1. Reducing Noise and System Costs by Managing Switching Power Supplies as Real-Time Processes Subash Sachidananda & Dr. Alex Dean Dept. of ECE - NC State University

2. EMI- and Energy-Aware Scheduling of Switching Power Supplies in Hard Real- Time Embedded Systems Subash Sachidananda & Dr. Alex Dean Dept. of ECE - NC State University RTAS 2011

3. Overview 0 Goal 0 Switch mode power supplies (SMPS) 0 Real-time scheduling 0 Our work Energy Optimization Switching Power Supplies Embedded Systems Real-Time Systems Energy Optimization

4. Application Domain - Embedded Systems

5. Goal 0 Reduce power and energy used by embedded computing systems in a cost-effective way 0 Basics 0 Two parts: Static and dynamic 0 P = S P V CC 2 + C P V CC 2 f Clock 0 Power  V 2 0 Energy is power * time 0 Competing pressures for energy optimization 0 Shut off unused subsystems 0 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

6. Switching Power Converters 0 Function 0 Efficient conversion of voltage up (boost) or down (buck), or both (buck-boost) 0 Benefits 0 Can run circuitry at lowest feasible voltage 0 Can scale voltage dynamically as needed to support changing clock speed 0 Battery voltage variations across discharge curve do not affect operating point of circuit

7. SMPS Extends Battery Life

8. Boost Converter Operation 0 Switch (transistor) S turns on 0 Current starts flowing through inductor L and S 0 Switch S turns off 0 Current flowing through L now goes through diode to charge C and power load

9. 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, radiated 0 Very sensitive to PCB layout: trace length, capacitor placement 0 Can be 100 mV

10. 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]

11. Reducing Harmonics 0 Methods 0 Redesign PCB and test. Repeat until acceptable 0 Use high-quality (expensive) capacitors 0 Limit slew rate of switches, use sinusoidal drive 0 Change to balanced topology 0 Insert inverse of harmonic 0 Drawbacks 0 More complex hardware design raises cost, size, mass 0 Some methods reduce efficiency

12. System Schedule and Noise

13. Real-Time System Analysis 0 Problem statement 0 We have a system of periodic software tasks running on a processor 0 How do we make sure all tasks meet their deadlines (are schedulable)? 0 Approaches 0 Use response-time analysis 0 When does the last task finish in the worst case? 0 Use a utilization-based test 0 How much of the processor’s time could we use?

14. Real-Time System Model 0 Assumptions 0 Single CPU 0 T ContextSwitch = 0 0 tasks are periodic with period  i 0 Deadline D i = period  i 0 No data dependencies between tasks 0 Constant process execution time T i Burns & Welling

15. Scheduling Approaches 0 Dimensions to task scheduling 0 Static vs. dynamic task ordering 0 Preemptive vs. non-preemptive 0 Prioritized vs. non-prioritized 0 Fixed vs. dynamic priority 0 Common scheduling approaches for real-time systems 0 Dynamic task ordering 0 Preemption among tasks 0 Priority assigned based on 0 Task frequency (“rate monotonic”, RM), or 0 Deadline frequency (“deadline monotonic” DMS), or 0 Earliest deadline first (EDF)

16. Utilization-Based Schedulability Tests 0 Utilization: Fraction of time processor is busy 0 Easy for EDF: Schedulable if U < 100% 0 Harder for RMS/DMS 0 Schedulable if utilization U < U max

17. Our Contributions 0 Goal 0 Ensure that noisy SMPS will not switch while a noise- sensitive task is running 0 Make-and-Take Approach 0 Put the SMPS under control of the task scheduler 0 Enhance the real-time scheduling model math to include SMPS activity

18. Task Scheduler Controls SMPS Get ready task T i Stop SMPS, measure V supply Restart SMPS Is task T i rivalrous with SMPS? Run task T i Is V supply > V Threshold,i ? Wait until V supply > V Threshold,i Stop SMPS Yes No

19. Real-time Model Updated 0 Start with execution time T i for each task i 0 Add in time if needed to run power supply to charge capacitor 0 For non-rivalrous tasks, T i * = T i 0 For rivalrous tasks, T i * = T i + T SMPS,i 0 Simple yet remarkably powerful

20. Experimental Evaluation 0 Build a system and see …

21. Hardware 0 QSK62P MCU board 0 16-bit, 24 MHz, 32K SRAM, 64K ROM 0 3-5V operation 0 Boost converter 0 Dirt-Cheap Value- engineered 0 450 kHz switching freq. 0 3.7 V input (Lithium cell) 0 Spec: 3.8 V to 4.8 V V supply

22. Application Software 0 Tasks t 1 -t 3 sample analog values (pressure, temperature, audio) and are sensitive to SMPS noise 0 Add in corresponding SMPS active time requirement 0 Task t 4 transmits data out UART, is not sensitive

23. Schedulability Analysis 0 Use rate-monotonic priority ordering, preemptive fixed-priority scheduling 0 Utilization test 0 Initial task set: U = 0.050 0 After adding SMPS: U* = 0.128 0 Utilization bound for RMS = 0.766 0 So system is schedulable and will never miss a deadline

24. Mutual Exclusion Enforced 0 T1* ready to run after T3*, but V supply is too low 0 So SMPS runs first, then T1 runs 0 V supply is low, so scheduler turns it on and can also run T4

25. Task Released While SMPS On 0 T2* ready to run, but SMPS is running 0 Scheduler measures V supply decides it is high enough to shut off SMPS and run T2* to completion

26. SMPS Idle When Not Needed 0 Scheduler runs when task T3* is released 0 Determines V supply high enough to run T3* without SMPS

27. Future Work 0 Tighten up utilization bound 0 Enable more overlap of SMPS operation with noise- insensitive tasks 0 Enhance scheduler 0 Tighten up schedulability model 0 Support buck conversion 0 Support multiple voltage domains

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

29. Thank you! 0 alex_dean@ncsu.edu alex_dean@ncsu.edu 0 http://www.cesr.ncsu.edu/agdean http://www.cesr.ncsu.edu/agdean

30. Appendix

31. 1 - Task Scheduler Controls SMPS

32. Flowchart of task scheduler

33. SMPS Allows Low-Voltage Operation 0 Different Minimum Voltages

34. Supporting Preemptive vs. Non-preemptive