backend lecture 3

8/16/2019 Backend Lecture 3

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The University of Texas at AustinFoil # 1

The University of Texas at AustinEE 382M-8 VLSI-2 Page 1

EE-382M-8

VLSI–II

Early Design Planning:

Back End

Mark McDermott


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Backend EDP Flow

• The project activit ies wil l include:

– Determining the standard cell and custom library elements needed

to completely do the design with APR tools.

– Detailed floor-plan of the block level components.

– A reasonably detailed top-level floorplan using the cluster abstracts.

– Approximate clock routing at the top-level

– Approximate Power-GND routing at the top level


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Standard Cell Library Effort

• Will be using a very minimal standard cell library for the project:~80+ cells

– Basic logic gates and buffers

– 1 set-reset flip-flop

• “ CMOS65_SubVt.lib” fi le was derived using a scaled 65nm .lib

file

– Need to validate the scaled numbers with HSPICE simulations.

– Need to validate power spreadsheet numbers using HSPICE:• S-D leakage currents

• Intrinsic power

– Need to validate area spreadsheet numbers


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Block Floorplanning Effort

• Objectives:

– Minimize area

– Determine best shape of the block

– Minimize total wire length

• Each team will do a detailed floorplan of their respective blocks.

The output will be a spreadsheet analysis showing the

contribution from each of the following:

– Power grid – Clocking

– Signal Routing

– Datapath area

– Random logic area

– White space


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The University of Texas at AustinFoil # 6 The University of Texas at AustinEE 382M-8 VLSI-2 Page 6

Integration Effort

• The integration team will be responsible for: – Doing a floor plan of the top level of the chip

– Characterizing the top-level routing delays and determining theassertions and constraints for each cluster. They will be working

with each cluster to optimize the constraints. – Designing the clock routing structure:

– Determining the clock generation implementation (block diagrams)

– Determining the clock regeneration circuitry (block diagrams)

– Determining the reset logic. – Designing the power grid.

– Determining the power estimation for the global clock and signalrouting.

– Generating the power budget for each cluster. – Generating the area budget for each cluster.


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Layout Implementation Options

SPARC-T1


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Layout Density & Die Size = Performance

• Higher density layout leads to

smaller block sizes

• Smaller block sizes lead toshorter wires

• Shorter wires can lead to higher

frequency

• Shorter wires can also lead to

higher IPC by requiring fewer

transmission pipe stages

Layout #1

Layout #2

A B’

A

C

CB

A C

Schematic

Floorplan

B’

The layout of Block B affects the

timing of the path from A to C


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• Synthesis – Random Logic Macro (RLM) – Cell layout comes from a shared cell library

– Automated cell selection and placement

– Automated routing between cells

• Structured Custom (SC/SDP)

– Cell layout comes from a shared cell library

– Manual cell selection and placement

– Automated routing between cells

• Custom Design (CD)

– Cell layout is unique for each application

– Manual cell selection and placement

– Manual routing between cells

Increasing

Design Effort

(And Density)


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CD SC RLM

ARTL Coding M M M

Logic Minimization M M A

Cell Placement M M A

Device Sizing M A A

Layout M A A

A = Automatic

M = Manual

CD SC RLM

Timing Best Better WorstDensity Best Better Worst

Design Time Worst Better Best

• RLM saves time incircuit design andlayout

• SC saves time inlayout.

• RLM and SDP makerevisions easier.


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Datapath and Block Floorplanning Procedure

• Step 1 - Identify feedthrus for RLM or SC/DP block

• Step 2 - Look for opportunities for track sharing

• Step 3 - Define the bitpitch of the block

• Step 4 - Review the metal plan within the cell• Step 5 - Review and plan the clock routing and placement

• Step 6 - Plan the critical cell placement locations

• Step 7 - Estimate the area of the cells and the block

• Step 8 - Review the power grid


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Feed-through or Over-the-cell (OTC) Routes

• Metal tracks routed over RLM, Datapath or custom block

• The block is neither the driver or a receiver of the signals

• Feedthrus use up metal tracks which impacts the internal

signals of the block• Carefully review datapath connectivity to account for them

Bypass

ALU 0

ALU 1

ALU 2

Sources Results

Receiver

Driver

Feedthrus

for ALU0


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• Step 1 - Identify feedthrus








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Step 2: Track Sharing

• Minimizes the number of unique tracks in layout byopportunistically sharing tracks where possible

• Often allows for the smallest possible bitpitch

• Allows for metal layers to be more efficiently utilized

• Can help improve performance by shortening distances

• Should always be explored to improve layout efficiency and

performance


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Bypass$

ALU 0

ALU 1

ALU 2

Sources Results

First, check outside your

block to see if there

are any candidates

for track sharing

Receiver

Driver


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Next, check inside your

block to see if there

are any candidates

for track sharing

LRBL RRBL

IE_BYC_DATA IE_RF_DATA

Metal 2Metal 4


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Step 2: Track Sharing Example


