Impact of Adaptive Voltage Scaling on
Aging-Aware Signoff
Tuck-Boon Chan, Wei-Ting Jonas Chan and
Andrew B. Kahng
VLSI CAD LABORATORY, UC San Diego
Outline
•
•
•
•
Introduction: BTI Aging and AVS
Signoff Problem
Observations and Proposed Heuristics
Experimental Results
1
Outline
•
•
•
•
Introduction: BTI Aging and AVS
Signoff Problem
Observations and Proposed Heuristics
Experimental Results
2
Intro: Bias Temperature Instability (BTI)
|ΔVth| increases when device is on (stressed)
|ΔVth| is partially recovered when device is off (relaxed)
NBTI: PMOS
PBTI:NMOS
|Vgs|
ON
OFF
ON
OFF
time
Device aging (|ΔVth|)
accumulates over time
[VattikondaWC06]
3
Intro: Adaptive Voltage Scaling (AVS)
• Accumulated BTI  higher |ΔVth|  slower circuit
• AVS can be used to compensate for performance degradation
Circuit
On-chip
aging
monitor
Circuit
performance
Without AVS
With AVS
target
time
Voltage
regulator
Circuit
performance
Closed-loop AVS
Vdd
time
4
Outline
•
•
•
•
Introduction: BTI Aging and AVS
Signoff Problem
Observations and Proposed Heuristics
Experimental Results
5
Problem: Signoff Corner Definition
• Timing signoff: ensure circuit meets performance target under PVT
variations & aging
• Conventional signoff approach:
– Analyze circuit timing at worst-case corners
– Fix timing violations, re-run timing analysis
• With
BTIaging
agingand
andAVS,
AVS,
what
is the Vvoltage
worst-cast
With BTI
the
worst-case
corner
is not
dd of the
corner
obviousfor timing analysis?
Vlib for circuit performance estimation
Min Vdd
Min
Vdd
VBTI for
aging
Max
estimation
Vdd
Slowest circuit
Less aging
Max Vdd
?
Slowest circuit
Too
Worst-casepessimistic
aging
Not applicable
(Optimistic)
Faster circuit
Worst-case aging
?
6
Derated Library Characterization & AVS
• VBTI = Voltage for BTI aging estimation
• Vlib = Voltage for circuit performance estimation (library
characterization)
• VBTI and Vlib are required in signoff
• Good VBTI and Vlib selection should consider expected BTI + AVS
• Aging and Vfinal are unknowns before circuit implementation
Step 1
VBTI
|Vt|
Vlib
?
Vfinal
Step 2
Derated
library
Step 3
Circuit
implementation and
signoff
BTI degradation
and AVS
circuit
7
Library Characterization for AVS
• VBTI = Voltage for BTI aging estimation
• Vlib = Voltage for circuit performance
estimation
final
lib (library
BTI
characterization)
• VBTI and Vlib are required in signoff
No obvious
• VBTI and Vlib depend on aging during AVS
guideline to define
• Aging and Vfinal are unknowns before
VBTI and Vlib
circuit implementation
Inconsistency among V , V & V
• What is the design overhead when
timing libraries are not properly
characterized?
• What are
guidelines
to
define
BTIand
Step 1
Step 2
Step 3
AVS-aware
Circuit
V
|V | signoff corners that
Derated
implementation and
library
guarantee
timing correctness
with
V
signoff
little design overhead?
BTI degradation
V
circuit
?
and AVS
BTI
t
lib
final
8
Previous Works
• There are many previous works on BTI + AVS
– [Basoglo10] [Kumar11] [Mintarno11] …
– No discussion of signoff for a circuit with BTI + AVS
• Previous works assume a circuit is signed off with
timing libraries without BTI degradation
• Then analyze BTI + AVS effects on circuit timing
– If circuit timing fails to meet requirements 
design iteration + signoff  longer design time
– An example of timing failure: AVS requires Vdd >
maximum allowed voltage to compensate aging
9
Outline
•
•
•
•
Introduction: BTI Aging and AVS
Signoff Problem
Observations and Proposed Heuristics
Experimental Results
10
“Chicken and Egg” Loop
• “Chicken and egg” loop in signoff
– Derated library characterization is related to BTI + AVS
– AVS affected by circuit implementation
• Timing constraints, critical paths, etc.
– Circuit is affected by library characterization
Vfinal
Circuit
Vlib , VBTI
Derated Libraries
11
Observation #1
• BTI is a “front-loaded”
phenomenon
• 50% BTI aging happens
within the 1st year of
circuit lifetime (total
lifetime = 10 years)
[Chan11]
Vfinal
≈70% Vdd increment in 1 year
(remaining 30% over 9 years)
• Most Vdd increment
happens in early lifetime
• Gap between Vdd and
Vfinal reduces rapidly
12
Heuristics #1
• Model BTI degradation with Vfinal throughout lifetime
– Aging of a flat Vfinal ≈ aging of an adaptive Vdd
– But slightly pessimistic
VBTI = Vlib ≈ Vfinal
NBTI
Vdd
PBTI
time
13
Vfinal Estimation
• Problem: Vfinal is not available at early design stage
(design has not been implemented)
• Vfinal = Vdd @ end of life (to compensate BTI aging)
– Gates along critical path
?
– Timing slack at t = 0
?
