Using System-on-a-Chip as a
Vehicle for VLSI Design Education
Andrew Laffely and Wayne Burleson
Electrical and Computer Engineering
University of Massachusetts Amherst
{alaffely,burleson}@ecs.umass.edu
This material is based upon work supported by the National Science Foundation under Grant No. 9988238
and SRC Tasks 766 and 1075
Burleson, UMASS
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Challenges in VLSI Education
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Advancing Processing Technology
Higher level design tools
Realistic yet tractable design projects
Preparation for jobs in semiconductor
and other sectors.
Making best use of faculty/student time
and university resources
Burleson/UMASS
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ECE 559/659: VLSI Design Project
(10 grads, 20 seniors)
Course Objectives:
• Learn design process for a complex VLSI in
deep sub-micron CMOS
• Learn VLSI design skills and tools, including
working in teams
• Learn about a particular application
component and its VLSI implementation
• Learn to present formal design reviews using
oral, written, graphical and web-based
techniques
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Key Aspects of the Course
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aSoC (home-grown SoC platform)
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Graduate and undergraduate teamwork
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Provides a unifying framework to class
Allows for subdivision but inter-relation of projects
Interesting cutting edge architecture based on NSF- and
SRC-funded research at UMASS and elsewhere
Covers many aspects of VLSI Design
Realistic constraints on area, timing, power and I/O
Graduate students provide leadership, motivation and
experience
Commercial tools and design flow
Review-based evaluation
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Oral and web-based reports for 4 different reviews:
proposal, feasibility, implementation, integration
Burleson/UMASS
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Adaptive System-on-a-Chip (aSoC)
Tile
mProc
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Multiplier
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Communication
Interface
North
FPGA
Tiled architecture with
mesh interconnect
Multiplier
East
West
Allows for heterogeneous
cores
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ctrl
South
Core
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Differing sizes, clock
rates, voltages
Low-overhead core
interface for
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Point to point
communication pipeline
On-chip bus substitute
for streaming
applications
Based on static
scheduling
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Fast and predictable
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Communication Interface
Core
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Core-ports
North
North
South
East
South
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East
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West
West
Inputs
Local Config.
Crossbar
Decoder
North to South & East
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Outputs
Controller
Custom design to
maximize speed
and reduce power
Local
Frequency
& Voltage
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Core-ports
Crossbar
Controller
Instruction
memory
Local frequency
and voltage supply
PC
Instruction Memory
Burleson/UMASS
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Class Projects
SoC Infrastructure1,3
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Communication
Interface
• Interconnect3
• Power Distribution
• Clock System
• Power Management
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1
2
3
Used in PhD Dissertation
Used in Masters Thesis
Used in Publications
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Cores
Motion estimation for
video encoding2,3
• AES Cryptography3
• Cache2,3
• Huffman Coding
• 3D Graphics1,2,3
• Discrete Cosine
Transform2,3
• Smart Card2,3
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Design Flow
http://vsp2.ecs.umass.edu/vspg/658/TA_Tools/design_flow.html
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Architecture to Layout
Architecture: Block diagram of system and behavioral description
Logic: Gate level or schematic description
• Circuit: Transistor sizing
• Layout: Floorplanning, clock and power distribution
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Tools
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VerilogXL: behavioral representation
VTVT: standard cell library
Synopsys: standard cell gate level netlist generation
Silicon Ensemble: standard cell netlist to layout
Cadence LayoutPlus: schematic and layout design
NCSU CDK: design and extraction rules
Cadence Layout vs. Schematic: layout verification
HSPICE: circuit simulator
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aSoC Implementation and Integration
2500 l
.18m TSMC technology
Full custom
3000 l
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Advanced Signaling Techniques
(building on SRC-funded work)
Differential current sensing
Booster Insertion
Multi-level current signaling
Phase coding
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Circuit Level Simulation (HSPICE)
Evaluating Subsystems with realistic models
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Capacitance, resistance and inductance
Process variations
Process generations
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Interconnect Characterization:
Comparing delay and power of signaling techniques for different
tile sizes at 250nm, 180nm, 130nm, 100n
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Voltage Scaling Approach
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Core-ports
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Single buffer for each
stream to cross
clock/voltage barrier
between core and
interface
Reading/Writing
success rates indicate
core utilization
Input blocked: Core
too slow
• Output blocked:
Core too fast
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Controller
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Interprets core-port
success rates to adjust
local clock and voltage
Core
Buffer
Processing
Pipeline
Local Local
Vdd Clock
Input
Core-port
Output
Core-port
Clock
Blocked
Blocked
and
Supply
Controller
Interconnect
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Vdd Selection Criteria
Normalized Core Critical Path Delay vs. Vdd
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Normalized
Delay 10
1/8 Speed 8
6
1/4 Speed 4
1/2 Speed
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As Vdd decreases delay increases
exponentially
Use curve to match available clock
frequencies to voltages
The voltage and frequency change
reduces power by 79%, 96%, and
98.7%
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P = aC(Vdd)2f
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Max Speed
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0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
0.73 1.16
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Voltage
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Clock Distribution
Tile
• Tiled architecture extends life
of globally synchronous
systems
• Precise H-tree implementation
• Load is small and equal at each
branch
• Skew can be reduced by 70%
with advanced deskew circuits1
64 tile aSoC
70nm
100nm
130nm
180nm
Chip Area
(9.24mm)2
(13.3mm)2
(17.2mm)2
(23.8mm)2
Frequency
5 GHz
2 GHz
1 GHz
0.5 GHz
Power
126 mW
240 mW
445 mW
784 mW
Mean Skew
41 ps
50 ps
92 ps
70.6 ps
Percent Skew
21 %
10 %
9%
4%
S. Tan et al. “Clock Generation and Distribution for the First IA-64 Microprocessor”
IEEE JSSC, Nov. 2000
Burleson/UMASS
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Power Distribution
• Heterogeneous cores may
require multiple power supply
voltages
• Tile structure enables
uniform interwoven grid
• Larger grid for higher current
demands
Gnd
Vml
Vl
Vmh
• Reduced resistance
• Higher capacitance
Vh
64 tile aSoC
Vh
Vmh
Vml
Vl
Voltage
1.8V
1.16V
0.73V
0.6V
Current
per Core
110mA
25mA
13mA
7mA
Total Power
12.1 W
1.86 W
607 mW
269 mW
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Architecture Evaluation
(Motion Estimation)
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Array-based
architecture
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Memory
Pipelined ME
FIFOs
Parameterized
search window
size
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Address
Generation
Unit
Full search
Choose 16x16 or
8x8 windows
Reduce power
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Processing
Element
Array
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Modify Existing Designs
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Take existing Verilog code or hardware and improve
or change functionality (e.g. add motion estimation
algorithms, provide AES key-length flexibility)
Evaluate changes in performance and overhead
- Old PE Layout
- New PE Layout
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Conclusions
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Advancing Process Technology
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Higher level design tools
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Re-use existing projects and provide unifying themes
Preparation for jobs in semiconductor and other sectors.
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Combine synthesis and custom techniques
Realistic yet tractable design projects
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Target .18u for affordable fab but also do scaling studies
Focus on system design and appropriate levels of abstraction
Teach how to learn new tools
Making best use of faculty/student time and university resources
Leverage research
Combine grad and undergrad
• Re-use materials, tools
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