vdd and gnd power analysis

8/17/2019 VDD and GND Power Analysis

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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


2/21


Announcements

Homework #8 Due Today

Project 2 Introduction Monday


3/21


Today’s Lecture

Power Rail Failure Modes

Rail Analysis with EPS/Voltage Storm


4/21


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


5/21


A Typical Off-Chip I/O Plan


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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


7/21


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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Problem 1: IR-Drop

Also called "Voltage Droop"

Change in local supply voltage

Can cause intermittenttiming failures

Should static or dynamic analysis be used?


9/21


Voltage Storm Static Analysis

Source: Cadence Encounter Power System

User Guide (wrapper for Voltage Storm and

other tools)


10/21


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


11/21


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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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


13/21


Today’s Lecture

Power Rail Failure Modes

Rail Analysis with EPS/Voltage Storm


14/21


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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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








worst-case IR-drop of

0.336 mV

number of nodes

in extracted RC network


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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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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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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








worst-case IR-drop of

0.336 mV

number of instances


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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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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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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

vdd and gnd power analysis

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