Leakage and Dynamic Glitch Power Minimization Using MIP

for Vth Assignment and Path Balancing

Yuanlin Lu and Vishwani D. AgrawalAuburn University

ECE Dept., Auburn, Alabama, USA

PATMOS’05, Leuven, Belgium, September 21-23, 2005

2

Problem Statement Design a CMOS Circuit :

with dual-threshold devices to globally minimize subthreshold leakage

with delay elements to eliminate all glitches to maintain specified performance

Allow Performance-Power tradeoff

3

CL

Dynamic Switching Power Dynamic Switching Power ++ Short Circuit Power Short Circuit Power ++ Leakage PowerLeakage Power

Power Consumption in CMOS Circuits

4

Leakage and Delay Increasing Vth can exponentially decrease Isub

But, gate delay increases at the same time

where α models channel effects(long channel α = 2, short channel α = 1.3)

While using dual Vth techniques, must consider the tradeoff between leakage reduction and performance degradation

thdd

ddpd

VV

CVT

T

ds

T

thgsT

eff

effoxsub V

V

nV

VVeV

L

WCuI exp1exp8.120

5

Some Previous References on Leakage Reduction and Glitch Power Reduction

Leakage Power Minimization by Dual-Vth CMOS Devices Heuristic Algorithms (locally optimal solution)

Q. Wang and S. B. K. Vrudhula, "Static Power Optimization of Deep Submicron CMOS Circuits for Dual VT Technology," Proc. ICCAD, 1998, pp. 490-496.

L. Wei, Z. Chen, M. Johnson and K. Roy, “Design and Optimization of Low Voltage High Performance Dual Threshold CMOS Circuits,” Proc. DAC, 1998, pp. 489-494.

Linear Programming (globally optimum solutions) D. Nguyen, A. Davare, M. Orshansky, D. Chinney, B. Thompson and K. Keu

tzer, “Minimization of Dynamic and Static Power Through Joint Assignment of Threshold Voltages and Sizing Optimization,” Proc. ISLPED, 2003, pp. 158-163.

F. Gao and J. P. Hayes, “Gate Sizing and Vt Assignment for Active-Mode Leakage Power Reduction,” Proc. ICCD, 2004, pp. 258-264

Dynamic Glitch Power Elimination by Linear Programming T. Raja, V. D. Agrawal and M. L. Bushnell, “Minimum Dynamic Power CMO

S Circuit Design by a Reduced Constraint Set Linear Program,” Proc. 16th International Conference on VLSI Design, 2003, pp. 527-532.

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New MIP: A Mixed Integer Linear Programfor Leakage and Glitch Power Reduction

Objective Function:

Minimize {Total leakage + No. of glitch

suppressing delay elements}

Alternative objective function (linear approximation):

Minimize {Total leakage + Total glitch

suppressing delay}

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Objective Function

Minimize { Σ Xi ILi + (1-Xi)IHi all gates i

+ Σ Σ Δdij } all gates i→ j

Where Xi = 1, gate i has low Vth, leakage = ILi

Xi = 0, gate i has high Vth, leakage = IHi

Δdij = delay inserted between gates i and j

for glitch suppression

Xi = [0,1] is integer, Δdij is real variable

ILi and IHi are constants for gate i determined by SPICE

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MIP: Variables and ConstantsEach gate has four variables and four constants:

Integer Variable: Xi: [0,1], specifies gate threshold voltage

Continuous-valued Variales: Ti: latest time at which the output of gate i can produce an

event after the occurrence of an event at primary inputs. ti: earliest time at which the output of gate i can produce a

n event after the occurrence of an event at primary inputs. Δdi,j: delay of inserted delay element at the jth input of gate i.

Constants Determined by Spice Simulation ILi and IHi: Leakage currents for low and high thresholds DLi and DHi: Delays for low and high thresholds

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MIP - Constraints Circuit delay constraint for each PO i:

Tmax can the delay of critical path or clock period specified by the circuit designer

Glitch suppression constraint for each gate i:

Constraints (g-2,3,4) make sure that Ti - ti < di for each gate, so glitches are eliminated

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MIP - gate constraints explained Constraints 1 & 2 let T2

be the largest arrival time at gate 2 output

Constraints 3 & 4 let t2 be the earliest arrival time at gate 2 output

Constraint 5 makes sure that T2- t2 < d2

D2 can be a larger delay (high Vth) or a smaller delay (low Vth)

0

3

1

2

(t2,T2)(t0,T0) 1,2d

2,2d

222222

22222,22

22221,202

22222,22

22221,202

1

10

1

10

1

tTDXDX

DXDXdt

DXDXdtt

DXDXdT

DXDXdTT

HL

HL

HL

HL

HL

(1)

(2)

(3)

(4)

(5)

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Power-Delay Tradeoff ExampleA 14-Gate Full Adder

A

B

C

Co

S

A

B

C

Co

S

A

B

C

Co

S

Unoptimized Circuit Optimized Circuit @ Tmax=Tc

Optimized Circuit @ Tmax=1.25 Tc

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Choices for a Delay Element Two cascaded-inverter buffer - consumes additional subt

hreshold leakage and dynamic power: All delay buffers are on non-critical paths and are assi

gned high Vth, to reduce leakage overhead Transmission gate (on state) – increases resistance

Smaller area overhead No subthreshold leakage Possible capacitance increase Used before

T. Raja, V. D. Agrawal and M. L. Bushnell, “Variable Input Delay CMOS Logic for Low Power Design,” Proc. 18th International Conference on VLSI Design, January 2005, pp. 598-605.

