© chandu visweswariah, 2004new challenges in ic design1 new challenges in ic design … with a...
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© Chandu Visweswariah, 2004 New Challenges in IC Design 1
New Challenges in IC Design… with a focus on variability …
SBCCI 2004 Panel Discussion
Chandu VisweswariahResearch Staff Member
IBM Thomas J. Watson Research CenterYorktown Heights, NY
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© Chandu Visweswariah, 2004 New Challenges in IC Design 2
Where will performance come from?• Technology scaling
– will require ever-more exotic materials and tricks
– will yield diminishing performance enhancements
• Performance will come from– multiple processors/cores– packaging options (3D ICs, silicon carrier)– better memory hierarchies– better tools– better compilers
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© Chandu Visweswariah, 2004 New Challenges in IC Design 3
Outline
• Challenge #1: migrate from corner-based timing to statistical timing
• Challenge #2: adopt robust design methodologies and practices
• Challenge #3: simultaneous timing and (leakage) power sign-off
• Challenge #4: stop targeting worst-case design; rather, design adaptive circuits that can recover from low-probability problems
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© Chandu Visweswariah, 2004 New Challenges in IC Design 4
The march of technologyP
erfo
rman
ce
Technology generation
Is this worth a huge
investment?
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1: Corner-based vs. statistical timing
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© Chandu Visweswariah, 2004 New Challenges in IC Design 6
Benefit of statistical timing• n = # independent sources of variation (say 9)
= total variability in critical path delay (say 5%)
• Fractional increase in frequency with a 3 sign-off instead of 3n sign-off
• Assumes sources of variation are roughly equally significant
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Corner-based vs. statistical
0
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1% 2% 3% 4% 5% 6% 7% 8% 9% 10%
Sigma as fraction of cycle time
% f
requ
ency
dif
fere
nce
7
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© Chandu Visweswariah, 2004 New Challenges in IC Design 8
2: Robustness
• Reduce sensitivity of performance to variations; examples:– N/P mistracking: avoid too many tall N or tall P
stacks in critical paths– Gate/wire mistracking: use equal fractions of
gates and wires in data and clock paths– ACLV/OCV: use compact layouts so that
capturing and launching paths are close by
– Vt mistracking: use equal fractions of low Vt transistors in critical data and clock paths
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© Chandu Visweswariah, 2004 New Challenges in IC Design 9
Q&A• Q: Where will the models come from?• A: IDMs have an advantage
• Q: What will the models look like?• A: Analytic forms are more conducive than table-based
delay modeling formats
• Q: Can timers handle the capacity?• A: Yes; 2.1M gate design timed in 69 minutes with 10.9
GB memory; 1.1M gate design timed in 110 minutes (dominated by load time) with 4.3 GB memory
• Q: How will it be phased in?• A: (a) true 3 sign-off (b) implicit robustness credit
(c) explicit robustness targeting(d) at-speed test for yield/speed tradeoffs
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© Chandu Visweswariah, 2004 New Challenges in IC Design 10
BEOL early-mode variability on ASIC part
0.0E+00
5.0E-03
1.0E-02
1.5E-02
2.0E-02
-230 -168 -105 -42
Slack (ps)
Prob
abili
ty
Pessimismreduction
-3
slac
k: -
162
ps
Exh
au
stiv
e c
orn
er a
nal
ysis
: -2
25 p
s
*Early mode; variability in 7 metal levels
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© Chandu Visweswariah, 2004 New Challenges in IC Design 11
Tooleaky
3: Simultaneous power/timing sign-off
Tooslow
Pro
babi
lity
Vt
Goodchips
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© Chandu Visweswariah, 2004 New Challenges in IC Design 12
4: Adaptive circuits• Key idea: we are penalizing performance by
covering low-probability problems• Instead, recover from the low-probability
problems adaptively– sensor circuits: to sense temperature, a late signal, a
wrong logical value, Vt, a mistracking situation– actuator circuits: to change back-gate bias, change Vdd,
repeat a computation, gate the clock, throttle instruction issue
• Adaptive circuits protect against static variability, dynamic variability and single-event upsets
• A wealth of design, CAD, methodology and verification problems suggest themselves!
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© Chandu Visweswariah, 2004 New Challenges in IC Design 13
Conclusion
• Variability is causing a number of problems– leakage power– timing closure– excessive pessimism– worst-case design kills scaling benefit
• Paradigm shifts are periods of opportunity: One person’s headache is another’s windfall!