VLSI Physical Design

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VLSI Physical Design Automation

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Other Design Styles: FPGA
• Field Programmable Gate Array
• First introduced by Xilinx in 1984.
• Pre-fabricated devices and interconnect, which are
programmable by user.
• Advantages:
– short turnaround time.
– low manufacturing cost.
– fully testable.
– re-programmable.
• Particularly suitable for prototyping, low or medium-
volume production, device controllers, etc.

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Comparison of Design Styles
Full-Custom
Standard
Cell
Gate Array FPGA
Cell size variable fixed height fixed fixed
Cell type variable variable fixed
programma
ble
Cell placement variable in row fixed fixed
Interconnections variable variable variable
programma
ble
Fabrication
layers
all layers
all
layers
routing
layers only no layers

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Comparison of Design Styles
Full-Custom Standard Cell Gate Array FPGA
Area compact
compact to
moderate
moderate large
Performance high
high
to moderate
moderate low
Design cost high medium medium low
Time-to-market long medium medium short

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Programming Technologies
• SRAM to control pass transistor / multiplexer
• EPROM – UV light Erasable PROM
• EEPROM – Electrically Erasable PROM
• Antifuses – One time programmable
• They are different in ease of manufacturing,
manufacturing reliability, area, ON and OFF
resistance, parasitic capacitance, power consumption,
re-programmability.

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Typical FPGA Architecture
• Consists of: Logic modules, Routing resources, and
I/O modules.
Logic Module
IO Module
Routing Tracks
& Switch boxes

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FPGA Architecture Examples
Logic
Module
Array-based Model Row-based Model
Sea-of-Gates Model Hierarchical Model
Routing
resources
overlayed
on logic
modules

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Two Types of Logic Modules
Look-Up Table (LUT) based:
• A block of RAM to store the truth table.
• A k-input 1-output functions needs 2k bits.
• k is usually 5 or 6.
Multiplexer based: e.g., f=ABC+ABC
C
B
A
A
B
f

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Two Types of Switchboxes
• First Type:
• Second Type:

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Several Segmentation Models
• Non-Segmentation Model:
• Uniform Segmentation Model:
1 4 0 0 2 0 0 3 0 5 0 0 0 0
0 0 0 1 0 0 4 0 2 0 3 0 0 5
Connecting
Not connecting
1 4 0 0 2 0 0 3 0 5 0 0 0 0
0 0 0 1 0 0 4 0 2 0 3 0 0 5
Fuse or
Programmable
switch

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Several Segmentation Models
• Uniform Staggered Segmentation Model:
• Non-uniform Staggered Segmentation Model:
1 4 0 0 2 0 0 3 0 5 0 0 0 0
0 0 0 1 0 0 4 0 2 0 3 0 0 5
1 4 0 0 2 0 0 3 0 5 0 0 0 0
0 0 0 1 0 0 4 0 2 0 3 0 0 5

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Comparison of Segmentation Models
• The segmented model provides better utilization of
routing resources.
• However, segmented model uses more fuses or
programmable switches.
• The delay of a net is directly proportional to the # of
fuses or programmable switches in the route
– Manhattan-distance based delay model does NOT work
anymore
– The segmented model is slower in general

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Physical Design of FPGAs
• Very different from other design styles
• Architecture dependent:
– LUT or Multiplexer in logic modules
– Type of switchboxes used
– Type of segmentation model used
– ......
• Physical Design:
– Partitioning
– Floorplanning/Placement
– Routing

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Partitioning
• Want to partition the circuit such that each partition
(cluster) can be implemented by a logic module.
• Also called Clustering.
• # of I/O pins, not cluster sizes, is important.
(For multiplexer based logic modules, functionality of
clusters is also important.)
Example:
Using 4-LUTs

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Placement
• Assign clusters formed during partitioning to logic
modules of FPGA.
• The problem is the same as gate-array placement.

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Routing
• Global routing:
– Similar to global routing in other design styles.
– Minimize wire length and balance densities.
• Detailed routing:
– Very different from other design styles.
– Different algorithms for different segmentation models.
– Channels and switchboxes have fixed capacities.

