Signal Integrity Methodology on 300 MHz SoC using ALF

libraries and toolsWolfgang Roethig, Ramakrishna Nibhanupudi, Arun Balakrishnan, Gopal Dandu

Steven McCormick, Vinay Srinivas, Robert Macys, Dhiraj Sogani, Kevin Walsh

Introduction

• Signal integrity becomes dominant factor for design closure in 180nm technology and below

• Conventional design method for SI– multiple point tools for analysis– repair manually or by scripts

• New design method for SI– Same tool for analysis and optimization– Unified signal integrity library

• Signal integrity library contents using the Advanced Library Format (ALF) will be explained

• Results on 333 MHz SoC design will be shown

Conventional crosstalk-aware STA

P&R data

delay calculation

SDF

pessimistic!

pessimistic!time window calculation

Extra delay due toaggressor / victim overlap

Refine time windowsTime

windowsaccurate? no

Do another design iterationyes

no

STA

Timingo.k.?

done

yes

Issues with crosstalk-aware STA• Iterative STA due to Chicken-and-egg problem

– To calculate crosstalk-effects on delay, time windows must be known

– To calculate time windows, delay must be known• Conventional STA can only handle one time window

per clock cycle– Pessimistic assumption for crosstalk– Overestimation of multi-aggressor effects

• New crosstalk-aware STA– Delay and noise calculation integrated in STA– Supports multiple activity windows per clock cycle

New STA with activity windows• Multiple activity windows per clock cycle• Individual slew rates for each activity window• Reduces timing uncertainty• More accurate calculation of coupling effects on

delay and noise

A

B

YA

BY

One clock cycle

Accurate evaluation of crosstalk effects

-27.

5

-25.

0

-22.

5

-20.

0

-17.

5

-15.

0

-12.

5

-10.

0

-7.5

-5.0

-2.5

-

50

100

150

200

250

# paths

Negative Time Slack

VictimAggressor

VictimAggressor

Min/Max windows overlap: crosstalk delay is estimated

Activity windows do not overlap: no crosstalk delay occurs

No time windows: pessimistic

Min/Max windows: still pessimistic

Crosstalk neglected: optimistic

Activity windows: accurate

Other signal integrity effects• Noise

– Crosstalk generates spurious waveforms on supposedly quiet signal lines

– May cause unintended flip flop switch– Functional failure

• Electromigration– High electrical current inside cells– May break vias, contacts wires– The higher the frequency, the higher the damage

Analysis is not enoughAnalysis is not enough

Prevention and repair must be providedPrevention and repair must be provided

Conventional Signal Integrity Flow

Routing change

Timing-driven placement

Extraction

Routing

netlist floorplan

Timingo.k.?

no

yes

Noiseo.k.?

no

yes

EMo.k.?

no

yes

done

Placement change

Netlist / floorplanchange

Issues with conventional SI flow

• Trial-and error approach• Multiple point tools do separate SI checks

– Crosstalk-aware timing– Noise– Signal electromigration (EM)– No common library

• Check and repair is done in different tools• Mutual unawareness of SI effects

– Timing repair may cause noise violation– EM repair may cause timing violation

• Unpredictable number of design iterations

New signal integrity design flow

ALFALF libraryTimingNoise

Electromigration

Pre-route optimization

Routing

Post-route optimization

Extraction

done

netlist floorplan

Initial placement

Accurate timing library• Tool results with .lib and ALF compared with SPICE

Del ay correl ati on wi th ALF

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0SPICE [nsec]

Phys

ical

Stu

dio

[nse

c]

Del ay correl ati on wi th . l i b

0.00.51.01.52.0

2.53.03.54.04.5

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0SPICE [nsec]

Phys

ical

Stu

dio

[nse

c]

Error criterionAverageStd deviationMax - Min

.lib+ 3.9 %+/- 5.0 % 17.4 %

ALF+ 0.5 %+/- 2.2 % 11.1 %

ALFALF.lib

ALF is more accurate, less pessimistic than .libALF is more accurate, less pessimistic than .lib

Timing and noise waveforms• Timing waveform shaped by aggressor driver

resistance• Noise waveform shaped by victim driver resistance• Accurate characterization of driver resistance is key

aggressor

victim

Timing waveform@ driving point

Timing waveform@ coupling point

Noise waveform

Driverresistance

ALFALF

Accurate noise modeling• Noise propagation for combinatorial cells• Dynamic noise margin for sequential cells• Greatly reduces pessimistic noise violations

output noise peak

inputnoise peak

inputpulse width

output load cap

inputpulse width

dynamic noise margin

output load cap

static noise margin

ALFALF

• Conditions for EM damage inside cell represented by abstract vector

• Each vector has associated max. frequency

Signal Electromigration

AB

Y

1

23

4 5

ALFALF

A B

Y

1: (10 A)2: (01 A)3: (01 Y)4: (10 A -> 10 Y)5: (01 B -> 10 Y)

Signal Electromigration Flow• ALF library contains max frequency = f(slew, load)

for each EM characterization vector in cell• Global activity file (GAF) contains actual switching

frequency for each design instance vector• GAF is generated by event-driven or probabilistic

simulation• EM violation, if max frequency < actual frequency• For optimization, EM frequency limit is transformed

into max cap. limit = f(slew) for given frequency• After net list change, actual frequency is locally

propagated through inserted buffers (ECO GAF)

Design result

Design stepAutomatic floor planTotal P&RTotal ExtractionTotal Optimization

Runtime2 H20 H16 H24 H

Design dataCell instancesWorst time slack# timing violations# SI violations

initial448K-10.7 ns860024 500

final467K0.0 ns00

130 nm 333MHz, 167MHz et al8.5mm*8.5mm3003.5 M

TechnologyClock frequencyDie sizeHard macrosTotal gate count

Conclusion• ALF provides comprehensive signal integrity support

– Timing, noise, electromigration• ALF enables better crosstalk-aware STA

– Accurate ALF timing models eliminate the need for proprietary delay calculators

• ALF enables efficient signal integrity flow for ASIC and SoC designs– Iteration-free analysis and optimization– Sign-off quality

ALF is the library for next-generation toolsALF is the library for next-generation tools