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Breaking the Wall of Interconnect: Research

and Education

Chung-Kuan ChengCSE DepartmentUC San DiegoCkcheng at ucsd.edu

EDA Education and Research Workshop at ICCAD 2008

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Outlines

• Technology Trend

• Interconnect: RC Segments + Buffers

• Interconnect: Transmission Lines + Buffers (On-Chip)

• Research and Education Directions

• Hands-on Experiences

• Conclusion

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Technology Trend (ITRS 2007)

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Technology Trend

5

90nm 45nm 22nm

global

wire

m1

wire

global

wire

m1

wire

global

wire

m1

wire

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Technology Trend: Wire Scaling

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Technology Trend: Packaging

8Delay comparison of wires (metal 1 and global) vs. gates

Interconnect: RC Segments + Buffers

9Energy comparison of wires (metal 1 and global) vs. gates

Interconnect: RC Segments (Energy per Bit)

10Delay energy product comparison of wires (metal 1 and global) vs. gates

Interconnect: RC Segments (Delay Energy Product)

11Bandwidth comparison of wires (metal 1 and global) vs. gates

Interconnect: RC Segments (Bandwidth)

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Repeated RC Segments• Analytical formula based flow implemented in MATLAB [3]

• Parameters: • a=0.4, b=0.7: constants related to transistor switching model• f=1: ratio of diffusion capacitance to gate capacitance of transistor• g=1.34: P/N ratio of transistor width

• Technology variables:• r0: output resistance of min-sized inverter; rw: wire resistance per unit length• cmos: min-sized NMOS gate capacitance; cw: wire capacitance per unit length

• Design objects: min-d, min-dp, min-d2p

node r0(kOhm) cmos(fF) rw(Ohm/um)* cw(pF/m)* Vdd(V)

Pitch(nm)

90nm 11.87 0.140 0.262 148.2 1.1 410

65nm 12.68 0.079 0.907 133.9 1.0 210

45nm 9.09 0.076 2.099 128.3 1.0 135

32nm 10.36 0.039 3.979 112.2 0.9 96

22nm 9.45 0.019 8.081 105.2 0.9 66 * Data are computed using formula in [3] based on wire parameters of ITRS report 2007 [1]

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Glossary

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Glossary (cont’d)

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Performance metrics of repeated wire

• Normalized delay:

• Normalized power (energy/bit):

– Leakage power factor:

• Normalized delay power product:• Bandwidth:

0 0(1 )(1 )(1 )stage nmos w

n w w inv w nmos invinv inv inv

d b g f r c br cdelay ar c l b g r c s

l l s

2(1.1 )(1 )(1 )stage leak nmos invn w dd

inv inv

p f g c spower c v

l l

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Optimum repeater insertion for minimizing delay

• Optimum repeater interval and size:

• Optimum delay and power

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Interconnection: Transmission Lines

Scheme 1: Repeated RC wire

Scheme 2: On-chip T-line driven by inverter chain

Scheme 3: On-chip T-line driven by inverter chain w/ resistive termination

2RL

2RL

Distributive RC wire

Transmission Line

Transmission Line

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Theory (Telegrapher’s Equation)

• Telegrapher’s equation:

),(),(),(

),(),(

),(

tzGVdt

tzdVC

dz

tzdIdt

tzdILtzRI

dz

tzdV

• Propagation Constant:

jCjGLjR ))((

• Wave Propagation: zjzeVzV 0)(

• Alpha and Beta corresponds to speed and phase velocity.

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• Operation Region– RC region:

– LC region:

• Two parameters used to verify the region[17] – Upper bound of wire length for lumped element modeling

– Corner frequency between RC and LC region

On-Chip Transmission Line (1)

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T-line Structure and Extraction

tan 0.00068

W

H

T

H

S

G

G

S2G S1

3.1r 62.2 10Cu cm

S3 G

• Single-ended strip-line configuration– Wire length: 5mm

• Extraction includes 3 adjacent wires to consider crosstalk– Use H=1.2um(2.4um) for C extraction and H=4.4um(8.8um) for L

extraction, to consider the worst case

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Experimental Settings

• Tools– 2D EM-field solver CZ2D from EIP tool suite of IBM

• Frequency dependent RLGC tabular model– HSPICE with predictive transistor model

• Synopsys level3 MOSFET model– Design flow is implemented in MATLAB

• Case configuration– Study and compare the performance metrics of 3 schemes at 45nm

node– Using the worst-case input pattern -+- to simulate delay/power. – Optimize under 3 object functions: min-d/min-dp/min-d2p

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Wire Bandwidth and Eye-Opening

Effect of driver impedance on wire bandwidth

Effect of driver impedance and termination resistance on wire output eye-opening

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Performance Comparison• Rs:10ohm, Wire:16X for scheme 2 and 3

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Effect of RS on Step Response

Wire 16X, w/ termination resistance Rload=220ohm

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Effect of Rload on Step Response

Wire 16X, w/ termination resistance RS=10ohm

(Optimal Value)

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Eye-diagrams of T-line scheme

• w/ termination resistance

• Rload=220ohm

• Optimal solution of min-ddp

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Interconnect Dominated Designs: Research Directions

• Analysis: Simulation from wires, circuits, to systems– Wires and device elements: modeling, extraction and measurement – Circuits:

