ultra-dynamic voltage scaling using sub-threshold
TRANSCRIPT
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Ultra-Dynamic Voltage Scaling Using Sub-threshold
Operation and Local Voltage Dithering in 90nm CMOS
Benton Calhoun and Anantha ChandrakasanMassachusetts Institute of Technology
ISSCC 2005
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Ultra-DVS Motivation
• Basics of subVT operation0 0.2 0.4 0.6 0.8 1
10−8
10−6
10−4
10−2
100
Nor
mal
ized
I D
Normalized VGS
Sub-threshold operation: Slow,lower power, minimum energy operation becomes possible
Strong Inversion Operation: fast, power-hungry
Unite the regions
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DVS
Dynamic Voltage Scaling (DVS)Time Constraint
0 0.2 0.4 0.6 0.8 10
0.2
0.4
0.6
0.8
1
Rate (normalized frequency)
Nor
mal
ized
Ene
rgy
Max rate
Sleep
Dynamic Voltage Scaling with infinite levels savesenergy per sample when the workload varies
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DVS
Max rate
Sleep
0 0.2 0.4 0.6 0.8 10
0.2
0.4
0.6
0.8
1
Rate (normalized frequency)
Nor
mal
ized
Ene
rgy
Quantizing the voltages decreases complexity, but reduces savings if VDD is constant for time constraint
Time Constraint
Quantized DVS
Gutnik and Chandrakasan, An Efficient Controller for Variable Supply-Voltage Low Power Processing, Symposium on VLSI Circuits, 1996
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0 0.2 0.4 0.6 0.8 10
0.2
0.4
0.6
0.8
1
Rate (normalized frequency)
Nor
mal
ized
Ene
rgy
Dithering within the time constraint givesaverage energy close to the optimum
Dithering
Need to switch VDD in the timescale of the changing workload:
Existing approaches use 100s of µsecs to switch
Voltage DitheringTime Constraint
Gutnik and Chandrakasan, An Efficient Controller for Variable Supply-Voltage Low Power Processing, Symposium on VLSI Circuits, 1996
Quantized rates
Required rates
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0 0.2 0.4 0.6 0.8 10
0.2
0.4
0.6
0.8
1
in (V)
out (
V)
Sub-VT OperationPREVIOUS WORK:
Swanson & Meindl, JSSC,1972Sub-VT model & theoretical limits
Vittoz & Fellrath, JSSC,1977 Analog sub-VT circuits
Vittoz, ISSCC Plenary Talk, 1994
Soeleman & Roy, ISLPED, 1999
Wang & Chandrakasan, ISSCC, 2004
90nm sub-VTInverter VTCs
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0 0.2 0.4 0.6 0.8 1 1.2
10−15
10−14
10−13
10−12
VDD
(V)
E/o
p (J
)
Sub-VT Minimum Energy Operation
+=
+=
−
−
th
DD
th
DD
nVV
DPgeffeffDD
nVV
DDgDPeffDDeffTotal
eLKCWCV
eVKCLWVCE
2
22LEAKAGEACTIVETOTAL EEE +=
DDDOFFDDTOTAL TVIVCE +⋅= 2
Gate Leakage Energy
90nm simulationfor 32-bit CLA addershowing minimum energy per operation
Total Leakage Energy
Total Energy
Active Energy
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0 0.2 0.4 0.6 0.8 1 1.2
10−15
10−14
10−13
10−12
VDD
(V)
E/o
p (J
)
Reason for Min. Energy Point
Leakage Energy increasesPrimarily because of the Larger delay in sub-threshold
0 0.2 0.4 0.6 0.8 1 1.210
−9
10−8
10−7
10−6
10−5
10−4
VDD
(V)
Delay
Leakage Current
Gate Leakage Energy
Total Leakage Energy
Total Energy
Active Energy
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• Test chip to show combination of sub-threshold operation with DVS– Sub-threshold Circuit Considerations– Local Voltage Dithering (LVD)– Ultra-Dynamic Voltage Scaling
Demonstrating UDVS
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90nm Test Chip Circuit
32-bit Kogge-Stone
adder
SwitchingControl
Ring oscillatorClock
VDD high(VDDH)
VDD low(VDDL)
Local Voltage Dithering (LVD)- Headers select VDD- Local Block Level
Headers
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Test Chip Die Photo
90nm6LM
Adder
AccumulatorHeader Size
Acc. 0 Acc. 1 Acc. 2 Acc. 3
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32-bit Kogge-Stone Adder Block
Sum calculation
Propagate, Generate creationPath throughadder treehas invertingstages of dot operators
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Kogge-Stone Adder Circuits
G0
G0
G1
G1
P1
P1
P0P1
P0P1
G0
G0
G1
G1
P1
P1
G=G0P1+G1
P=P0P1
P=P0+P1
G=G1(P1+G0)
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WeakWeak
Sub-VT Sizing Considerations
0 20 40 60 80 100 120
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
time (ns)
Ratioed FF
Non-ratioed FF
WeakWeak
Strong
Strong
N1 N2 N3
N1 N2 N3
N1
N2
N3
N3, non-ratioed
