VLSIVLSIVLSIVLSIProf. Vojin G. OklobdzijaProf. Vojin G. Oklobdzija

References References (used for creation of the presentation (used for creation of the presentation material):material):

[1] Mead, Conway, “[1] Mead, Conway, “Introduction to VLSI SystemsIntroduction to VLSI Systems”, Addison ”, Addison Wesley Publishing.Wesley Publishing.

[2] Glasser, Dobberpuhl, “[2] Glasser, Dobberpuhl, “The Design and Analysis of VLSI The Design and Analysis of VLSI CircuitsCircuits”, Addison Wesley Publishing.”, Addison Wesley Publishing.

[3] Weste, Eshraghian, “[3] Weste, Eshraghian, “Principles of CMOS VLSI DesignPrinciples of CMOS VLSI Design”, Addison ”, Addison Wesley Publishing.Wesley Publishing.

[4] Shoji, “[4] Shoji, “CMOS Digital Circuits Technology”CMOS Digital Circuits Technology”, Prentice Hall., Prentice Hall.

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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Historical OverviewHistorical Overview• nMOS era: 1970-85• Pass-transistor design• CMOS existed early but took off 1985 on• Domino CMOS, 1982

– NORA– DCVSL

• CPL, DPL– DCVS-PG– SRPL– LEAP

• SOI-CMOS

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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n-MOS Design Era 1970-85n-MOS Design Era 1970-85

LSI started with nMOS:• pass-transistor design experience:

- Flourished at the beginning of the nMOS era(popularized by Mead-Conway book)- Allows high density layout and compact

design style- Fast: outperforming gate based design- Low in power

• Drawbacks:– Not compatible with existing design tools– Exhibiting testability and reliability

problems

Review of CMOSReview of CMOSReview of CMOSReview of CMOS

Prof. Vojin G. OklobdzijaProf. Vojin G. Oklobdzija

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS BasicsCMOS Basics

A B

Vdd

0

1B

A

10

1 1

1 0

)( A B+

( )BA

Function F and its Dual

Karnaugh Mapof Function F

FF

Vss

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS BasicsCMOS Basics

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS BasicsCMOS Basics

B A

C

D

A

B C D

1

1 0 0 0

0000

1001

100

( )BA +C+Dcovering zeroes :

( )+D A CB

covering ones :

A

B

D

C

F

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS BasicsCMOS BasicsA complex path example:

VDD

A B

C DE

A

B

C

DE

Output

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS BasicsCMOS BasicsMore complex blocks are realizable in CMOSPrimitive gates:

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS Deficiencies:CMOS Deficiencies: Muli-Input NOR Muli-Input NOR function function in CMOS is slowin CMOS is slowVarious remedies:

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS Deficiencies and CMOS Deficiencies and RemediesRemedies

A

B

B

A B + BA

A

BA B + BA

XOR Faster, one-levelrealizations of XOR

function

(a) (b)

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS Deficiencies and CMOS Deficiencies and RemediesRemedies

AB + BA

A

B

XNOR

AB + BA

Faster, one-levelrealizations of XOR

function

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS BasicCMOS Basic

Inverter Transfer Inverter Transfer function:function:

Logic voltage levels are Logic voltage levels are VVOHOH and V and VOLOL

and Vand VILIL and V and VIHIH

The inverter transfer The inverter transfer function liefunction lie

within the shaded regionwithin the shaded region

VDD

VOH

VIH

Vout

VIL

VOL

0VOL VIL

Vin

VIH VOH VDD

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS Basic: Inverter CMOS Basic: Inverter CharacteristicCharacteristicp “ON”

n “OFF’’p ”OFF”n ”ON”

VDD

VDD

0.5VDD

0

0 0.5VDD VDD + VtpVtn

AB

C

DE

BOTH p & n “ON”

LeakageLeakageCurrentsCurrents

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS Basic: Inverter CMOS Basic: Inverter CharacteristicCharacteristic

+ VDD

t

t

0

+ VDD

0.9 VDD

0.1 VDD

VDD

Vin(t) 1T

2TV0(t)

LC

VDD

t

dt

ft rt

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS Basic: Inverter CMOS Basic: Inverter CharacteristicCharacteristic

Transistors during the Transistors during the transitiontransition

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS Basic: Inverter CMOS Basic: Inverter SwitchingSwitching

CL

t = 0

VDDp-DEVICE

n-DEVICE

SATURATION :

VO

Ic

CL

VDDp-DEVICE

n-DEVICE

LINEAR :

VO

Ic

CL

t = 0

VDDp-DEVICE

n-DEVICE

SATURATION

VO

Ic

p-DEVICE

n-DEVICE

LINEAR

Rc

CL

VDD

VO

Ic

Rc

inDDO VVV inDDO VVV 0

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS Basic: Power CMOS Basic: Power • During the static During the static

state there is no state there is no currentcurrent

• Current is only Current is only present during present during transistion:transistion:

- Short circuit current Short circuit current (crow-bar current)(crow-bar current)

- Charging and Charging and discharging of the discharging of the output capacitoroutput capacitor

- Leakage CurrentLeakage Current

t

t

t

VDD

VDD

VDD

0

0

0

V

V

I

tp

tftr

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS Basic: Power CMOS Basic: Power

This is an E=mc2 of low-power designThere are three ways to control power:

- Reducing Power-Supply Voltage (most effective !!)

- Reducing the switching activity k (various ways)

- Reducing CL (technology scaling etc.)

- Reducing the required frequency of operation (?)

PCMOS=kCLV2DDfo

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS Basic: Delay CMOS Basic: Delay • Which one of the three designs is the fastest ?• How can we find this out without simulation ?