• Spice simulation of whole circuits• Power analysis• Timing analysis (LOCV)

– Systems: • Buses and interfaces• Function and logic analysis• Rapid prototyping and emulation• Multi-domain analysis: EM, thermal, mechanical, biological analysis

• Synthesis: Design from wires, buses, layouts, modules, to networks– RC segments, transmission lines, photonic communication– Power systems, clock distributions, signal buses– Physical layout: floorplanning, placement and routing– Function module synthesis– Network architectures

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Interconnect Dominated Designs: Education

• Computer Sciences– Software Engineering– Algorithms and Numerical Methods– Logic and Arithmetic Designs– Computer and Network Architectures– Distributed Computation

• Electrical Engineering– Physics (Photonics)– EM Waves– Circuit Theory– VLSI Designs

• Motivations and Methodologies

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Education: Motivations and Methodologies

• Data Mining: Literature, Patents, Products, Packages, Research Groups

• Problem Solving – Statement of the Problems– Hands-on Experience– Debugging

• Communication– Teaming– Networking– Broadcasting

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Education: Hands-on Experience

• Y. Zhu, T. Weng, C.K. Cheng, "Enhancing Learning Effectivelness in Digital Design Courses by Programmable Logic Boards," to appear in IEEE Trans. on Education.

• T. Weng, Y. Zhu, and C.K. Cheng, "Digital Design and Programmable Logic Boards: Do Students Actually Learn More?" ASEE/IEEE Frontiers in Education Conf., Session S1H-1, pp. 1-6, 2008.

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Education: Hands-on Experience• Introduction

• Class Information

• Teaching Experience

• Students Feedback

• Final Exam Results

• Conclusion

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Hands-on Experience: Example• Digital design is an essential part of the

CS curriculum• Challenges in teaching

– Lack of previous hardware class work– Lack of interest in hardware among CS

students

• EDA Software Process vs Hardware Execution

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Hands-on Experience: Example• Concerns

– Whether CS students have proper background?– How difficult to teach VHDL and integrate it into

courses?– Board prices & Teaching load

• Benefits– Students can gain better understanding, grasp full

design implementation cycle (Zema 1998, Areibi 2001, Vera et al. 2006, etc.)

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Class Structure• Introductory digital logic design course in

CS program at UCSD– Combinational & sequential logic– Standard modules: decoders, MUX…– System design

• Associated labs– Altera Quartus II software– UP-2 CPLD board (100 US$)– Schematic & VHDL designs

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Lab Assignments• 1. Combinatorial Circuit Design:

5 basic circuits (adder, multiplexer)

• 2. Sequential Circuit Design: Shift registers, counters, clock

• 3. Finite State Machines:traffic light, train controller, grey counter

• 4. CPU Design

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Teaching Goals & Methodologies

• 1. Utilize the PLD board to promote learning for the students– Design labs to reflect the teaching materials

• 2. Design labs that are enjoyable and educational for the students– Design labs that are novel and have a

degree of fun to them

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The PLD Board

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Students Feedback• Survey structure

– 16 statements to evaluate (1: strongly disagree, 5: strongly agree)

– 14 short-answer questions

• Conducted in the end of the term

• 35 returned out of 38 students (92.1%)

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Students Feedback 1 – PLD board usefulness

• 86% enjoyed using the PLD board

• 9% enjoyed running simulations

• 80% felt that they learned digital logic from using the board

• 23% felt they learned from using the simulations

• 77% agree PLD board helped them learn digital design

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Students Feedback 1 – PLD board usefulness

• 63% stated that they would not have learned the contents as well without the board

• 26% said they would have spent a lot of time and effort to grasp the knowledge without the board

• 4% thought the boards were not useful

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Students’ Words• “I thought it was really neat to be able to see

the number 32 instead of 10000 with high and lows in a timing diagram”

• “it (the PLD board) allowed me to apply concepts learned in the class in a real world situation and understand how people in the industry go about using these concepts to solve these problems”

• “(The CPU design project is enjoyable) because I got to build an actual CPU so that I could get a feel of what low level programming is like”

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Students’ Words (cont.)• “This class is very useful and I have learned a

lot from it, especially lab 4 because I really got to see how things are done”

• “At first I thought it would just be another class to drag myself to everyday. However, I now have a great appreciation for digital design. So much so that I am considering a career on it.”

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Final Exam Score Comparison• Compare the final exam results on

CSE140 (Digital Design Theory) between Fall 2005 and Spring 2007

• Similar course structures, similar final exams, same instructor

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Final Exam Score Comparison

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Observations• The second tier students benefit most• Survey comments indicate that the use of board

solidified many of the concepts• From students who scored 80-90

– 100% agreed that they learned more from the board– All thought tutorials are useful– All but one student said the course was more

interesting than they originally expected

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Observations• We investigated whether using a programmable

logic board helps CS students• We introduce PLD boards in our labs• Students answered a detailed survey

– They enjoyed the boards– Those struggled in the class benefit most– Tutorials are important

• Exam results comparison indicates that middle score range students improved a lot from the boards

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• Technology Trend: Interconnect Dominance

• Expansion of Literatures and Tools

• Classic Theories and New Problems Covering Multiple Disciplines

• Motivations and Methodologies

• Hands on Experiences

Summary

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Thank you!Q & A