EE141 1 EE141 1 EE141-S04 EE141 EE141- Spring 2004 Spring 2004 Digital Integrated Digital Integrated Circuits Circuits Lecture 27 Lecture 27 Power distribution Power distribution Resistive interconnect Resistive interconnect EE141 2 EE141-S04 Administrative Stuff Administrative Stuff Make-up lecture on Monday 4-5:30pm Special office hours of Prof. Rabaey today 5-6:30pm at BWRC Homework 9 posted – due next Th 5pm Poster presentations next Tu. Sign up for time slot (office door of Prof. Rabaey). Poster template on web-site.
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EE141
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EE141EE141-- Spring 2004Spring 2004Digital Integrated Digital Integrated CircuitsCircuits
Administrative StuffAdministrative StuffMake-up lecture on Monday 4-5:30pmSpecial office hours of Prof. Rabaey today 5-6:30pm at BWRCHomework 9 posted – due next Th 5pmPoster presentations next Tu. Sign up for time slot (office door of Prof. Rabaey). Poster template on web-site.
We have already learned how to drive RC interconnectImpact of resistance is commonly seen in power supply distribution:
IR dropVoltage variations
Power supply is distributed to minimize the IR drop and the change in current due to switching of gates
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RI Introduced NoiseRI Introduced Noise
M1
X
I
R9
RDV
f pre
DV
VDD
VDD 2 DV9
I
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Resistance and the Power Resistance and the Power Distribution ProblemDistribution Problem
Source: Cadence
•• Requires fast and accurate peak current predictionRequires fast and accurate peak current prediction•• Heavily influenced by packaging technologyHeavily influenced by packaging technology
BeforeBefore AfterAfter
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Power DistributionPower Distribution
Low-level distribution is in Metal 1Power has to be ‘strapped’ in higher layers of metal.The spacing is set by IR drop, electromigration, inductive effectsAlways use multiple contacts on straps
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Power and Ground DistributionPower and Ground Distribution
GND
VDD
Logic
GND
VDD
Logic
GND
VDD
(a) Finger-shaped network (b) Network with multiple supply pins
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3 Metal Layer Approach (EV4)3 Metal Layer Approach (EV4)3rd “coarse and thick” metal layer added to the
technology for EV4 designPower supplied from two sides of the die via 3rd metal layer
2nd metal layer used to form power grid
90% of 3rd metal layer used for power/clock routing
Metal 3
Metal 2
Metal 1
Courtesy Compaq
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4 Metal Layers Approach (EV5)4 Metal Layers Approach (EV5)4th “coarse and thick” metal layer added to the
technology for EV5 designPower supplied from four sides of the die
Grid strapping done all in coarse metal90% of 3rd and 4th metals used for power/clock routing
Metal 3
Metal 2
Metal 1
Metal 4
Courtesy Compaq
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2 reference plane metal layers added to thetechnology for EV6 designSolid planes dedicated to Vdd/Vss
Significantly lowers resistance of gridLowers on-chip inductance
6 Metal Layer Approach 6 Metal Layer Approach –– EV6EV6
Metal 4
Metal 2Metal 1
RP2/Vdd
RP1/Vss
Metal 3
Courtesy Compaq
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ElectromigrationElectromigration (1)(1)
Limits dc-current to 1 mA/µm
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ElectromigrationElectromigration (2)(2)
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The Impact of The Impact of ResistivityResistivity
CN-1 CNC2
R1 R2
C1
Tr
Vin
RN-1 RN
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
0.5
1
1.5
2
2.5
time (nsec)
volt
ag
e (
V)
x= L/10
x = L/4
x = L /2
x= L
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
0.5
1
1.5
2
2.5
time (nsec)
volt
ag
e (
V)
x= L/10
x = L/4
x = L /2
x= L
Diffused signal Diffused signal propagationpropagation
Delay ~ LDelay ~ L22
The distributed The distributed rcrc--lineline
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The Global Wire ProblemThe Global Wire Problem
ChallengesNo further improvements to be expected after the introduction of Copper (superconducting, optical?)Design solutions
Use of fat wiresInsert repeaters — but might become prohibitive (power, area)Efficient chip floorplanning
Towards “communication-based” design How to deal with latency?Is synchronicity an absolute necessity?
( )outwwdoutdwwd CRCRCR693.0CR377.0T +++=
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Interconnect:Interconnect:# of Wiring Layers# of Wiring Layers
# of metal layers is steadily increasing due to:
• Increasing die size and device count: we need more wires and longer wires to connect everything
• Rising need for a hierarchical wiring network; local wires with high density and global wires with low RC
Copper is planned in full sub-0.25 µm process flows and large-scale designs (IBM, Motorola, IEDM97)With cladding and other effects, Cu ~ 2.2 µΩ-cm vs. 3.5 for Al(Cu) ⇒40% reduction in resistance
For a given technology and a given interconnect layer, there exiFor a given technology and a given interconnect layer, there exists sts an optimal length of the wire segments between repeaters. The an optimal length of the wire segments between repeaters. The delay of these wire segments is delay of these wire segments is independent of the routing layer!independent of the routing layer!
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INTERCONNECTINTERCONNECT
Dealing with InductanceDealing with Inductance
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L L di/dtdi/dt
Impact of inductance on supply voltages:• Change in current induces the change in voltage• Longer supply lines have larger L
CL
V’DD
VDD
L i(t)
VoutV in
GND ’
L
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L L di/dtdi/dt: Simulation: Simulation
t
t
t
vout
iL
vL
20mA
40mA
5V
0.2V
0.0
1.02.0
3.0
4.0
5.0
Vou
t(V)
0
10
20
I L (m
A)
2 4 6 8 10t (nsec)
-0.3
-0.1
0.1
0.3
0.5
VL(V
)
tfall = 0.5 nsec
tfall = 4 nsec
Signals Waveforms for Output Driver connected To Bonding Pads(a) vout; (b) iL and (c) vL.
The Results of an Actual Simulation are Shown on the Right Side.
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Choosing the Right PinChoosing the Right Pin
ChipL
L ´
Bonding wire
Mountingcavity
Leadframe
Pin
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Decoupling CapacitorsDecoupling Capacitors
Decoupling capacitors are added: • on the board (right under the supply pins)• on the chip (under the supply straps, near large buffers)
– 160 Vdd/Vss bondwire pairs on the WACC minimize inductance
Source: B. Herrick (Compaq)
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EV6 WACCEV6 WACC
587 IPGA
MicroprocessorWACC
Heat Slug
389 Signal - 198 VDD/VSS Pins389 Signal Bondwires
395 VDD/VSS Bondwires
320 VDD/VSS Bondwires
Source: B. Herrick (Compaq)
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Design Techniques to address L Design Techniques to address L di/dtdi/dt
Separate power pins for I/O pads and chip coreMultiple power and ground pinsPosition of power and ground pins on packageIncrease tr and tfAdvanced packaging technologiesDecoupling capacitances on chip and on board
Advantages:Advantages:1. Shorter wires within blocks1. Shorter wires within blocks2. Block address activates only 1 block => power savings2. Block address activates only 1 block => power savings
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Block Diagram of 4 Block Diagram of 4 MbitMbit SRAMSRAM