# design pittfalls and fixes

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Introduction to

CMOS VLSIDesign

Lecture 16:Circuit Pitfalls

David Harris

Harvey Mudd College

Spring 2004

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Outline

Circuit Pitfalls

Detective puzzle

Given circuit and symptom, diagnose causeand

recommend solution

All these pitfalls have caused failures in real chips

Noise Budgets

Reliability

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Circuit

2:1 multiplexer

Symptom

Mux works when selected

D is 0 but not 1.

Or fails at low VDD.

Or fails in SF corner.

XD0Y

D1

S

S

Principle:

Solution:

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Circuit

2:1 multiplexer

Symptom

Mux works when selected

D is 0 but not 1.

Or fails at low VDD.

Or fails in SF corner.

XD0Y

D1

S

S

Principle: Threshold drop

X never rises above VDD-Vt

Vtis raised by the body effect

The threshold drop is most serious as Vtbecomes a greater

fraction of VDD.

Solution:

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Circuit

Latch

Symptom

Set f= 0

Eventually Q

spontaneously flips to 1

Principle:

Solution:

D Q

f

f

X

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Circuit

Latch

Symptom

Set f= 0

Eventually Q

spontaneously flips to 1

Principle: Leakage

X is a dynamic node holding value as charge on the node

Eventually subthreshold leakage may disturb charge

Solution:

D Q

f

f

X

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Circuit

Latch

Symptom

Set f= 0

Eventually Q

spontaneously flips to 1

Principle: Leakage

X is a dynamic node holding value as charge on the node

Eventually subthreshold leakage may disturb charge

Solution: Stabilize node with feedback

Or periodically refresh node (requires fast clock,

not practical processes with big leakage)

D Q

f

f

X

f

f

Q

D X

f

f

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Circuit

Domino AND gate

Symptom

Precharge gate (Y=0)

Then evaluate

Eventually Y

spontaneously flips to 1

Principle:

Solution:

1

0Y

fX

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Circuit

Domino AND gate

Symptom

Precharge gate (Y=0)

Then evaluate

Eventually Y

spontaneously flips to 1

Principle: Leakage

X is a dynamic node holding value as charge on the node

Eventually subthreshold leakage may disturb charge

Solution:

1

0Y

fX

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Circuit

Domino AND gate

Symptom

Precharge gate (Y=0)

Then evaluate

Eventually Y

spontaneously flips to 1

Principle: Leakage

X is a dynamic node holding value as charge on the node

Eventually subthreshold leakage may disturb charge

Solution: Keeper

1

0Y

fX

1

0Y

f

X

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Circuit

Pseudo-nMOS OR

Symptom

When only one input is

true, Y = 0.

Perhaps only happens

in SF corner.

Principle:

Solution:

A B

YX

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Circuit

Pseudo-nMOS OR

Symptom

When only one input is

true, Y = 0.

Perhaps only happens

in SF corner.

Principle: Ratio Failure

nMOS and pMOS fight each other.

If the pMOS is too strong, nMOS cannot pull X low enough.

Solution:

A B

YX

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Circuit

Pseudo-nMOS OR

Symptom

When only one input is

true, Y = 0.

Perhaps only happens

in SF corner.

Principle: Ratio Failure

nMOS and pMOS fight each other.

If the pMOS is too strong, nMOS cannot pull X low enough.

Solution: Check that ratio is satisfied in all corners

A B

YX

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Circuit

Latch

Symptom

Q stuck at 1.

May only happen for

certain latches where

input is driven by a

small gate located far

away. Principle:

Solutions:

QD

f

f weak

X

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Circuit

Latch

Symptom

Q stuck at 1.

May only happen for

certain latches where

input is driven by a

small gate located far

away. Principle: Ratio Failure(again)

Series resistance of D driver, wire

resistance, and tgate must be much

less than weak feedback inverter.

Solutions:

QD

f

f weak

X

QD

f

f weakstronger

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Circuit

Latch

Symptom

Q stuck at 1.

May only happen for

certain latches where

input is driven by a

small gate located far

away. Principle: Ratio Failure (again)

Series resistance of D driver, wire

resistance, and tgate must be much

less than weak feedback inverter.

Solutions: Check relative strengths

Avoid unbuffered diffusion inputs where driver is unknown

QD

f

f weak

X

QD

f

f weakstronger

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Circuit

Domino AND gate

Symptom

Precharge gate while

A = B = 0, so Z = 0

Set f= 1

A rises Z is observed to

sometimes rise Principle:

Solutions:

B

A

Yf

X

Z

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Circuit

Domino AND gate

Symptom

Precharge gate while

A = B = 0, so Z = 0

Set f= 1

A rises Z is observed to

sometimes rise Principle: Charge Sharing

If X was low, it shares charge with Y

Solutions:

B

A

Yf

X

Z

BA

Yf

XC

x

CY

Z

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Circuit

Domino AND gate

Symptom

Precharge gate while

A = B = 0, so Z = 0

Set f= 1

A rises Z is observed to

sometimes rise Principle: Charge Sharing

If X was low, it shares charge with Y

Solutions: Limit charge sharing

Safe if CY>> CX

Or precharge node X too

B

A

Yf

X

Z

BA

Yf

XC

x

CY

Z

Y

x Y DD

x Y

CV V V

C C

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Circuit

Dynamic gate + latch

Symptom

Precharge gate while

transmission gate latch

is opaque

Evaluate

When latch becomes

transparent, X falls Principle:

Solution:

0

Xf

Y

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Circuit

Dynamic gate + latch

Symptom

Precharge gate while

transmission gate latch

is opaque

Evaluate

When latch becomes

transparent, X falls Principle: Charge Sharing

If Y was low, it shares charge with X

Solution:

0

Xf

Y

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Circuit

Dynamic gate + latch

Symptom

Precharge gate while

transmission gate latch

is opaque

Evaluate

When latch becomes

transparent, X falls Principle: Charge Sharing

If Y was low, it shares charge with X

Solution: Buffer dynamic nodesbefore

driving transmission gate

0

Xf

Y

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Circuit

Latch

Symptom

Q changeswhile latch is

opaque

Especially if D comes

from a far-away driver

Principle:

Solution:

QD

weakV

DD

VDD

GND

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Circuit

Latch

Symptom

Q changes while latch is

opaque

Especially if D comes

from a far-away driver

Principle: Diffusion Input Noise Sensitivity

If D < -Vt, transmission gate turns on

Most likely because of power supply noise or coupling on D

Solution:

QD

weakV

DD

VDD

GND

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