© Digital Integrated Circuits 2nd and F. Brewer 2003, 2011 Design Methodology ECE 224a CMOS VLSI Design Lab F. Brewer

© Digital Integrated Circuits2nd and F. Brewer 2003, 2011 Design Methodology

ECE 224a CMOS VLSI Design LabECE 224a CMOS VLSI Design Lab

F. Brewer


ECE 224aECE 224a

Fabricate a real design MMI/Cadence/Mentor/Synopsys Tools

MMI Full Custom (Cell, Array, Data-Path) Cadence/Synopsys P&R (digital path)

Not a first class in VLSI 124a or equivalent required, 124d is good plan

Review Essential Concepts FET, Diode, Transient Model (Elmore), Sizing Layout/Design Rules: Wire Planning, Gradient

Variation, Tricks of Trade


Class LogisticsClass Logistics

Homework (out wed, due 1 week) Quizzes (3 in-class) No Final Design Proposal Design Review Submitted Project Report


The First Integrated Circuits The First Integrated Circuits

Bipolar logic1960’s

ECL 3-input GateMotorola 1966


Intel 4004 Micro-ProcessorIntel 4004 Micro-Processor

19711000 transistors1 MHz operation


0.6um 3M 3.3-5V bulk CMOS P1/P2 CAP Poly Resistor HV Implants (up to

40V!) 2.25mm2

1.5mmx1.5mm 9 week design

cycle, 3 person

ECE224a ProjectECE224a Project


Current State of AffairsCurrent State of Affairs

High-End Technology (32-22nm) still a driver Limited to large design efforts ($$$NRE) Small number of Players

– FPGA: Actel, Lattice, Xilinx, Altera– Processor: AMD, Intel, IBM– SOC: Conexant, Cisco, Juniper, Nintendo…– Structured ASIC: NEC, Fujitsu, Hitatchi, Samsung

Most Design Starts > 0.09um! Mixed Signal Applications Mature Technology – Lower NRE and Risk High Potential for Innovative Design/Architecture


224a Project Limits224a Project Limits

Get 1 1.5x1.5mm design/2-3 students 1500 Standard Cell Gates 50kbits ROM/5kbits SRAM 64 Comparators/ 15 Op-Amps 40-48 pins (at least 8 used for Pwr/Gnd) 100Mhz practical large swing (3.3V) limit

– 800+MHz differential 300mV

3.3 or 5V default, 12V possible


Design ScheduleDesign Schedule

9 week design flow 1 week project definition 3 weeks schematic/simulation + test design 2 weeks layout 2 weeks design verification and tweak Tape Out

– Must be DRC, LVS Clean– Must have Full Die Simulation/Sanity– Must have test plan and agree to physical test


Survival GuideSurvival Guide

Choose Team to Complement Skills! No more than 3. 2 is fine, 1 if enough project slots

Under-Specify/Over-Deliver If you cannot finish basic design in 1 week simplify

design! Basic Design through layout before adding features! Make decisions early, stick to them Use expert resources: Professors, experienced

students Goal: Have Fun! --


What to make?What to make?

Mixed Signal Designs Rock Pure Digital 1-bit signal processing Analog Sensors/Digital Output Good Choice

– Temperature, Light, Magnetic, RF, Field, voltage, current, time, phase…

Digital Synthesis/Power also good– Sound (even music!), RFID/Xmit, motor driver/controller,

PLL (clock synthesis or other…), Display (LCD) or LED Tricky Small Designs

– Journal of Consumer Circuits, JSSC about 10-15 years ago (0.5um in vogue), LFSR tricks…


What to What to NOTNOT make: make:

MicroProcessor 4-bit possible (8 bit tiny MIPS – won’t fit w/o reg)

– 1 success in 22 years, 5 months design time No non-volatile Memory

– (design some is good, but hard, project!)

Digital Multiplier/Adder/Function Block Space, Pins (How to test!), Why?

Generic OpAmp How to test/characterize? If you have a use in mind – it is not generic!


Design Design MethodologyMethodology


Evolution in ComplexityEvolution in Complexity


The Design Productivity ChallengeThe Design Productivity Challenge

Source: ITRS’97

A growing gap between design complexity and design productivity

58%/Yr. compound

Complexity growth rate

21%/Yr. compound

Productivity growth rate

19

81

Logic

Tra

nsi

stors

per

Chip

(K

)

Pro

duct

ivit

y (

Tra

ns.

