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Low Power Microprocessors

Advanced Digital IC Design

Low Power Microprocessors

Gao Wei & Tian Youge

Low Power Microprocessors, Gao Wei & Tian Youge, 2012/ 2/ 16

Outline

Why is Low-Power Important? Low Power Technology Conclusions

Low Power Microprocessors, Gao Wei & Tian Youge, 2012/ 2/ 16

References

Why is Low-Power Important? Battery

Last longer Last longer More powerful

Thermal Issues

Cooling Reliability

Low Power Microprocessors, Gao Wei & Tian Youge, 2012/ 2/ 16

Save Energy—it’s limited after all !

Background

Control, I /O Clock Memory Datapath

[ 1]

Low Power Microprocessors, Gao Wei & Tian Youge, 2012/ 2/ 16

Where should the emphasis on power reduction be?

2

Low Power Technology

Clock Reconfigurable Cache Asynchronous Logic Dynamic Voltage Scaling

Low Power Microprocessors, Gao Wei & Tian Youge, 2012/ 2/ 16

y g g Gray Code

Clockclock gating

Circuit without clock gating[ 2] latch-based clock gating circuit [ 2]

register

Low Power Microprocessors, Gao Wei & Tian Youge, 2012/ 2/ 16

g g[

]

EN= 1 EN= 0

Clock variable frequency clock [ 2]

Frequency Multiplier CPUPLL

Multiplier Processor Core Clock 16MHz

Multiplier SysPLL

Multiplier Divider Multiplier Divider

Gated clock a special

Soc/Memory Clock Peripheral Clock Peripheral Clock Peripheral Clock 32kHz

Low Power Microprocessors, Gao Wei & Tian Youge, 2012/ 2/ 16

Gated clock a special case of the variable frequency clock.

Reconfigurable Cache[ 3]

Low Power Microprocessors, Gao Wei & Tian Youge, 2012/ 2/ 16

Cache Configuration Dynam ic Selector

Tw o virtual levels

L1-activated

part

L2-unactivated

part

3

Reconfigurable Cache

[ 4]

Low Power Microprocessors, Gao Wei & Tian Youge, 2012/ 2/ 16

Reconfigurable cache state transition diagram

Outline

Asynchronous Logic y g Dynamic Voltage Scaling

Low Power Microprocessors, Gao Wei & Tian Youge, 2012/ 2/ 16

Gray Code

Types of Sequential Circuit

Synchronous Asynchronous

The same clock signal is applied to each flip-flop. Changes in state occur when the clock changes state from An asynchronous circuit is a circuit in which the parts are largely autonomous. Not governed by a clock

Low Power Microprocessors, Gao Wei & Tian Youge, 2012/ 2/ 16

the clock changes state from

• ne level to another.

Not governed by a clock circuit or global clock signal.

Asynchronous Logic

Benefits:

70% lower power consumption compared to synchronous design. Less stress on the power distribution network.

Low Power Microprocessors, Gao Wei & Tian Youge, 2012/ 2/ 16

4

Asynchronous CPU

In 2004, Epson , p manufactured the world's first bendable microprocessor called ACT11, an 8-bit asynchronous chip .[ 5]

Low Power Microprocessors, Gao Wei & Tian Youge, 2012/ 2/ 16

Dynamic Voltage Scaling P = ACV2f + tAVI short f + VI leak

Capacitive Power Dissipation – ACV2f Capacitive Power Dissipation ACV f dynamic – processor activity influences power loss C – Output Capacitance V – Supply Voltage

Low Power Microprocessors, Gao Wei & Tian Youge, 2012/ 2/ 16

A – Gate Activity f – System Frequency Most dominant term in the power equation

Dynamic Voltage Scaling

Overvolting:

To increase computer performance, or in rare To increase computer performance, or in rare cases, to increase reliability

Undervolting:

To conserve power, particularly in laptops and

• ther mobile devices where energy comes

Low Power Microprocessors, Gao Wei & Tian Youge, 2012/ 2/ 16

• ther mobile devices, where energy comes

from a battery and thus is limited.

Dynamic Voltage Scaling

DVS work steps

P di t

Transform Compute

Collect signals Compute the system load Next period performance

Predict Frequency

Voltage

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Need a power management chip to adjust the small voltage

5

Dynamic Voltage Scaling

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Measured Energy Savings for DVS

Binary codes

Natural binary codes, these two positions would be right next to each other:

… 011 100 ...

Low Power Microprocessors, Gao Wei & Tian Youge, 2012/ 2/ 16

The problem: it is very unlikely that switches will change states exactly in synchrony.

Gray Code

Dec Gray Binary 0 000 000

The gray code solves

1 001 001 2 011 010 3 010 011 4 110 100 5 111 101 6 101 110

g y this problem by changing only one switch at a time, so there is never any ambiguity of position.

Low Power Microprocessors, Gao Wei & Tian Youge, 2012/ 2/ 16

6 101 110 7 100 111

Gray Code

Advantages:

Changes by only one bit as it sequences from

• ne number to the next.

Reduce the switching activity at the address lines by 30~ 50% compared to using normal binary code addressing

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binary code addressing.

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Conclusions

The development trends of low power microprocessor: microprocessor:

On system-level design——Higher level of abstraction. Co-design of hardware and software.

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More research on asynchronous circuits.

References

[ 1] D.W. Dobberpuhl et. al, “A 200-MHz 64b Dual Issue CMOS Microprocessor,” IEEE JSSC”, vol 27, No. 11, Nov 1992 pp 1555-l 566 1992, pp 1555 l 566. [ 2] J. Schutz, “A 3.3V 0.6um BiCMOS SuperScalar Microprocessor”, ISSCC DigTech. Papers, pp 202-203, Feb 1994. [ 3] Yang J.Gupta R Energy Efficient Frequent Value Data Cache Design 2002. [ 4] Vijaykrishman N.Kandemir M.Irwin M J Energy-driven Integrated Hardware software Optimization Using

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Integrated Hardware-software Optimization Using Simp; ePower 2000. [ 5] Karaki, A flexible 8b asynchronous microprocessor based on low-temperature poly-silicon TFT technology, 2005.

Advanced Digital IC Design

Thank you! Thank you! & Questions?

Low Power Microprocessors, Gao Wei & Tian Youge, 2012/ 2/ 16