signal integrity methodology on 300 mhz soc using alf libraries and tools
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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
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