/Sta

ff-M

onth

)

100

1,000

10,000

100,000

1,000,000

10,000,000

100

1,000 10,000

100,000

1,000,000

10,000,000

100,000,000

10

19

85

19

89

19

93

19

97

20

01

20

05

20

09


Scaling?Scaling? Technology shrinks by 0.7/generation With every generation can integrate 2x more

functions per chip; chip cost does not increase significantly

Cost of a function decreases by 2x But …

How to design chips with more and more functions? Design engineering population does not double every

two years… Physical design constraints more and more difficult to

surmount Diminishing Returns for Design Dollars


The Custom Approach The Custom Approach

Intel 4004

Courtesy Intel


Transition to Automation and Regular StructuresTransition to Automation and Regular Structures

Intel 4004 (‘71)Intel 4004 (‘71)Intel 8080Intel 8080 Intel 8085Intel 8085

Intel 80286Intel 80286 Intel 80486Intel 80486Courtesy Intel


Automating DesignAutomating Design

Exploitation By Algorithms Regular Structures Logic Synthesis Regularization of Connection

Floorplanning (Localization of function) System Level Performance/Power/Cost Allocation of Physical Resources

Communication/Interconnect Hierarchy based on Sensitivity to Latency Wires to Link Protocols


A System-on-a-Chip: ExampleA System-on-a-Chip: Example

Courtesy: Philips


Design MethodologyDesign Methodology

• Design process traverses iteratively between three abstractions: behavior, structure, and geometry• Trick: automate these steps


Implementation ChoicesImplementation Choices

Custom Semi-custom

Digital Circuit Implementation Approaches

Standard CellsCompiled Cells Macro Cells

Cell-based

Pre-diffused(Gate Arrays)

Pre-wired(FPGA's)

Array-based

Hard Macros(RAM/ROM)


Implementation StrategiesImplementation Strategies

Data-Path 1D tiling, custom in depth

Cell based logic Technology confined to cells (area) 2D via 1D cell rows, automatic P&R

2D Arrays (Memory, CAM, CCD, MPY) Dense but very constrained Design time consuming!


2-d Cell Based: “Hard” Modules2-d Cell Based: “Hard” Modules

25632 (or 8192 bit) SRAMGenerated by hard-macro module generator


1-d Cell-based Design (standard cells)1-d Cell-based Design (standard cells)

Routing channel requirements arereduced by presenceof more interconnectlayers

Functionalmodule

(RAM,multiplier,)

Routingchannel

Logic cellFeedthrough cell

Rows ofcells


Concepts of Placement Concepts of Placement

Standard cells are placed in “placement rows”

Cells in a timing-critical path are placed close together to reduce routing related delays (Timing Driven)

Placement rows can be abutting or non-abutting


Concepts of Routing Concepts of Routing

Connecting between metal layers requires one or more “vias”

Metal Layers have preferred routing directions Metal 1 (Blue) Horizontal Metal 2 (Yellow) Vertical Metal 3 (Red) Horizontal


Concept of Routing TracksConcept of Routing Tracks

Metal routes must meet minimum width and spacing “design rules” to prevent open and short circuits during fabrication

In grid based routing systems, these design rules determine the minimum center-to-center distance for each metal layer (Track/Grid spacing)

Congestion occurs if there are more wires to be routed than available tracks


Grid-Based Routing SystemGrid-Based Routing System

Metal traces (routes) are built along and centered around routing tracks

Each metal layer has its own tracks and preferred routing direction Metal 1 – Horizontal Metal 2 – Vertical

Track and pitch information can be located in the technology file Design Rules


Standard Cell — Old ExampleStandard Cell — Old Example

Automation Height fixed Width variable Channel routing

Optimization Place by annealing

to minimize wire-length and net criticality

[Brodersen92]


Standard Cell – The New GenerationStandard Cell – The New Generation

Cell-structurehidden underinterconnect layers

Same basic scheme-- more layers-- wires over cells-- power/clock plan-- leave spaces forFiller/bypass andBuffer cells


Standard Cell - ExampleStandard Cell - Example

3-input NAND cell(from ST Microelectronics):C = Load capacitanceT = input rise/fall time


““Soft” MacroModulesSoft” MacroModules

Synopsys DesignCompiler


Gate Array — Sea-of-gatesGate Array — Sea-of-gates

rows of

cells

routing channel

uncommitted

VD D

GND

polysilicon

metal

possiblecontact

In1 In2 In3 In4

Out

UncommitedCell

CommittedCell(4-input NOR)


Sea-of-gate Primitive CellsSea-of-gate Primitive Cells

NMOS

PMOS

Oxide-isolation

PMOS

NMOS

NMOS

Using oxide-isolation Using gate-isolation


Sea-of-gatesSea-of-gates

Random Logic

MemorySubsystem

LSI Logic LEA300K(0.6 m CMOS)

