Surfliner: Distortion-less Electrical Signaling for Speed of Light On-chip CommunicationHongyu Chen, Rui Shi, Chung-Kuan Cheng

Computer Science and EngineeringUniversity of California, San Diego

David M. HarrisHarvey Mudd College

Outline• Introduction• Surfliner

• Overview• Theory• Implementation• Simulation Results

• Applications• Conclusions

Introduction• On-chip Global Interconnect trend

• Major Concerns: Speed, Power, Cost

0

40

80

120

160

200

180 150 130 100 90 80 70 65 57 50Process Technology Node (nm)

Dela

y (p

s)

1mm Global I nterconnect with Scattering(source: I TRS Roadmap 2004)

FO4 I nverter Delay (Estimated by0.36*Ldraw)

1mm distortionless Transmission Line (Speedof Light)

Introduction• ITRS roadmap: Wall of Global Interconnect

• Delay (Speed of Light 5ps/mm)• Power Density (> ½)• Clock Skew:Variations (5GHz)

Wire optimization is getting harder and harder !

Introduction• Existing on-chip signaling techniques

• Pre-emphasis and Equalization, Dally, VLSI 1998

• Modulating to High-frequency domain; S. Wong, JSSC 2003.

• Non-linear transmission line; Afshari, Hajimiri, CICC 2003.

• Differential pair with discharging scheme, Horowitz, SVLSI 2003

Previous Works

Modulating to High-frequency domain; S. Wong, JSSC 2003.

Previous works

Differential pair with discharging scheme, Horowitz, SVLSI 2003

Surfliner - Overview• Speed-of-the-light on-chip communication

• < 1/5 Delay of Traditional Wires• Low Power Consumption

• < 1/5 Power Consumption• Robust against process variations

• Short Latency• Insensitive to Feature Size

Surfliner - Theory

RΔl LΔlGΔl CΔl

RΔl LΔlGΔl CΔl …

i(z,t)

RΔl LΔl RΔl LΔl

Differential Lossy Transmission Line Surfliner

RΔl LΔlCΔl

RΔl LΔlCΔl …

i(z,t)

RΔl LΔl RΔl LΔl

Current loss through shunt capacitance Frequency dependent phase velocity (speed) and attenuation

Add shunt conductance to compensate current loss R/G = L/CFlat from DC Mode to Giga HzTelegraph Cable: O. Heaviside in 1887.

Theory (Telegrapher’s Equation)• Telegrapher’s equation:

),(),(),(

),(),(),(

tzGVdttzdVC

dztzdI

dttzdILtzRI

dztzdV

• Propagation Constant:

jCjGLjR ))((

• Wave Propagation: zjzeVzV 0)(

• Alpha and Beta corresponds to attenuation and phase velocity. Both are frequency dependant

Theory (Distortionless Line)• Set G=RC/L• Frequency Independent speed and attenuation:

LCCLR ,//

• Characteristic impedance: (pure resistive)

CLZ /0 • Phase Velocity (Speed of light in the media)

cLCv /1• Attenuation:

zZR

ezA 0)(

Digital Signal Response

Eye Diagram

• Injected 1.0V voltage falls to 365mv over a 2cm wire

120 stage, 2.1ps jitter

Speed, Power, Variations• Speed of Light: 5ps/mm or 50ps/cm• Power: 10mW at >GHz• Conductance variation = 10%, f=10MHz~1

0GHz• Phase velocity variation < 1%• Attenuation variation < 1%

Implementation• Add shunt conductance between differential w

ires

• Resistors realized by serpentine unsilicided poly, diffusion resistors, or high resistive metal

Simulation Results• Configuration of wires

• Characteristic Impedance (at 10GHz) : 39.915 Ohm

• Inductance: 0.22nH/mm Capacitance: 141fF/mm

• Attenuation: 253mv magnitude at receiver’s end (assuming 1V at sender’s end)

• Using Microstrip (free space above the wires): impedance can be improved to 52.8Ohm

Simulation Results (Settings)• Agilent ADS Momentum extract 4-port S-

parameters• HSpice: Transient analysis• Assume 1023 bit pseudo random bit

sequence (PRBS)• 15GHz clock• 10% of clock period transition slope for each

rising and falling edge

Simulation Results

4 Stages 120 Stages

Simulation ResultsJitter and silicon area usage

#Stages 4 10 20 40 80 120 160Jitter (ps) 27 9.5 5.4 4.2 3.9 2.1 2.08

Area (um2) 0.52 3.25 13.0 52 208 468 832

Power w/ different width and separation

(w, s) (um) (3,3) (4,4) (5,4) (10,5)Power (mW) 4.98 3.62 3.02 2.13

Attenuation 0.307 0.415 0.496 0.60

Applications of Surfliner1.Clock distributions

2. Data communications: Buses Between CPUs, DSPs, Memory Banks

Application of Surfliner3. High Performance Low Power Wafer Packaging

IC IC IC

Distortionless On-Wafer Transmission Lines for Data Communication and Clock Distributions

CoupledCapacitors

Differential Driver Sense-amps

Conclusions• Feasibility of Implementing Distortionless

Transmission Line for On-Chip Communication

• Advantages of Surfliner• Speed of Light• Low Power (independent with data rate)• High throughput

• Limitation of Surfliner• Require wide metal wires• Static power consumption

Conclusions• Applications

• Global data communication• High Speed Clock• Wafer Scale Packaging

• Future Directions• Explore the design space of wire configurations

and sender/receiver circuitry• Quantify the design trade-offs for surfliner• Innovative communication architectures