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Structured ASIC
• New buzz word, but essentially gate array
– Mask reconfigurable
– Not field reconfigurable
• Between FPGA and standard cells
– Balance delay/performance and mask cost
• Only programmable once
– by vias (e.g., Via-Programmable Gate Array – VPGA)

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Physcial Design Automation
of MCMs and SiPs

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MCM and SiP
• Multi-Chip Module
• System in package (SiP)
– Different package styles
– Thermal consideration for 3D
• Alternative packaging approach for high performance
systems.
• Similar to PCB and IC layout problems, but
– PCB layout tools cannot handle the dense and complex wiring
structure of MCM.
– IC layout tools cannot handle the complex electrical, thermal
and geometrical constraints.

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Example: Pentium
Substrate size:
32mmx32mm
Package size:
43mmx43mm
(4 times smaller)

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Partitioning
• Partitioning a circuit so that each sub-circuit can be
implemented into a chip.
• MCM may contain as many as 100 chips.
• Need to consider timing constraints and thermal
constraints
• In addition, also need to consider traditional I/O
constraints and area constraints.

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Placement
• # of components is much less as compared to IC
placement.
• However, need to consider timing constraints and
thermal constraints (as bare chips are placed close to
each other).
• Routing is done in routing layers, not between chips.
• So no routing region needs to be allocated.

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Routing
• Main objective is to satisfy timing constraints.
• Another objective is to minimize # of routing layers, not to
minimize routing area.
– Cost is directly proportional to # of layers
• Crosstalk, skin effect and parasitic effect are important
considerations.
• Wires are of smaller pitch and more dense than PCB layout.

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EE382 V -- Conclusions

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What Have Been Taught?
• Introduced different problems in Physical Design.
• Numerous algorithms which are different in terms of
– design styles
– objectives
– constraints
– techniques
– optimality
– efficiency
– robustness
– .....

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What Is Important?
• Understand the problems
– How to formulate the problems, represent the constraints,
solutions, etc.
– Reasonable assumptions/abstractions
• Know fundamental algorithms to solve the problems.
• However, the world keeps on changing:
– technology
– objectives
– constraints
– requirement on solution quality
– computational power
• It is more important to learn how to think
– formulate the problem
– solve it smartly

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Problem Solving Techniques
• Greedy Algorithm
• Simulated Annealing/Genetic Algorithm
• Mathematical Programming
– Linear, Quadratic, Integer Linear, geometric, posynomial, …
• Dynamic Programming
• Reduction to graph problems
– min-cut, max-cut, shortest path, longest path, bipartite matching,
minimum spanning tree, etc.
• Divide-and-Conquer
• Many different heuristics
• ....

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System Specification
Micro-Architectural
Specification
Timing & Relationship
Between Units
RTL (in HDL)
Netlist
Architectural Design
Functional Design
Logic Design
Circuit Design
VLSI Design Cycle

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Netlist
Layout
Mask
Packaged Chips
Physical Design
Fabrication
Packaging
And Testing
VLSI Design Cycle

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Conventional Physical Design Cycle
Partitioning
Floorplanning & Placement
Routing

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Technology Trend and Challenges
Source:
ITRS’03
 Interconnect determines the overall performance
 In addition: noise, power => Design closure
 Furthermore: manufacturability => Manufacturing closure

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New Trends in Physical Design
• For nanometer IC designs, interconnect dominates
• New physical effects
– Noise: coupling, P/G noise
– Power: leakage, power/voltage islands
– Manufacturability: yield, printability
– Reliability, …
• More and more vertical integration
– Logic synthesis coupled with physical design
– Interconnect optimizations & design planning
– Physical design as a bridge between lower level modeling and
higher level optimization/planning
• Existing CAD algorithms are far away from optimal

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Check points
 Problem solving skills on underlying physical
design algorithms
 Know what’s behind the scene of CAD tools
 Know the trend and critique ability if given a new
research paper
 Project study of a topic of your choice and
implementation (through class project)
 Presentation skill
 Paper writing and job preparation