Copyright Agrawal, 2007 ELEC6270 Fall 07, Lecture 12 1 ELEC 5270/6270 Fall 2007 Low-Power Design of...

Copyright Agrawal, 2007Copyright Agrawal, 2007 ELEC6270 Fall 07, Lecture 12ELEC6270 Fall 07, Lecture 12 11

ELEC 5270/6270 Fall 2007ELEC 5270/6270 Fall 2007Low-Power Design of Electronic CircuitsLow-Power Design of Electronic Circuits

Pass Transistor Logic: A Low Power Pass Transistor Logic: A Low Power Logic FamilyLogic Family

Vishwani D. AgrawalVishwani D. AgrawalJames J. Danaher ProfessorJames J. Danaher Professor

Dept. of Electrical and Computer EngineeringDept. of Electrical and Computer EngineeringAuburn University, Auburn, AL 36849Auburn University, Auburn, AL 36849

[email protected]://www.eng.auburn.edu/~vagrawal/COURSE/E6270_Fall07/course.html


Low-Power Logic StylesLow-Power Logic Styles

Pass transistor logicPass transistor logicDynamic logicDynamic logicDomino logicDomino logicAdiabatic and charge recovery logicAdiabatic and charge recovery logicAsynchronous logicAsynchronous logicLogic restructuringLogic restructuring


Pass Transistor Logic (PTL)Pass Transistor Logic (PTL)

Requires fewer transistorsRequires fewer transistorsSmaller areaSmaller areaReduced capacitanceReduced capacitanceReduced energy and powerReduced energy and power


CMOS AND GateCMOS AND Gate

F = ABA

B

A

B

F = AB


Pass Transistor AND GatePass Transistor AND Gate

A

B

F = AB

0

Need 4 transistors instead of 6 for CMOS AND gate.


CMOS OR GateCMOS OR Gate

F = A + BA

B

A

B

F = A + B


Pass Transistor OR GatePass Transistor OR Gate

1

B

F = A + B

A

Need 4 transistors instead of 6 for CMOS OR gate.


Reduced Voltage SwingReduced Voltage Swing

VDD = 2.5V

IN

OUT

n transistors, W/L = 0.5μ/0.25μ p transistors, W/L = 1.5μ/0.25μ

Vx = VDD – Vtn


Spice SimulationSpice Simulation

Time, ns

0 0.5 1.0 1.5 2.0

3.0

2.0

1.0

0.0

Vo

ltage

, VIN

OUT

Vx

J. M. Rabaey, A. Chandrakasan and B. Nokolić, Digital IntegratedCircuits, Upper Saddle River, New Jersey: Pearson Education, 2003.


Voltage Transfer Characteristic Voltage Transfer Characteristic (VTC) of AND Gate(VTC) of AND Gate

A

B

F = AB

0

n transistors, W/L = 0.5μ/0.25μ p transistors, W/L = 1.5μ/0.25μ


VTC: Spice SimulationVTC: Spice Simulation

Vin, V

0 0.5 1.0 1.5 2.0 2.5

3.0

2.0

1.0

0.0

F, V

B = VDDA = 0 → VDD

A = VDD, B = 0 → VDDA = B = 0 → VDD

VDD – Vtn


EnergyEnergy

VDD = 2.5V

0 → VDD

CL

T TE0→1 = ∫ P(t) dt = VDD ∫ i(t) dt

0 0

VDD-Vtn

= VDD ∫ CL dVout = CL VDD (VDD – Vtn) < CL VDD2

0

If this voltage is insufficient for turning the pMOSTransistor in inverter off, leakage power will be consumed.

Vout

i(t)


Energy: PTL vs. CMOSEnergy: PTL vs. CMOS

PTL consumes less dynamic power than PTL consumes less dynamic power than static CMOS Logic.static CMOS Logic.

PTL leakage may be higher when output is PTL leakage may be higher when output is low, because the reduced voltage level low, because the reduced voltage level may be insufficient to turn the PMOS may be insufficient to turn the PMOS transistor in the inverter off.transistor in the inverter off.


Ways to Reduce LeakageWays to Reduce Leakage

Level restorationLevel restorationMultiple-threshold transistorsMultiple-threshold transistorsTransmission-gate logicTransmission-gate logic


Level RestorationLevel Restoration

A=1

B=1

CL

Vout

VDD Level restorer

Level restorer device should be weaker than the nMOS pass transistor. Otherwise, VDD → 0 transition at Vout will be impossible.

0


Multiple-Threshold TransistorsMultiple-Threshold TransistorsUse zero-threshold pass-transistors.Use zero-threshold pass-transistors.Use high-threshold transistors in all other Use high-threshold transistors in all other

gates.gates.This can cause leakage through multiple This can cause leakage through multiple

gates.gates.


Leakage Through Zero-Threshold Leakage Through Zero-Threshold TransistorsTransistors

0

1

1

0

Zero or low-threshold transistors

Leakage current path


Transmission-Gate LogicTransmission-Gate Logic Provides both power and ground levels.Provides both power and ground levels. Good design, except needs more transistors. Good design, except needs more transistors.