T. Raja, V. D. Agrawal and M. L. Bushnell, “Variable Input Delay CMOS Logic Design for Low Dynamic Power Circuits,” PATMOS’05.

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Delay Element Implementation

Buffer

(a)

Vdd

GND

1 1 000

Vdd

GND

0 1 1 0a

(b)

Gate LeakageSubthreshold

(a) Transmission Gate (b) Buffer

Delay Element SubthresholdLeakage (pA)

Transmission Gate

High Vth 0

Low Vth 0

Buffer (Two Cascaded Inverters)

High Vth 409

Low Vth 20800

* size of buffer:

W/L: N1:315/70

P1:630/70

N2:175/70

P2:350/70

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Leakage reduction and performance tradeoff 27 , 70nm℃

CircuitNumber

ofgates

CriticalPathDelay

Tc

(ns)

UnoptimizedIleak (μA)

Optimized Ileak (μA)

(Tmax= Tc )

LeakageRed.

%

Sun OS 5.7 CPUsecs.

Optimized forIleak (μA)

(Tmax=1.25Tc )

Leakage Red.

%

SunOS 5.7CPUsecs.

C432 160 0.751 2.620 1.022 61.0 0.42 0.132 95.0 0.3

C499 182 0.391 4.293 3.464 19.3 0.08 0.225 94.8 1.8

C880 328 0.672 4.406 0.524 88.1 0.24 0.153 96.5 0.3

C1355 214 0.403 4.388 3.290 25.0 0.1 0.294 93.3 2.1

C1908 319 0.573 6.023 2.023 66.4 59 0.204 96.6 1.3

C2670 362 1.263 5.925 0.659 90.4 0.38 0.125 97.9 0.16

C3540 1097 1.748 15.622 0.972 93.8 3.9 0.319 98.0 0.74

C5315 1165 1.589 19.332 2.505 87.1 140 0.395 98.0 0.71

C6288 1177 2.177 23.142 6.075 73.8 277 0.678 97.1 7.48

C7552 1046 1.915 22.043 0.872 96.0 1.1 0.445 98.0 0.58

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Leakage Reduction and Performance As we allow increase of Tmax fr

om the smallest value Tc, more leakage power can be saved, because more gates can be assigned high Vth.

But, the trend slows down. When Tmax ≈ 1.3 Tc, the reducti

on trend saturates, because almost all gates have been assigned high Vth, and there is no more optimization space left.

The maximum leakage reduction can be 98%.

1 1.1 1.2 1.3 1.4 1.50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Normalized Critical Path Delay

Nor

mal

ized

Lea

kage

Pow

er

C432

C880

C1908

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Comparing Dynamic & Leakage Power Leakage (increases with temperature):

Determined by Spice simulation of gates at 90ºC Added up for all gates of circuit optimized by MIP

Dynamic power (depends on node activity and capacitance): Node capacitances for optimized circuit estimated Gate delays determined by Spice simulation of gates Activity determined by event driven discrete-time simulato

r using 1,000 random vectors applied with 120% Tc clock period

c

ii

iddinvdyn

dyn T

FOTVC

T

EP

2.11000

5.0 2

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Leakage, Dynamic and Total Power Comparison 90 , 70nm℃

CircuitName

No. of

Gates

Leakage Power Dynamic Power Total Power

Pleak1

(uW)

Pleak2 (u

W)Leakage

Reduction Pdyn1 (u

W)

Pdyn2 (u

W)Dynamic

Reduction Ptotal1

(uW)

Ptotal2

(uW)Total

Reduction

C432 160 35.77 11.87 66.8% 101.0 73.3 27.4% 136.8 85.2 37.7%

C499 182 50.36 39.94 20.7% 225.7 160.3 29.0% 276.1 200.2 27.5%

C880 328 85.21 11.05 87.0% 177.3 128.0 27.8% 262.5 139.1 47.0%

C1355 214 54.12 39.96 26.3% 293.3 165.7 43.5% 347.4 205.7 40.8%

C1908 319 92.17 29.69 67.8% 254.9 197.7 22.4% 347.1 227.4 34.5%

C2670 362 115.4 11.32 90.2% 128.6 100.8 21.6% 244.0 112.1 54.1%

C3540 1097 302.8 17.98 94.1% 333.2 228.1 31.5% 636.0 246.1 61.3%

C5315 1165 421.1 49.79 88.2% 465.5 304.3 34.6% 886.6 354.1 60.1%

C6288 1189 388.5 97.17 75.0% 1691.2 405.6 76.0% 2079.7 502.8 75.8%

C7552 1046 444.4 18.75 95.8% 380.9 227.8 40.2% 825.3 246.6 70.1%

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Conclusion A new mixed integer linear programming technique

Simultaneous minimization of leakage (dual-Vth) and elimination of glitches (path delay balancing)

Global tradeoff between power and performance Experimental results shows that 96%, 40% and

70% reduction in leakage, dynamic (glitch) and total power, respectively.

Future directions: Include gate sizing for switching capacitance reduction a

nd leakage reduction Allow dual-supply voltages for reduction of power

components Robust optimization for process variations

Thank You All !

Questions?