Low power vlsi

1. LOW POWER DESIGN
METHODS
V.ANANDI
ASST.PROF,E&C
MSRIT,BANGALORE

2. Course Objective
Low-power is a current need in VLSI
design.
Learn basic ideas, concepts and methods.
Gain hands-on experience.

3. 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

4. Introduction
Why is it a concern?
Business & technical needs
Semiconductor processing technology
Power Consumption of VLSI Chips

5. NEED FOR LOW POWER
More transistors are packed into the chip.
Increased market demand for portable
devices.
Environmental concerns

6. 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

7. 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

8. 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

9. Low-Power Design Techniques
Functional and architectural methods
Clock suppression
Clock frequency reduction
Supply voltage reduction
Power down
Algorithmic and Software methods

10. Power Dissipation in CMOS
Logic (0.25µ)
%75 %5
%20
Ptotal (0→1) = CL VDD
2 + tscVDD Ipeak + VDDIleakage
CL
VDD VDD

11. Degrees of Freedom
The three degrees of freedom are:
Supply Voltage
Switching Activity
Physical capacitance

12. Components of Power
Dynamic
Signal transitions
Logic activity
Glitches
Short-circuit
Static
Leakage Ptotal = Pdyn + Pstat
= Ptran + Psc + Pstat

13. CMOS Dynamic Power
Dynamic Power = Σ 0.5 αi fclk CLi VDD
2
All gates i
≈ 0.5 α fclk CL VDD
2
≈ α01 fclk CL VDD
2
where α average gate activity factor
α01 = 0.5α, average 0→1 trans.
fclk clock frequency
CL total load capacitance
VDD supply voltage

14. Dynamic Power
VDD
Ground
CL
R
R
Dynamic Power
= CLVDD
2/2 + Psc
Vi
Vo
isc

15. 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.

16. Dynamic Power Reduction
Reduce power per transition
Reduced voltage operation – voltage scaling
Capacitance minimization – device sizing
Reduce number of transitions
Glitch elimination

17. Glitch Power Reduction
Design a digital circuit for minimum
transient energy consumption by
eliminating hazards

18. Static (Leakage) Power
Dynamic
Signal transitions
Logic activity
Glitches
Short-circuit
Static
Leakage

19. Leakage Power
IG
ID
Isub
IPT
IGIDL
n+ n+
Ground
VDD
R

20. 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

21. 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.

22. Low-Power System Design
State encoding
Bus encoding
Finite state machine
Clock gating
Flip-flop
Shift register
Microprocessors
Single processor
Multi-core processor

23. Clock-Gating in Low-Power Flip-Flop
D Q
D
CK

24. 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

25. A Multicore Design
Multiplier
Core 1
Multiplier
Core 5
Reg
Reg
Reg
Reg
5
to
1
mux
Multiphase
Clock gen.
and mux
control
Input
Output
200MHz
CK
200MHz
40MHz
40MHz
40MHz
Multiplier
Core 2
Core clock frequency = 200/N, N should divide 200.

26. 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

27. 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.