Courtesy LSI Logic


The return of gate arrays?The return of gate arrays?

metal-5 metal-6

Via-programmable cross-point

programmable via

Via programmable gate array(VPGA)

[Pileggi02]

Exploits regularity of interconnect


Pre-wired Arrays:Pre-wired Arrays:Classification of prewired arrays (or field-programmable devices): Based on Programming Technique

Fuse-based (program-once) Non-volatile EPROM based RAM based

Programmable Logic Style Array-Based Look-up Table

Programmable Interconnect Style Channel-routing Mesh networks


Fuse-Based FPGAFuse-Based FPGA

antifuse polysilicon ONO dielectric

n+ antifuse diffusion

2

From Smith’97

Open by default, closed by applying current pulse


Array-Based Programmable LogicArray-Based Programmable Logic

PLA (flexible – sizing) PROM (dense) PAL (uniform load)

I 5 I 4

O0

I 3 I 2 I 1 I 0

O1O2O3

Programmable AND array

ProgrammableOR array I5 I4

O0

I3 I2 I1 I0

O1O2O3

Programmable AND array

Fixed OR array

Indicates programmable connection

Indicates fixed connection

O0

I3 I2 I1 I0

O1O2O3

Fixed AND array

ProgrammableOR array


Programming a PROMProgramming a PROM

f0

1 X 2 X 1 X 0

f1NANA

: programmed node


Rent’s RuleRent’s Rule

Rent described a relation between the number of components in a subsystem and the number of wires to connect it.

The rule was developed for large digital systems, but is reflected in all human design (hierarchy)

A Rent coefficient of 0.5 corresponds to a planar scalable design – i.e. the perimeter (where wires go) is grows to support the area of a planar figure.

rDT NKN


r=0.45K=0.82

r=0.63K=1.4

RRent’s Rule:ent’s Rule:

100 1,000 10,000 100,000 1,000,00010

100

1,000

10,000

static ram

dynamic ram

microprocessors

gate arrays

high performance computers

chip level

board level

[Bakoglu, 1987]

r=0.5K=1.9

r=0.12K=6

rDT NKN


Design FlowDesign Flow


Design Flow - OverviewDesign Flow - Overview

Generic VLSI Design Flow from System Specification to Fabrication and Testing

Steps prior to Circuit/Physical design are part of the FRONT-END flow

Physical Level Design is part of the BACK-END flow

Physical Design is also known as “Place and Route”

CAD tools are involved in all stages of VLSI design flow

Different tools can be used at different stages due to EDA common data formats*


Where does the Gate Level Netlist come from?Where does the Gate Level Netlist come from?


FloorplanningFloorplanning

2D layout: Area does not correspond to architecture Communication on boundaries – wire length


Design Must Be Floorplanned Before P&RDesign Must Be Floorplanned Before P&R

Floorplan of design: Core area defined with large macros placed Periphery area defined with I/O macros placed Power and Ground Grid (Rings and Straps) established

Utilization: Percentage of the core used by placed standard cells and macros

typically 80-85%


I/O Placement and Chip Package RequirementsI/O Placement and Chip Package Requirements

Some Bond Wire requirements:

No Crossing

Minimum Spacing

Maximum Angle

Maximum Length


Guidelines for a Good FloorplanGuidelines for a Good Floorplan

A few quick iterations of place and route with timing checks may reveal the need for a different floorplan


Defining the Power/Ground Grid and BlockagesDefining the Power/Ground Grid and Blockages

Purpose of Grid is to take the VDD and VSS received from the I/O area and distribute it over the core area

Blockages can also be added in the floorplan to prohibit standards cells from being placed in those areas

Loading IR drop and noise issues

Sometimes need Guard rings around critical regions


Design Flow – Timing Driven PlacementDesign Flow – Timing Driven Placement

Astro™ optimizes, places, and routes the logic gates to meet all timing constraints

Balancing design requirements Timing Area Power Signal Integrity


Timing ConstraintsTiming Constraints

Astro™ needs constraints to understand the timing intentions Arrival time of inputs Required arrival time at outputs Clock period

Constraints come from the Logic Synthesis tool SDC (Synopsys Design

Constraints) format


Cell and Net DelaysCell and Net Delays

Astro™ calculates delay for every cell and every net

To calculate delays, Astro™ needs to know the resistance and capacitance of each net Uses geometry of net and Look Up Tables to estimate the

resistances and capacitances


Timing Driven PlacementTiming Driven Placement

Timing Driven Placement places critical path cells close together to reduce net RC

Prior to routing, RC are based on Virtual Routes

What if critical paths do not meet timing constraints with placement?