– Circuit activity (BTI aging) ✔
• BTI aging depends on circuit activity
– Assume DC or AC stress in derated library
characterization
14
Observation and Heuristic #2
• Observation #2: Vfinal is not sensitive to gate types
• Heuristic #2: use average Vfinal of different gate types
– Vfinal is a function of timing slack
– Assume timing slack = 0
10mV
15
Proposed Library Characterization Flow
Obtain Vheur (average
of standard cells)
• Heuristic #2: obtain Vheur
by averaging Vfinal of
different cells
Obtain derated library
with VBTI = Vlib = Vheur
• Heuristic #1: use a “flat”
Vheur to estimate BTI
degradation
Signoff circuit with
derated library
16
Outline
•
•
•
•
Introduction: BTI Aging and AVS
Signoff Problem
Observations and Proposed Heuristics
Experimental Results
17
A Reference Signoff Flow
• Basic idea: keep a consistent VBTI ,
VLIB and Vdd throughout circuit
lifetime
• Signoff flow:
– Estimate aging at each time
step
– Update circuit timing and Vdd
– Repeat until t = tfinal
– Modify circuit and start over if
Vfinal > maximum allowed
voltage
• No overhead in timing analysis,
but very slow
Many STA runs
and library
Vstep: AVS voltage step
Vfinal: converged voltage
18
Technology and Benchmark Circuits
• NANGATE library with 32nm PTM technology
• Signoff for setup time violation
• Temperature = 125C
• Process corner = slow NMOS and PMOS
• BTI degradation = {DC, AC}
Supply voltages
Circuit
C5315
c7552
AES
Frequency (GHz)
1.38
1.25
0.89
MPEG2
1.05
Vmax
Vinit
Vheur1 (DC)
1.05V
Vheur1 (AC)
Vheur2 (DC)
Vheur2 (AC)
0.95V
0.90V
0.97V
0.95V
0.93V
19
Experiment Setup
• Characterize different derated libraries
• Evaluate impact of library characterization
• Seven testcases
1 : VBTI = Vlib = Vinit  Ignore AVS
2 : Most pessimistic derated library
3 : VBTI = Vlib = Vmax  Extreme corner for AVS
4 : VBTI = Vfinal  Do not overestimate aging but ignores AVS
5 : No derated library (reference)
6 : Proposed method with α=0
7 : Proposed method with α=0.03
Case
Vlib(V)
1
Vinit
2
Vinit
VBTI (V)
Vinit
Vmax
3
Vmax
Vmax
4
Vinit
5
N/A
Vfinal
N/A
6
7
Vheur1
Vheur1
Vheur2
Vheur2
20
Results for DC Scenario
Good
corners
Optimistic signoff corner
• AVS increases supply voltage
aggressively to compensate aging
• Consume more power
• May fail to meet timing if desired
supply voltage > Vmax
1 : VBTI = Vlib = Vinit  Ignore AVS
2 : Most pessimistic derated library
3 : VBTI = Vlib = Vmax  Extreme
corner for AVS
4 : Vbti = Vfinal  Do not
overestimate aging but ignores
AVS
5 : No derated library (reference)
6 : Proposed method with α=0
7 : Proposed method with α=0.03
Pessimistic signoff corner
• Ovestimate aging and/or
underestimate circuit
performance
• Large area overhead
21
Results for AC Scenario
Good
corners
• Similar results as in the DC scenario
• Design overheads due poorly characterized libraries (#1 to #4)
are smaller compared to the DC scenario
22
Power vs. Area for All Designs
• Overlay all data points (4 designs x {DC, AC})
Circuit signed off using
our derated libraries
Circuit signed off using
other derated libraries
“Knee” point for balanced
area and power tradeoff
23
Conclusions
• Voltage for aging estimation (VBTI), library
characterization (Vlib) and operation (Vfinal) are
inconsistent
• Poorly-characterized libraries lead to circuit area or
power overheads
• We propose a flow to characterize a derated library
– Heuristic #1: approximate Vlib = VBTI ≈ Vfinal
– Heuristic #2: use replica circuits to estimate Vfinal
• Circuits implemented with our derated libraries have
similar area and power as those implemented from a
reference flow
24
Future Works
• A comprehensive aging- and AVS-aware library characterization
including PVT corners
• Consider hold time violation due to degradation in clock
distribution network
• Re-examine signoff corners with AVS
– Do we still need to signoff at the worst-case corners?
25
Thank you!
26
Implementation of Reference Signoff Flow
• Create new libraries for eachConventional STA
time step is too slow
• Alternative implementation
• Pre-characterize libraries
for different VBTI, Vlib
• Interpolate power,
leakage, and delay using
the pre-characterized
libraries
27
Interpolation Results
• Compare values from actual libraries vs. interpolation
• Interpolation errors are negligible
28
BTI Model
• Use BTI model in [Vattikonda06]
• Fitting parameters are
characterized with published
data in [Zafar06]
NBTI
[Zafar06]
29
BTI Aging and AVS
NBTI and PBTI degrade circuit performance over lifetime
Two variables of aging severity
•
Supply voltage:
» Higher VDD speeds up BTI aging
•
Activity:
» Stressed: |Vth| of transistor increases when it is on
» Relaxed: Part of the |Vth| increment is recovered when
transistor is off
Degradation vs. Operation Modes
Degradation
•
•
Max VDD
Max VDD
Max VDD
Adaptive VDD
Adaptive VDD
Signal
probability
AC
Adaptive VDD
DC
Transistor stress
time
30
Braking the Loop
• VBTI = Vlib = Vfinal to avoid overly pessimistic or optimistic
• Heuristic: estimate VBTI, Vlib with circuit replica
Circuit Replica
Vlib , VBTI
Derated Libraries
Vlib= VBTI ≈ Vfinal
Vfinal
Circuit
31