Energy Management in Embedded Systems

1. Energy Management in Embedded Systems Aurobinda Routray, Associate Professor Department of Electrical Engineering Indian Institute of Technology Kharagpur

2. Why Power Efficiency in Low Power • Stand Alone Systems • Battery Driven • Battery capacity is limited • It is possible to decrease the Battery discharge rate by Intelligent use of its power • DVS: stands for Dynamic Voltage Switching • Hardware: reconfiguration and intelligent clock throttling • Software: Code Size Minimization and Run Time optimization

3. Which Systems need it • Cell Phones • Sensor Networks • Pervasive Computing • Ubiquitous Computing • All kinds of real time embedded systems • What is an real time embedded system ? • Miniature Robots • Of Course EVs

4. Pervasive Computing

5. nickel-cadmium battery Electrochemistry • A fully charged NiCd cell contains: a nickel hydroxide positive electrode plate. a cadmium negative electrode plate. a separator. and an alkaline electrolyte (potassium hydroxide). NiCd batteries usually have a metal case with a sealing plate equipped with a self-sealing safety valve. The positive and negative electrode plates, isolated from each other by the separator, are rolled in a spiral shape inside the case. The chemical reaction which occurs in a NiCd battery is: 2 NiO(OH) + Cd + 2 H2O ↔ 2 Ni(OH)2 + Cd(OH)2 This reaction goes from left to right during discharge, and from right to left during charge. The alkaline electrolyte (commonly KOH) is not consumed in this reaction and therefore its Specific Gravity, unlike in Lead- Acid batteries, is not a guide to its state of charge.

6. Battery specifications • Energy/weight 40–60 Wh/kg • Energy/size 50–150 Wh/L • Power/weight 150W/kg • Charge/discharge efficiency 70%–90% • Self-discharge rate 10%/month • Time durability • Cycle durability 2000 cycles • Nominal Cell Voltage 1.2 V

7. Nickel-metal hydride battery A nickel-metal hydride battery, abbreviated NiMH, is a type of rechargeable battery similar to a nickel-cadmium (NiCd) battery but has a hydrogen-absorbing alloy for the negative electrode instead of cadmium. As in NiCd batteries, the positive electrode is nickel oxyhydroxide (NiOOH). A NiMH battery can have two to three times the capacity of an equivalent size NiCd and the memory effect is not as significant. However, compared to the lithium-ion battery, the volumetric energy density is lower and self-discharge is higher.

8. Electrochemistry • The negative electrode reaction occurring in a NiMH battery is as follows: H2O + M + e− ↔ OH− + M-H. The electrode is charged in the right direction of this equation and discharged in the left direction. • Nickel oxyhydroxide (NiOOH) forms the positive electrode and the corresponding reaction is: Ni(OH)2 + OH− ↔ NiOOH + H2O + e−. • The "metal" in the negative electrode of a NiMH battery is actually an intermetallic compound. Many different compounds have been developed for this application, but those in current use fall into two classes. The most common is AB5, where A is a rare earth mixture of lanthanum, cerium, neodymium, praseodymium and B is nickel, cobalt, manganese, and/or aluminum. Very few batteries use higher-capacity negative material electrodes based on AB2 compounds, where A is titanium and/or vanadium and B is zirconium or nickel, modified with chromium, cobalt, iron, and/or manganese, due to the reduced life performances [3]. Any of these compounds serves the same role, reversibly forming a mixture of metal hydride compounds. • When overcharged at low rates, oxygen produced at the positive electrode passes through the separator and recombines at the surface of the negative. Hydrogen evolution is suppressed and the charging energy is converted to heat. This process allows NiMH batteries to remain sealed in normal operation and to be maintenance-free. • NiMH batteries have an alkaline electrolyte, usually potassium hydroxide.

