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LOW POWER DESIGN
METHODS
V.ANANDI
ASST.PROF,E&C
MSRIT,BANGALORE
Course Objective
Low-power is a current need in VLSI
design.
Learn basic ideas, concepts and methods.
Gain hands-on experience.
Contents
 Introduction
 Dynamic power
Short circuit power
Reduced supply voltage operation
Glitch elimination
 Static (leakage) power reduction
 Low power systems
State encoding
Processor and multi-core design
 Books on low-power design
Introduction
Power Consumption of VLSI Chips
Why is it a concern?
Business & technical needs
Semiconductor processing technology
NEED FOR LOW POWER
More transistors are packed into the chip.
Increased market demand for portable
devices.
Environmental concerns
Meaning of Low-Power Design
 Design practices that reduce power
consumption at least by one order of magnitude;
in practice 50% reduction is often acceptable.
 General considerations in low-power design
Algorithms and architectures
High-level and software techniques
Gate and circuit-level methods
Power estimation techniques
Test power
Topics in Low-Power
 Power dissipation in CMOS circuits
 Device technology
 Low-power CMOS technologies
 Energy recovery methods
 Circuit and gate level methods
 Logic synthesis
 Dynamic power reduction techniques
 Leakage power reduction
 System level methods
 Microprocessors
 Arithmetic circuits
 Low power memory technology
 Test power
 Power estimation methods and tools
Low-Power Design Techniques
Circuit and gate level methods
Reduced supply voltage
Adiabatic switching and charge recovery
Logic design for reduced activity
Reduced Glitches
Transistor sizing
Pass-transistor logic
Pseudo-nMOS logic
Multi-threshold gates
Low-Power Design Techniques
Functional and architectural methods
Clock suppression
Clock frequency reduction
Supply voltage reduction
Power down
Algorithmic and Software methods
Power Dissipation in CMOS
Logic (0.25µ)
Ptotal (0→1) = CL VDD2 + tscVDD Ipeak + VDDIleakage
VDD
VDD
CL
%75
%20
%5
Degrees of Freedom
The three degrees of freedom are:
Supply Voltage
Switching Activity
Physical capacitance
Components of Power
Dynamic
Signal transitions
Logic activity
Glitches
Short-circuit
Static
Leakage
Ptotal =
=
Pdyn + Pstat
Ptran + Psc + Pstat
CMOS Dynamic Power
Dynamic Power
=
Σ 0.5 αi fclk CLi VDD2
All gates i
≈ 0.5 α fclk CL VDD2
≈ α01 fclk CL VDD2
where
α
α01
fclk
CL
VDD
average gate activity factor
= 0.5α, average 0→1 trans.
clock frequency
total load capacitance
supply voltage
Dynamic Power
isc
R
VDD
Dynamic Power
Vo
Vi
CL
R
Ground
= CLVDD2/2 + Psc
Summary: Short-Circuit Power
 Short-circuit power is consumed by each
transition (increases with input transition time).
 Reduction requires that gate output transition
should not be faster than the input transition
(faster gates can consume more short-circuit
power).
 Increasing the output load capacitance reduces
short-circuit power.
 Scaling down of supply voltage with respect to
threshold voltages reduces short-circuit power.
Dynamic Power Reduction
Reduce power per transition
Reduced voltage operation – voltage scaling
Capacitance minimization – device sizing
Reduce number of transitions
Glitch elimination
Glitch Power Reduction
Design a digital circuit for minimum
transient energy consumption by
eliminating hazards
Static (Leakage) Power
Dynamic
Signal transitions
Logic activity
Glitches
Short-circuit
Static
Leakage
Leakage Power
IG
Ground
VDD
R
n+
Isub
IPT
IGIDL
n+
ID
Leakage Current Components
Subthreshold conduction, Isub
Reverse bias pn junction conduction, ID
Gate induced drain leakage, IGIDL due to
tunneling at the gate-drain overlap
Drain source punchthrough, IPT due to
short channel and high drain-source
voltage
Gate tunneling, IG through thin oxide
Reducing Leakage Power
 Leakage power as a fraction of the total power
increases as clock frequency drops. Turning
supply off in unused parts can save power.
 For a gate it is a small fraction of the total power;
it can be significant for very large circuits.
 Scaling down features requires lowering the
threshold voltage, which increases leakage
power; roughly doubles with each shrinking.
 Multiple-threshold devices are used to reduce
leakage power.
Low-Power System Design
State encoding
Bus encoding
Finite state machine
Clock gating
Flip-flop
Shift register
Microprocessors
Single processor
Multi-core processor
Clock-Gating in Low-Power Flip-Flop
D
CK
D
Q
Power Reduction in Processors
Hardware methods:
Voltage reduction for dynamic power
Dual-threshold devices for leakage reduction
Clock gating, frequency reduction
Sleep mode
Architecture:
Instruction set
hardware organization
Software methods
Multiphase
Clock gen.
and mux
control
40MHz
Reg
40MHz
Output
Reg
Multiplier
Core 2
5 to 1 mux
Input
Reg
40MHz
Multiplier
Core 1
Reg
A Multicore Design
200MHz
Multiplier
Core 5
200MHz
CK
Core clock frequency = 200/N, N should divide 200.
Challenges
 Development of low Vt, supply voltage and design
technique
 Low power interconnect and reduced activity approaches
 Low-power system synchronization
 Dynamic power-management techniques
 Development of application-specific processing
 Self-adjusting and adaptive circuits
 Integrated design methodology
 Power-conscious techniques and tools development
 Severe supply fluctuations or current spikes
REFERENCES on Low-Power Design
 A. Chandrakasan and R. Brodersen, Low-Power Digital CMOS Design,
Boston: Springer, 1995.
 A. Chandrakasan and R. Brodersen, Low-Power CMOS Design, New York:
IEEE Press, 1998.
 J. M. Rabaey and M. Pedram, Low Power Design Methodologies,
Boston: Springer, 1996.
 K. Roy and S. C. Prasad, Low-Power CMOS VLSI Circuit Design, New
.
York: Wiley-Interscience, 2000
 G. K. Yeap, Practical Low Power Digital VLSI Design, Boston:Springer, 1998.
 Tutorial on low power by Vishwani.D.Aggarwal VDAT’06 Symposium on low
power design methodologies.