A

S

B

S’A’ + SB’

Inverting multiplexer


Transmission-Gate XORTransmission-Gate XOR

A AB’+A’B

B


A Logic LibraryA Logic LibraryCellCell

Number of transistorsNumber of transistors

CMOSCMOS TGLTGL

OR02OR02 66

MUX21MUX21 1212 66

AND02AND02 66

XOR2XOR2 1212 66

AND03AND03 88

AOI32AOI32 1010

OAI21OAI21 66

OAI32OAI32 1010

AO21AO21 88

NOR04NOR04 88

OR03OR03 88

NOR02_2xNOR02_2x 44

OAI221OAI221 1010

NAND02_2xNAND02_2x 44

AOI321AOI321 1212

INV02INV02 22

DFFRDFFR 3434


Synthesis of PTLSynthesis of PTL

AA BB CC ZZ

00 00 00 00

00 00 11 00

00 11 00 00

00 11 11 11

11 00 00 00

11 00 11 11

11 11 00 11

11 11 11 11

Shannon’s expansion:

Z = AB + BC + AC = A(B+BC+C) + A’(BC) = A(B+C) + A’BC = A[B+B’C] + A’[BC]

A 1 0

1 0 1 0B

C C1 0

Z


Pass-Transistor CellPass-Transistor Cell


Synthesis of Z = A’B + B’C + A’C’Synthesis of Z = A’B + B’C + A’C’

B

0A

C

Z

C = 1, Z = A’B + B’B = 1, Z = A’B = 0, Z = 1

C = 0, Z = A’


Synthesis of Z = A’ + BC’ + B’CSynthesis of Z = A’ + BC’ + B’C0

B

B’

A

A’

C C’

Z

A = 1, Z = BC’ + B’CB = 1, Z = C’B = 0, Z = C

A = 0, Z = 1


Synthesis of Z = AB’C’ + A’B’CSynthesis of Z = AB’C’ + A’B’C

A’

A

B’

B

C’ C 1

Z

A = 1, Z = B’C’B = 1, Z = 0B = 0, Z = C’

A = 0, Z = B’CB = 1, Z = 0B = 0, Z = C


CPL: Complementary Pass-CPL: Complementary Pass-Transistor LogicTransistor Logic

Every signal and its complement is Every signal and its complement is generated.generated.

Gates are static, because the output is Gates are static, because the output is connected to either VDD or GND.connected to either VDD or GND.

Design is modular; same cell can produce Design is modular; same cell can produce various gates by simply permuting the various gates by simply permuting the input signals.input signals.

Also called differential pass-transistor logic Also called differential pass-transistor logic (DPL)(DPL)


A CPL CellA CPL Cell


CPL Cell Used As AND/NANDCPL Cell Used As AND/NAND

A

B

A’

B’

B B’

Z = AB

Z’ = (AB)’


CPL Cell Used As OR/NORCPL Cell Used As OR/NOR

A

B

A’

B’

B’ B

Z = A + B

Z’ = (A + B)’


CPL Cell Used As XOR/XNORCPL Cell Used As XOR/XNOR

A

A’

A’

A

B’ B

Z = AB’ + A’B

Z’ = AB + A’B’


CPL vs. CMOSCPL vs. CMOS

CPL requires fewer transistors.CPL requires fewer transistors.Useful for modular (array) circuits like Useful for modular (array) circuits like

adders, multipliers, barrel shifter, etc.adders, multipliers, barrel shifter, etc.CPL operation can be faster and energy CPL operation can be faster and energy

efficient.efficient.Following example is taken from:Following example is taken from:

M. E. Elrabaa, I. S. Abu-Khater, and M. I. M. E. Elrabaa, I. S. Abu-Khater, and M. I. Elmasry, Elmasry, Advanced Low-Power Digital Circuit Advanced Low-Power Digital Circuit TechniquesTechniques, Springer, 1997, Chapter 2., Springer, 1997, Chapter 2.


Example: 4-Bit Carry Select AdderExample: 4-Bit Carry Select Adder

A_1 B_1Adder cell

S1’ S0’ C0’ C0 C1’ C1

A_2 B_2Adder cell

S1’ S0’ C0’ C0 C1’ C1

M M M

M M M

MMMM

S_1

C’_0

C_0 S_2

A_3 B_3Adder cell

S1’ S0’ C0’ C0 C1’ C1

A_4 B_4Adder cell

S1’ S0’ C0’ C0 C1’ C1

M M M

M M M

MMMM

S_3

C’_2

S_4

C_4

C’_4


CMOS Carry-Select Adder CellCMOS Carry-Select Adder Cell

S1’ S0’ C0’ C0 C1’ C1

Ai Bi


CPL Adder CellCPL Adder Cell

Ai Bi

S1’ S0’ C0’ C0 C1’ C1


CPL Multiplexer CellCPL Multiplexer Cell

Ci

Ci’

in1 in2

in1 in2

Ci

Ci’

M

out


32-Bit Adders in 0.832-Bit Adders in 0.8μμ, 3.3V, 3.3V

Type of designType of design Type of logicType of logicEnergyEnergy

μμW/MHzW/MHz

DelayDelay

nsns

Minimum Minimum transistor sizetransistor size

CMOSCMOS 90.090.0 11.011.0

CPLCPL 65.065.0 10.010.0

Transistor sizing Transistor sizing for delay for delay

optimizationoptimization

CMOSCMOS 93.093.0 10.510.5

CPLCPL 72.072.0 7.57.5


ReferencesReferencesG. R. Cho and T. Chen, “On the Impact of G. R. Cho and T. Chen, “On the Impact of

Technology Scaling on Mixed PTL/Static Technology Scaling on Mixed PTL/Static Logic,” Logic,” Proc. IEEE Int. Conf. Computer Proc. IEEE Int. Conf. Computer DesignDesign, 2002., 2002.

R. Zimmermann and W. Fichtner, “Low-R. Zimmermann and W. Fichtner, “Low-Power Logic Styles: CMOS Versus Pass-Power Logic Styles: CMOS Versus Pass-Transistor Logic,” Transistor Logic,” IEEE J. Solid State IEEE J. Solid State CircuitsCircuits, vol. 32, no. 7, pp. 1079-1090, July , vol. 32, no. 7, pp. 1079-1090, July 1997.1997.

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