9. Nickel-metal hydride battery • Battery specifications • Energy/weight 30–80 Wh/kg • Energy/size 140–300 Wh/L • Power/weight 250–1000 W/kg • Self-discharge rate 30%/month (temperature dependant) • Cycle durability 500–1000 • Nominal Cell Voltage 1.2 V

10. Lithium Ion-Batteries A more advanced lithium-ion battery design is the lithium polymer cell. Electrochemistry The anode of a conventional Li-ion cell is made from carbon, the cathode is a metal oxide, and the electrolyte is a lithium salt in an organic solvent. The underlying chemical reaction that allows Li-ion cells to provide electricity is: Li1-xCoO2 +LixC6<=>C6+LiCoO2 It is important to note that lithium ions themselves are not being oxidized; rather, in a lithium-ion battery the lithium ions are transported to and from the cathode or anode, with the transition metal, Co, in LixCoO2 being oxidized from Co3+ to Co4+ during charging, and reduced from Co4+ to Co3+ during discharge.

11. Battery specifications • Energy/weight 160 Wh/kg • Energy/size 270 Wh/L • Power/weight 1800 W/kg • Self-discharge rate 5%-10%/month • Time durability (24-36) months • Cycle durability 1200 cycles • Nominal Cell Voltage 3.6 / 3.7 V

14. Smart Battery System A Smart Battery generally contains one or more secondary battery cells, an analog monitoring chip, a digital controller chip, various discrete diodes, transistors, passive components, and a redundant safety monitor chip. All are used to monitor voltage, current, and temperature of the cells and manage proper discharge and charging of the battery pack within desired safety limits.

17. Dynamic Energy Management • Achieved through Low Power Idle and Sleep Modes

20. Typical State Transitions for Power Saving

21. Power Management in Pentium M

22. Various States • Normal • Auto Halt: when the processor executes halt instruction. If the system asserts STPCLK interrupt it comes out of this state • Stop-Grant State: when the STPCLK is asserted it comes to this state • Halt Grant Snoop Stage • Sleep • Deep Sleep • Deeper Sleep

23. Clock Throttling in Pentium-M

24. Intel 90 nm – Pentium M Processor (2 MB cache)

25. Power Density in Pentium M by Infra-Red Emission Microscopy

26. Dynamic Energy Management • Achieved through Low Power Idle and Sleep Modes • Because of packaging density the static power consumption is increasing by 20% a year with a 0.13 micron technology. Expected to go up with 100nm technology • The processor should be allowed to run at different speeds • Energy savings can be achieved by reducing the processor’s supply voltage as the clock frequency is reduced.

27. Dynamic Voltage Switching • Dynamic Voltage Scaling (DVS) exploits the fact that the peak frequency of a processor implemented in CMOS is proportional to the supply voltage. An approximation to the power equation for a CMOS circuit is: Where: • P is the power consumed at supply voltage VDD • C(VDD)2fcis the Dynamic component due to switching (C is capacitance, fc is frequency) • VDDIQis the Static component due to leakage (IQ is leakage current)

30. A Typical DVS Scheme

31. Adaptive Voltage Scaling Adaptive Voltage Scaling (AVS) is a closed-loop control technique, which provides substantial improvement over DVS schemes. AVS simplifies voltage scaling by inherently compensating for process and temperature variations and eliminating the need for a frequency vs. supply voltage table. Implementation of this technique requires the use of hardware performance monitors co-located with the embedded processors that receive changing performance level requests from performance setting algorithms. These performance monitors are capable of accurately monitoring intra-die and inter-die process and temperature variations and communicating the information to external Energy Management Units (EMU) through standard interfaces.

32. A Typical Energy Management Solution in ARM Processor APB: Advanced Peripheral Bus IEM: Intelligent Energy Manager PWI: Power Wise Interface

33. Power Wise Interface (PWI) The Power Wise specification is a system-level approach to energy management that enables Adaptive Voltage Scaling (AVS) and state control for battery-powered devices. The Power Wise concept incorporates closed-loop AVS with a high-speed, serial-power-management bus to allow a processing engine to use the minimum voltage at any operating frequency, at any given time in the system, to minimize dynamic energy dissipation.

34. Adaptive Power Management The APC contains hardware performance monitors that monitor the power consumed by the processor and track the temperature and device-to-device process variations. The APC communicates to an off-chip Energy Management Unit (EMU) over a two-wire, bidirectional bus called the PowerWise Interface (PWI).