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Slide 1© W. Rhett Davis NC State University ECE 720 Spring 2013
ECE 720 – ESL & Physical Design
Lecture 25:
Power Rail AnalysisSpring 2013
W. Rhett Davis
NC State University
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Slide 2© W. Rhett Davis NC State University ECE 720 Spring 2013
Announcements
Homework #8 Due Today
Project 2 Introduction Monday
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Slide 3© W. Rhett Davis NC State University ECE 720 Spring 2013
Today’s Lecture
Power Rail Failure Modes
Rail Analysis with EPS/Voltage Storm
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Slide 4© W. Rhett Davis NC State University ECE 720 Spring 2013
The Power Rail Problem
Every time a gate switches, current flows
from the VDD rail or into the VSS/GND rail
Where does this current come from?
Vin Vout
CL
Vdd
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Slide 5© W. Rhett Davis NC State University ECE 720 Spring 2013
A Typical Off-Chip I/O Plan
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Slide 6© W. Rhett Davis NC State University ECE 720 Spring 2013
Simplified Power Rail Model
J(s) is a time-varying current source to model aggregatecurrent of many gates
Cd models wire capacitance, transistor source
capacitances, and added decoupling capacitance ("decap")
Quarter
power pad
4Lp
Vdd
Quarter
ground pad
d C x y
( ) J s x y
Rs Rs
Rs Rs
Rs Rs Rs
Rs
4Lp
Source:
Huang, et al,
Elec. Comp.
& Tech. Conf.
2007
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Slide 7© W. Rhett Davis NC State University ECE 720 Spring 2013
Simplified Power Rail Model
Inductors model bond wires Typically very difficult to analyze by hand, due to large
number of resistors
Large matrix solvers or coarse analytical models needed
Quarter
power pad
4Lp
Vdd
Quarter
ground pad
d C x y
( ) J s x y
Rs Rs
Rs Rs
Rs Rs Rs
Rs
4Lp
Source:
Huang, et al,
Elec. Comp.
& Tech. Conf.
2007
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Slide 8© W. Rhett Davis NC State University ECE 720 Spring 2013
Problem 1: IR-Drop
Also called "Voltage Droop"
Change in local supply voltage
Can cause intermittenttiming failures
Should static or dynamic analysis be used?
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Slide 9© W. Rhett Davis NC State University ECE 720 Spring 2013
Voltage Storm Static Analysis
Source: Cadence Encounter Power System
User Guide (wrapper for Voltage Storm and
other tools)
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Slide 10© W. Rhett Davis NC State University ECE 720 Spring 2013
Problem 2: Ldi/dt Noise
Also called "Simultaneous Switching Noise (SSN)"
or "Ground Bounce"
Bond-wire inductance causes
global ringing in the rail, which
can significantly increasethe change in rail voltage
Dynamic analysis needed
in Cadence Voltage Stormto see these effects
Cadence claims this is necessary for sub 130nm techs
Source: Gary Charles
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Slide 11© W. Rhett Davis NC State University ECE 720 Spring 2013
Cell-Based Analysis w/ VCD Files
Experiments show reasonably
good prediction of SSN using
currents from cell-based powerestimates from VCD files
Source: van Heijningen, Badaroglu.et al
, JSSC 2002
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Slide 12© W. Rhett Davis NC State University ECE 720 Spring 2013
Problem 3: Electromigration
Mean Time to Failure
(Black's Equation: IEEE
Trans. Elec. Dev. 1969)
»
j – current density» Ea – activation energy
» T – temperature
» k – Boltzmann's const.
Static or Dynamic Analysis?
kT Ea
n
m
e j
AT MTTF
Caused by collisions of
electrons with metalatoms
Important to control
current density
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Slide 13© W. Rhett Davis NC State University ECE 720 Spring 2013
Today’s Lecture
Power Rail Failure Modes
Rail Analysis with EPS/Voltage Storm
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Slide 14© W. Rhett Davis NC State University ECE 720 Spring 2013
Power Pad Location Files
Needed as input to specify where the power comes in
from off-chip
VDD.ppl
VSS.ppl
VDD VSS
VDD 6.0 100.0 metal6
VSS 188.0 100.0 metal6
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Slide 15© W. Rhett Davis NC State University ECE 720 Spring 2013
VDD Rail IR-Drop Analysis
Results for ir (linear filters):
================================
*
* Data filtering results for IR drop:* Overall data minimum: 0.949664V
* Overall data average: 0.949806V
* Overall data maximum: 0.95V
*
* Filter 1: 39791 of 39791 data values fell into this filter.