Logic OptimizationsLogic Optimizations

These optimizations can be done during pre-place, in-place, or post-place stages of placement

Each optimization can be done separately or all done concurrently during placement (none – one – all)


The “Design Closure” ProblemThe “Design Closure” Problem

Synopsys

Iterative Removal of Timing Violations (white lines)Problem: no guarantee of convergence, can take weeks


Timing Closure and Mask VerificationTiming Closure and Mask Verification

Fail Timing

RTL ECO

Density, Order ECO

Constraint, Library ECO


Design Flow – Clock Tree SynthesisDesign Flow – Clock Tree Synthesis

All clock pins are driven by a single clock source

Large delay and transition time due to length of net

Clock signal reach some registers before others (Skew)


After Clock Tree SynthesisAfter Clock Tree Synthesis

A clock (buffer) tree is built to balance the output loads and minimize the clock skew

A delay line can be added to the network to meet the minimum insertion delay (clock balancing)


Gated - CTSGated - CTS

Clocks may not be generated directly from I/O

Power saving techniques such as clock-gating are used to turn of the clock to sections of the design


Effects of CTSEffects of CTS

Several (Hundreds/Thousands) of clock buffers added to the design

Placement / Routing congestion may increase

Non-clock cells may have been moved to less ideal locations

Timing violations can be introduced


Timing Driven RoutingTiming Driven Routing

Routing along the timing-critical path is given priority Creates shorter, faster connections

Non-critical paths are routed around critical areas Reduces routing congestion problems for critical paths Does not adversely impact timing of non-critical paths


Timing VerificationTiming Verification

Calibre PEX performs the layout parasitic extraction of the resistances and capacitances of all wires and devices in the design

Results in Augmented Spice Deck and in logic format decks such as SPEF (Standard Parasitic Extended Format) SPEF is an extended form of Standard Parasitic Format (SPF),

which enables the transfer of design specific resistances and capacitances from physical design to timing analysis and simulation tools

Primetime performs static timing analysis Detects timing violations by combining SPEF and netlist and checks

against the design timing constraints (setup and hold times)


Physical VerificationPhysical VerificationChecks the design for fabrication feasibility and physical

defects that could result in the design to not function properly 3 checks (DRC/ERC, and LVS)

Design Rule Checks (DRC) Verifies that design does not violate any fabrication rules

associated with the target process technology (metal width/space, antenna wires, fill ratio, etc)

Electrical Rules Checks (ERC) Verifies that there are no short or open circuits with power and

ground as well as resistors/capacitors/transistors with floating nodes (part of LVS)

Layout Versus Schematic (LVS) Final physical design matches the logical (schematic) version in

terms of correct connectivity and number of electrical devices


FabricationFabrication

Physical Design process is complete upon successful completion of timing, functional, and physical verification

The design can be “Taped-Out” and GDSII created for the manufacturer GDSII (Graphic Design System II) is a

binary format containing the physical geometry information of the design.

The shapes are assigned numeric attributes in the form of “Layer Number” and “Data Type” (Metal 1 => 100:0)

Fabrication and Test determine which chips can be implemented into the system (yield)


UCSB ToolsUCSB Tools

Tools from all the major vendors: Cadence (ON-Semi 0.6um via NCSU/OSU

SCMOS) Synopsys (Logic Synthesis, Simulation, Timing) Mentor (Calibre DRC/LVS/PEX) MMI (Full Custom, Array and DataPath)

Final Chip physical verification must be through Calibre full-chip DRC and LVS Full-chip extracted spice sanity check


HW 1HW 11. If 6” wafers cost $800/ea, 10” cost $1400 and 12” cost $2000

estimate the die cost of a 5mmx5mm die in each case given a uniform defect rate of 0.2/cm2. (Hint: A point defect in your chip kills it).

2. Analog circuits typically lag by several orders of magnitude is scale compared to digital ones. How does this relate to the complexity of composing two analog circuits to make a more complex behavior?

3. FPGA use is growing – in many cases replacing ASIC designs. Typically, such designs are 30-150x slower than ASIC designs of the same power. Why?

4. Why is Timing Closure a serious issue in highly constrained digital designs (big, fast, low-power, lots of pins)?

Documents

© Digital Integrated Circuits 2nd and F. Brewer 2003, 2011 Design Methodology ECE 224a CMOS VLSI Design Lab F. Brewer