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DESIGN OF LOW POWER ALU USING AREA EFFICIENT CARRY SELECT ADDER
Presented By

A. Ramakrishna Reddy
ES & VLSI Under the Guidance of

Mr.

K. Naresh.

Assistant Professor

1

OBJECTIVE To design Low Power ALU using Efficient Carry Select Adder (CSLA)

2

ALU
• Arithmetic operations – Addition, Subtraction. • Logical operations- AND, OR, XOR, XNOR, Increment and Decrement • Adder is the main block in ALU.

3

ONE BIT ALU

4

8-BIT ALU USING RIPPLE CARRY ADDERS
s2

s1 0 0 1 1 0 0 1

s0 0 1 0 1 0 1 0

operation AND XOR XNOR OR DECREMENT ADDTION SUBTRACTION

0 0 0 0 1 1 1

1

1

1

INCREMENT

5

REGULAR 16-BIT SQRT CSLA
A[15:11] B[15:11] A[10:7] B[10:7] A[6:4] B[6:4] 0 0 A[3:2] B[3:2] A[1:0] B[1:0] 0

15:11 RCA 15:11 RCA MUX 12:6

0 1

10 :7 RCA 10:7 RCA
1

6:4 RCA 6:4 RCA
1

3:2 RCA 3:2 RCA

1:0 RCA

Cin

1

CY

CY

MUX 10:5

C Y

MUX 8:4

CY MUX

6:3

C OUT

S[15:11]

S[10:7]
group4

S[6:4]
group3

S[3:2]
group2

S[1:0]
group1

group5

6

GROUP-2 IN REGULAR SQRT CSLA
• .

A[3:2] B[3:2]

2-bit RCA

0

1

2-bit RCA

Carry to the next group

MUX

Carry from previous group

S3 S2
7

EXISTING TECHNIQUE
• 2nd level RCA block is replaced by BEC Block

1

n-bit RCA

(n+1)-bit BEC

8

GROUP-2 IN EXISTING SQRT CSLA
A[3:2] B[3:2]

2-bit RCA

0

3-bit BEC

Carry to the next group

MUX

Carry from previous group

S3 S2

9

OVERHEADS IN 3-BIT BEC

• Number of NOT gates=1 • Number of XOR gates=2 • Number of AND gates=1

Total number of transistors required = 32
10

PROPOSED TECHNIQUE

n-bit BEC

n-bit SHM

SHM=SPECIAL HARDWARE WITH MULTIPLEXERS

11

16-BIT SQRT CSLA USING PROPOSED TECHNIQUE
A[15:11] B[15:11]
.

A[10:7]

B[10:7]

A[6:4] B[6:4]

A[3:2] B[3:2]

A[1:0] B[1:0]

0

0

0

15:11 RCA

10 :7 RCA

6:4 RCA

6-bit SHM

5-bit SHM

4-biT SHM

3:2 RCA 3-bit SHM

0

1:0 RCA

Cin

CY

MUX 12:6

CY

MUX 10:5

CY MUX

CY

8:4

MUX 6:3

C OUT

S[15:11]

S[10:7] group4

S[6:4] group3

S[3:2] group2

SUM[1:0] group1

group5

12

3-BIT SHM

X0= b0 X1= X0.b1+X0.b1 X2=(X1+b1).b2+X1.b1.b2

13

COMPARISION OF 3-BIT BEC AND SHM

Type of logic 3-bit BEC

Gates 2 -XOR 1-AND 1-NOT 3-MUX 3-NOT

Number of transistors 24 6 2 18 6

Total number of transistors 32

3-bit SHM

24

14

CRITICAL PATH DELAY FROM DSCH TOOL

Critical path delay
15

NUMBER OF TRANSISTORS AND POWER FROM MICROWIND TOOL

NUMBER OF TRANSISTORS

POWER DISSIPATION
16

COMPARISION OF SECOND LEVEL 2BIT RCA, 3-BIT BEC AND 3-BIT SHM

Logic for Second level RCA using CMOS BEC using CMOS SHM using CMOS

Number Critical path of delay(ns) transistors 56 32 24 1.900 1.200 2.350

Area ( um2) 1342 781 486

Power dissipation (uw) static dynam total ic 6.706 42.565 49.271 3.269 25.746 29.015 3.100 22.843 25.94 3

17

COMPARISION OF EXISTING AND PROPOSED TECHNIQUES FOR SECOND BLOCK
Design Type Number of transistor s 106 Critical path delay (ns) Area (µm2) Power dissipation (µw) static dynamic total 3.240 3465 21.005 106 127.0 05 118.7 62

RCA-BECMUX(CMOS ) RCA-SHMMUX(CMOS )

98

3.770

2996

20.138

98.624

18

GDI TECHNIQUE

19

FULL ADDER USING GDI TECHNIQUE
10 TRANSISTOR FA

8-TRANSISTOR FA
20

8-BIT ALU USING EFFICIENT CARRY SELECT ADDER

21

COMPARISION OF EXISTING AND PROPOSED ALU FOR 10-TRANSISTOR FA
MODEL(ALU) NUMBER OF TRANSISTO RS Critical path delay(ns) Area(µm) Power( mw)

8BIT ALU USING 10 TRANSISTOR RCA 8BIT ALU USING 10 TRANSISTOR CSLA

448

3.195

12384

0.204

508

1.865

24682

0.205

22

COMPARISION OF EXISTING AND PROPOSED ALU FOR 8-TRANSISTOR FA
MODEL(ALU) NUMBER OF TRANSISTO RS Critical path delay(ns) Area(µm) Power( mw)

8BIT ALU USING 8 TRANSISTOR RCA 8BIT ALU USING 8 TRANSISTOR CSLA

432

3.745

11832

0.221

494

2.070

20988

0.262

23

CONCLUSION
• Speed is increased 41.6% in case 10 transistor full adder. • 44.7% in case of 8-transistor full adder. • Satisfactory level of power consumption and propagation delay can be achieved using the proposed technology without the need to purchase new technology libraries, which may lead to design cost reduction. • The proposed work can be extended and carried further with an aim of increasing the number of bits and approach to new technology such as 0.08, 0.06 micron meter technology.

24

THANK YOU

25

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