* filtered data range: 0.949664V - 0.95V
* filtered data average: 0.949806V
* 366 values were in range 1: 0.949664V - 0.949706V
* 9060 values were in range 2: 0.949706V - 0.949748V
* 17298 values were in range 3: 0.949748V - 0.94979V
* 2837 values were in range 4: 0.94979V - 0.949832V
* 542 values were in range 5: 0.949832V - 0.949874V
* 1405 values were in range 6: 0.949874V - 0.949916V
* 4748 values were in range 7: 0.949916V - 0.949958V
* 3535 values were in range 8: 0.949958V - 0.95V
worst-case IR-drop of
0.336 mV
number of nodes
in extracted RC network
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Slide 16© W. Rhett Davis NC State University ECE 720 Spring 2013
IR Drop Analysis Plots
Plots are "zoomed out" so that rails cover layout
completely
Why are the areas of greatest IR drop toward
the center?
VSS
rail
VDDrail
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Slide 17© W. Rhett Davis NC State University ECE 720 Spring 2013
Useful Debugging Plots
Instance Total Power Plot (ip) – 100-1000 nW
– 10-100 nW – 1-10 nW
» show highest power towards
center and bottom
Instance Switching Power
Density (W/cm2) (ipd_s)
» shows more switching towards
the center of chip
» May be the cause of large IR-
drop towards center
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Slide 18© W. Rhett Davis NC State University ECE 720 Spring 2013
VDD Rail Effective Resistance
Results for reff:
===============
** Data filtering results for effective resistance:
* Overall data minimum: 16.7789Ohm
* Overall data average: 44.6578Ohm
* Overall data maximum: 91.6936Ohm
*
* Filter 1: 9194 of 9194 data values fell into this filter.* filtered data range: 16.7789Ohm - 91.6936Ohm
* filtered data average: 44.6578Ohm
* 158 values were in range 1: 82.3293Ohm - 91.6936Ohm
* 577 values were in range 2: 72.9649Ohm - 82.3293Ohm
* 1179 values were in range 3: 63.6006Ohm - 72.9649Ohm
* 977 values were in range 4: 54.2362Ohm - 63.6006Ohm
* 1410 values were in range 5: 44.8719Ohm - 54.2362Ohm
* 1480 values were in range 6: 35.5075Ohm - 44.8719Ohm
* 1511 values were in range 7: 26.1432Ohm - 35.5075Ohm
* 1902 values were in range 8: 16.7789Ohm - 26.1432Ohm
worst-case IR-drop of
0.336 mV
number of instances
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Slide 19© W. Rhett Davis NC State University ECE 720 Spring 2013
Effective Resistance Plots
Effective Resistance of rail, from instance to pad
Instance power not considered
Helps to identify locations where decap can help
» Decap can help up to a point, but adding additional
decap doesn’t help if resistance is too high.
VSS
rail
VDDrail
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Slide 20© W. Rhett Davis NC State University ECE 720 Spring 2013
Other Useful Plots
Electromigration Risk (er)
» Gaussian distribution assumed
around MTTF
» Electromigration Models
not generated
» Results appear to show areasof large current density
Tap Current (er)» Current at each instance's
supply pin
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Slide 21© W. Rhett Davis NC State University ECE 720 Spring 2013
Notes
Dynamic Analysis not supported by this flow
Switching activity saved as a TCF (Toggle
Count File – Cadence's version of SAIF)
» TCF file used instead of VCD if it exists
» allows discarding of VCD
No Decaps exist in our standard cell library
» Power pad position, rail number/width are the only
things that we can change, currently