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Step 2: Track Sharing Example


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• Step 1 - Identify feedthrus








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Bit Pitch Defining Width of Chip

AMD K5


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Step 3: Define the Bitpitch

• Fixed cell width chosen toallow easy assembly

• Most often determined by

metal usage within the

datapath

• Integration efficiency would

prefer one bitpitch per

project• Architectures lend

themselves to more unique

bit pitches

Bitpitch A

A

A

A

A

Vdd

Sig0

Sig1

Sig2

Sig3

Sig4

Sig5

Vss

Vdd

Sig0

Sig1

Sig2

Sig3

Sig4 Sig5

Vss


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Insure all blocks in a datapath stack follow the same bitpitch

B i t p i t c h # 2 Y µ

B y p a s s C a c h e

I n t e g e r R e g i s t e r

F i l e

A

L U 0

A

L U 1

A r i t h F l a g s

A G E N

- L D / S T A

S h i f t e r

W B

M u x

B i t

O p s

S y s t e

m U o p s

B i t p i t c h

# 1 X µ


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Bit Pitch Example: 3:2 Adder Bit Cell

Bitpitch

7.56u

M1

M4

M3 & M1


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Bit Pitch Example: 4 Bit Cells stacked

Bitpitch

7.56u BIT - 0

BIT - 1

BIT - 2

BIT - 4


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Bit Pitch Example: Tiled Datapath


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Bit Pitch Example: Swizzle

Don’t mix and match bit pitches to avoid swizzle channels

As buses get wider and the number of tracks per

bit gets higher the cost of swizzle channels grows

Swizzle

Channel


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• Wider bit pitches allow more upper level metal usage

• Narrower bit pitches allow shorter routes for orthogonal signals

• Balancing these conflicting objectives can be difficult

• Understand your local constraints and be aware of the tradeoffs


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• Step 1 - Identify feedthrus• Step 2 - Look for opportunities for track sharing







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Metal Planning

• Metal layer, width, spacing and shielding are negotiable – “ Negotiable” means you have to plead your case to the integration

leaders

• All of these impose a physical constraint for layout

• For your first attempt at convergence

– M1,M2 : Local routing – M3,M4, M5, M6 : Data and control

– M7,M8 : Power, Ground, Clock, Reset, etc

– Assume all nets are routed in M1&M2 within your block

– Assume your only shielding is on clocks and reset

– Assume the routes are minimum


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Metal Flow Planning

Avoid bi-directional dataflow

BAD GOOD

Data

Cntl

Data

Cntl

Data


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Shielding

• Intentionally routing signals to control the effective line-to-linecapacitance seen during switching.

• Requires designers to constrain the physical assembly done by

routing tools or physical design specialists (PDSs).

• Falls into one of three categories:

– Physical shielding - signals are routed next to a power rail

– Logical shielding - signals are routed by logically related signals

– Temporal shielding - signals are routed by temporally dist inct

signals


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Miller Coupling Factor

A

B

C

A

B

C

A

B

C

A

B

C

A

B

C

MCF = 1.5 One against, one quietMCF = 2.0 Both against

MCF = 0.5 One with, one quietMCF = 1.0 Both quiet MCF = 0.0 Both with


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No Shielding

• Signals are routed next to any neighboring signals• Neighbors can slow down (max delay) or speed up (min delay)

signal transitions through line-to-line coupling

• Variation can create design problems

• Most signals will not be shielded

Sig A Sig B Sig C

No ShieldMax MCF 2.0 Min MCF 0.0

A

B

C

A

B

C


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Physical Shielding

• Signals are routed next to at least one power rail• Helps both min delay and max delay

• Can be expensive in terms of metal usage

• Typically limited to most critical nets and clocks

Vss Sig A Sig B Vss Sig A Vss

Half Shield Full Shield

Max MCF 1.5

Min MCF 0.5

Max MCF 1.0

Min MCF 1.0


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Logical Shielding

• Signals are routed next to mutually exclusive neighbors• Also helps min delay and max delay

• Comparable results as physical shielding but lesser cost

• Encouraged in mux structures and arrays

Sel A Sel B Sel C

A

B

C

Sel A

Sel B

Sel C

Sel A

Sel B

Sel C

Max MCF 1.5

Min MCF 1.0


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Temporal Shielding

• Signals are routed next to signals that limit aggressors• Can help max delay or min delay or both

• Lesser cost than physical shielding, but more design effort

• Encouraged wherever possible but tricky

A

B

C

A

B

C

Max MCF 1.0

Min MCF 0.0

Ck

Ck

Ck

Sig A Sig B Sig C


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Variations of Clock Tree distribution networks