CK
CK
CK
CK
CK
DQ
D Q
Ratioed FF fails to write a 1 atstrong N, weak P corner at 400mV
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Preserving State in Sub-VT
0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.220
0.05
0.1
0.15
0.2
0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.220
0.05
0.1
0.15
0.2
N11
N20
N30 Q=1
0 10 1
N10
N21
N31 Q=010
Hold 1 Failure Hold 0 Failure
N3N1
VDD Q
VDD (V) VDD (V)
N3N1
VDD
Q
Vol
tage
(V)
Vol
tage
(V)
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Measured Sub-VT Operation
300mV Clock/1024
Data
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0.2 0.4 0.6 0.8 1 1.2
10−16
10−15
10−14
10−13
10−12
VDD (V)
Mea
sure
d T
otal
Ene
rgy
per
Cyc
le (
J)Minimum Energy Measurements
VDDopt = 330mVf = 50kHz
9XTotal Energy
Leakage Energy
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0.2 0.4 0.6 0.8 1 1.2
10−16
10−15
10−14
10−13
10−12
VDD (V)
Mea
sure
d T
otal
Ene
rgy
per
Cyc
le (
J)Minimum Energy & Temperature
VDDopt insensitiveto temperature
EL increases becauseVT decreases
VT(T)=VT(T0)-KTµ(T)=µ(T0)(T/T0)-M
ILEAK increases by same amount that TD decreases due to VT
T = 25,50,70,85oC
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Measured Standby Power Savings
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.110
−10
10−9
10−8
10−7
VDD (V)
MeasuredExtrapolated
Leakage Current
Standby Power 107X
Measured Flip-flop Retention Voltage: 110mV
35X
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Local Voltage Dithering (LVD)
0 0.2 0.4 0.6 0.8 10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Rate (normalized frequency)
Nor
mal
ized
Ene
rgy
per
sam
ple
(mea
sure
d)
0.2 0.4 0.6 0.8 110
3
104
105
106
107
108
109
VDD
(V)
Mea
sure
d rin
g os
cilla
tor
freq
uenc
y (H
z)
T = 25,50,70,85oC
340MHz, 1.1V
50kHz, 330mV
7kHz, 250mV
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Timing Overhead for LVD
ThresholdCounter<Threshold
Counter ClockGating
Start
Gate
Dither
Dither Dither
Clock
Clock/1024Clock/1024
VDDL=0.9V, rate=0.5 VDDL=0.6V, rate=0.04
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Switching Energy
For test chip,CNORM=3.7
LVD Circuit Issues
01
VDDH=1.0VVDDL=0.3V
1.0V
01
VDDH=1.0VVDDL=0.3V
1.0VTie header bulks to VDDH
VDDHVDDL CNORM normalizes ESW to E of circuit at VDDH
N is number of cycles spent at VDDL to break-even: 222
DDHNORMDDLDDH VCNVNV +≥
)( 22
2
DDLDDH
DDHNORM
VVVCN−
≥
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UDVS Results
10−5
10−4
10−3
10−2
10−1
100
10−6
10−5
10−4
10−3
10−2
10−1
100
Rate (normalized frequency)
Nor
mal
ized
Ene
rgy
per
sam
ple
(mea
sure
d)
No ditheringideal DVSDithered
2X
9X
Dithering close to ideal
Dither during high performance operation and switch to sub-threshold minimum energy operation when speed is not important
1.1V, 340MHz
0.8V, 100MHz
0.33V, 50kHz
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10−5
10−4
10−3
10−2
10−1
100
10−6
10−5
10−4
10−3
10−2
10−1
100
Rate (normalized frequency)
Nor
mal
ized
Ene
rgy
per
sam
ple
(mea
sure
d)No ditheringideal DVSDithered
10−5
10−4
10−3
10−2
10−1
100
10−6
10−5
10−4
10−3
10−2
10−1
100
Rate (normalized frequency)
Nor
mal
ized
Ene
rgy
per
sam
ple
(mea
sure
d)
No ditheringideal DVSDithered
UDVS Flexibility1.1V, 340MHz
Closer to ideal over high performance range
1.1V, 340MHz
0.8V, 100MHz
0.5V, 2.4MHz
0.8V, 100MHz
0.6V, 13MHz
Closer to ideal over full range
Choose dithered voltages to match application of interest
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Providing UDVS Voltages
Can implement UDVS with 3 headers OR two headers and an adjustable voltage supply
VDDH
VDDL- adjustable
UDVS Block
VDDHVDDL VDDM
UDVS Block
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UDVS System Example
Different blocks can voltage dither based on their own workload for optimal efficiency
VDDH
VDDL (adjustable)
UDVS Block UDVS BlockUDVS Block
Level conversion
Level conversion
Level conversion
Bus
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Conclusions
• Local voltage dithering allows near-optimum savings for high performance, variable rate systems
• Sub-threshold allows minimum energy operation• UDVS allows performance and energy scaling
over the full operational range of VDD
Acknowledgements: Funded by Texas Instruments and DARPA through a subcontract with MIT Lincoln Labs