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Energy Management
in Embedded Systems
Aurobinda Routray,
Associate Professor
Department of Electrical Engineering
Indian Institute of Technology Kharagpur
Why Power Efficiency in Low Power
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Stand Alone Systems
Battery Driven
Battery capacity is limited
It is possible to decrease the Battery discharge rate by
Intelligent use of its power
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DVS: stands for Dynamic Voltage Switching
Hardware: reconfiguration and intelligent clock throttling
Software: Code Size Minimization and Run Time
optimization
Which Systems need it
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Cell Phones
Sensor Networks
Pervasive Computing
 Ubiquitous Computing
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All kinds of real time embedded systems
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What is an real time embedded system ?
Miniature Robots
Of Course EVs
Pervasive Computing
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.
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
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 hydrogenabsorbing 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.
Electrochemistry
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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.
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Nickel oxyhydroxide (NiOOH) forms the positive electrode and
the corresponding reaction is: Ni(OH)2 + OH− ↔ NiOOH + H2O
+ e−.
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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.
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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
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.
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
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.
Dynamic Energy Management
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Achieved through Low Power Idle and Sleep
Modes
Typical State Transitions for Power Saving
Power Management in Pentium M
Various States
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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
Clock Throttling in Pentium-M
Intel 90 nm – Pentium M Processor (2 MB cache)
Power Density in Pentium M by
Infra-Red Emission Microscopy
Dynamic Energy Management
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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.
Dynamic Voltage Switching
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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)2fc is the Dynamic component due to switching (C is capacitance, fc is
frequency)
• VDDIQ is the Static component due to leakage (IQ is leakage current)
A Typical DVS Scheme
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.
A Typical Energy Management Solution in ARM Processor
APB: Advanced Peripheral Bus
IEM: Intelligent Energy Manager
PWI: Power Wise Interface
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.
Adaptive Power Management
The APC contains hardware performance
monitors that monitor the power consumed by the
processor and track the temperature and deviceto-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).