Tapered H-Tree

Target: Metallization and Gate topology uniformity


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Clock Routing

• Watch out for the clock, it’s your most critical net• Make sure the physical design treats it accordingly

• Help reduce clock power by eliminating unnecessary load

• Make sure the clock has enough via coverage• Leave room for decoupling capacitors and upsizing

• Don’t forget to account for clock routing overhead (full shield) in

your metal planning


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Clock Routing

BAD GOOD

UNNECESSARY

LOAD

Avoid unnecessary clock load to save active power


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Power/Clock Grid

• Clock grid is interleaved between VDD and VSS on metal6

Port1 Input Data Latch

LCB

LCB

Port0 Input Data Latch LCB

LCB

Port0 Read/Write CktLCB

Port0 Output LatchLCB

LCB

Port1 Output LatchLCBPort1 Read/Write Ckt

LCB

LCB

LCB

LCB

Bitcell

Array

Port1 Input Data Latch

LCB

LCB

Port0 Input Data LatchLCB

LCB

Port0 Read/Write Ckt LCB

Bitcell

Array

P or t 0 D e c

o d er

LCB

LCB

Port0 Output Latch LCB

LCBPort1 Output LatchPort1 Read/Write Ckt

LCB

LCB

LCB

LCB

LCB

LCB

LCB

Port0 Read/Write Ckt

P or t 1 D e c

o d er


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Clock Routing

Make sure there are enough vias to get power through

the clock network

INSUFFICIENT

VIA COVERAGE

SUFFICIENT

VIA COVERAGE


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Clock Routing

Remember to count clocks as ~5-7 tracks in your

wire planning!

Vdd Clock Vss

Be careful with gated clocks. Fine grain

clock gating tends to drastically increase

the number of unique clocks, significantly

increasing the metal usage.

No tools catch this before layout

1x 2x 1 x

1.5x 1.5x


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C ll Pl t


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Cell Placement

• Start with the critical path! – Place cells to limit the wire load on the crit ical path

– Move less critical blocks out of the way

• Place clock generators to limit clock wire load

– Again, place most critical clock LCBs first if area is tight

– Ideally there should be minimal side loads

• Consider track sharing opportunit ies when placing cells

– Cell placement can enable or disable track sharing

– Optimum placement generally follows data flow

C ll Pl t


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Cell Placement

LCB

Short

critical

path

No side

load

C ll Pl t d R ti


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Cell Placement and Routing

D t th d Bl k Fl l i P d


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A E ti ti


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Area Estimation

• All modules have an area budget in the floorplan

• That budget is only an educated guess

• Some guesses are high, and some are low

• You will need to enhance the quality of these estimates by more

accurately estimating the area of your modules

• While doing this you will reduce the amount of late surprises in

the design and also reduce post-layout effort by converging withaccurate parasitics


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Di Si E ti ti


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Die Size Estimation

Datapath and Block Floorplanning Proced re


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Power Grid


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Power Grid

Think of the grid as a straw

between the C4 and the devices.

Too many devices sucking through

the same straw or too narrow astraw can cause devices to starve

and the supply to dip or crater!

SAMPLE P /G d GRID


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SAMPLE Power/Ground GRID

Shielding takes up significant routing resources. Global M6 routes over the array should have minimal coupling noise

to array bitlines.

* Where is minimum critical dimension for width/space

S i g

S i g

S i g

S i g

VSS VDD VSS

S i g

48

S i g

V s s

V s s

V s s

V s s

(Full Shielding, MCF = 1.0)

4

Power Grid


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Power Grid

SCHEMATIC

VIEW

CELL LAYOUT

VIEW

RELATIVE CELL

PLACEMENTA

Bit 31

Bit 0

A

A

OUT

Power Grid


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Power Grid

A

SCHEMATIC

VIEW

CELL LAYOUT

VIEW

RELATIVE CELL

PLACEMENTA

Bit 31

Bit 0

A

When large, arrayed drivers pull

on the same rail, supply bounce

can occur degrading performance

and causing supply offset noise

OUT

Out

Current

Vdd

Vss

Power Grid


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Power Grid

• Be very careful arraying large drivers• Follow the % power guidelines for the power grid

• Try to keep temporal relationships between arrayed drivers

• Consider the physical impact on the grid by your design• Be prepared to make the grid more robust to compensate for

marginal grids

Summary


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Summary

• Early design planning and layout can have a signif icant impacton processor design

– Die size, profi t & power are impacted by layout density

– Schedule is impacted by implementation choices

• Floorplanning also signif icantly impacts circuit performance

– Shielding can help timing and noise sensit ive circuits

– Carefully f loorplanning critical paths can help reduce wire loads

– Reducing clock routing can reduce clock skew and clock power

Backup


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Backup

Wire and Resistance Calculator


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Wire and Resistance Calculator

ALPHA 21364


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ALPHA 21364

PPC 603


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PPC 603

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