After adding SMPS: U* = 0.1280Utilization bound for RMS = 0.7660So system is schedulable and will never miss a deadlineMutual Exclusion Enforced0T1* ready to run after T3*, but Vsupply is too low0So SMPS runs first, then T1 runs0Vsupply is low, so scheduler turns it on and can also run T4Task Released While SMPS On0T2* ready to run, but SMPS is running0Scheduler measures Vsupply decides it is high enough to shut off SMPS and run T2* to completionSMPS Idle When Not Needed0Scheduler runs when task T3* is released0Determines Vsupply high enough to run T3* without SMPSFuture Work0Tighten up utilization
View Full Document
Unlocking...