CL

Case 1

0a

7a

(a)

CL

Case 27a

4a

3a

0a

(b)

CL

Case 3

0a1a

3a2a

5a6a

4a

7a

(c)

Learn about Logical Effort !

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS Basic: DelayCMOS Basic: Delay

Cin1 Cin2Cout

DischargeId

Charge:Ic

DischargeId

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS Basic: DelayCMOS Basic: Delay

Delay can be approximated with:

RND7Cin1+RNORCin2+RND2Cout

CoutCin1 Cin2

Id Id

Ic

RND7

RNOR

RND2

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS Basic: DelayCMOS Basic: DelayDelay of a signal path in CMOS logic is

dependent on:• Fan-in of a gate

– Represented as a resistance of the pull-up/down transistor path of the gate

• Fan-out of a gate– Represented as a capacitive load at the

output

• Number of CMOS blocks in the path.• Wire delay connecting various blocks.

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS Basic: DelayCMOS Basic: DelayDelay of a signal path in CMOS logic can be

reduced by:• Making the transistors larger in order to

minimize resistance of a pull-up/down path in the gate

• Making the transistors smaller in order to minimize the capacitive load of each gate

• Reducing the number of CMOS blocks in the path.

• Bringing the blocks closer and/or choosing the less wire intensive topology.– Note that these requirements are often contradictory

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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CMOS Basic: DelayCMOS Basic: Delay• How to estimate delay and critical

timing in CMOS circuits ?• How to determine the proper transistor

sizing in order to make a compromise with contradicting requirements ?

• How to choose the right circuit topology ?

The Answer:“Logical Effort”

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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Pass-Transistor DesignPass-Transistor Design

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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Pass-Transistor DesignPass-Transistor DesignAnother way of looking at Karnaugh Map: AND function

0 10

1

0 1

B

A

1 0

A

A

B

B

F

F

B B

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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Pass-Transistor DesignPass-Transistor Design

Two-variable function

A

A

X

Y

F

X Y F0 0 00 1 A1 01 1 10 B AB01 B1B 0B 1B 0 A+BB 1BB BBB B B

B

B

B

B

B

A

BA

BABA

BABA

BA

BA

BA

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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Pass-Transistor DesignPass-Transistor Design“Threshold Voltage Drop” problem:

A

B=Vdd

B

Fmax = Vdd-Vth

A=Vdd

Vth+

-

Cout

Vdd

Vdd

Fmax = Vdd-Vth

Cout

Vth+

-Vth

+

-

Vdd

(a) (b)

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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Pass-Transistor DesignPass-Transistor Design

Solving the “Threshold Voltage Drop” problem in CMOS:

A=0V

In=VddFmax= Vdd

A=Vdd

Vth+

-

Cout

Vdd

Vdd

Cin

Vth+

-

(a) (b)

+

-Vdd

ON

+Vdd

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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Pass-Transistor DesignPass-Transistor Design

Function Generator

A A B B

P0

P1

P2

P3

F(A,B)

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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Pass-Transistor DesignPass-Transistor DesignFull 1-bit

AdderA

A

A

S

A

A

S

OC

OC

B B C C

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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Pass-Transistor DesignPass-Transistor Design

Compact ALU

Example (IBM PC/RT)Circ. 1984

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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Control Lines OutputControl

A - inputs B - inputs

Odd Even Odd Even

Operation K1 K2 Qn A A B B Odd Even

Arithmetic

A+B Add 0 0 0 0 1 1 0 0 1 1 0 0 1

A+B+1 0 0 1 0 1 1 0 0 1 1 0 0 1

A-B Subtract 0 0 1 0 1 1 0 1 0 0 1 0 1

B-A Subtract 0 0 1 1 0 0 1 0 1 1 0 0 1

B+1 Increment 0 0 1 1 1 0 0 0 1 1 0 0 1

+1 2s compl 0 0 1 1 1 0 0 1 0 0 1 0 1

A+1 Increment 0 0 1 0 1 1 0 1 1 0 0 0 1

+1 2s compl 0 0 1 1 0 0 1 1 1 0 0 0 1

Logical

1 1 1 0 0 0 0 0 0 0 0 0 0 0

B 1 1 0 0 0 0 0 0 1 0 1 0 0

1 1 0 0 0 0 0 1 0 1 0 0 0

1 1 0 0 1 0 1 0 1 0 1 0 0

1 1 0 0 1 0 1 1 0 1 0 0 0

1 1 0 1 0 1 0 0 0 0 0 0 0

1 1 0 1 0 1 0 0 1 0 1 0 0

0 1 1 0 0 0 0 0 0 0 0 0 1 1

1 1 0 1 0 1 0 1 0 1 0 1 1

A 1 1 0 1 0 1 0 0 0 0 0 1 1

1 1 0 0 1 0 1 0 1 0 1 1 1

1 1 0 0 1 0 1 1 0 1 0 1 1

1 1 0 1 0 1 0 0 1 0 1 1 1

0 1 0 0 1 0 1 0 1 0 1 1 1

0 1 0 0 1 0 1 1 0 1 0 1 1

0 1 0 1 0 1 0 0 1 0 1 1 1

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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Pass-Transistor Pass-Transistor DesignDesign

Compact ALU Example (IBM PC/RT)

A B

K2 A

B

K1

A B K2

IC

OC

OCK2

A

B

f

HV

HV

Carry Generator

Function Generator

Fall 2004 Prof. V.G. Oklobdzija: High-Performance System Design

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Using Pass-Transistor Design to Using Pass-Transistor Design to Speed-up AdditionSpeed-up Addition