Introduction to
CMOS VLSI
Design
Lecture 11:
Adders
David Harris
Harvey Mudd College
Spring 2004
Outline
q
q
q
q
q
q
q
Single-bit Addition
Carry-Ripple Adder
Carry-Skip Adder
Carry-Lookahead Adder
Carry-Select Adder
Carry-Increment Adder
Prefix Adder
11: Adders
CMOS VLSI Design
Slide 2
Single-Bit Addition
A
Half Adder
S=
B
Full Adder
S=
Cout
Cout =
S
Cout =
C
0
0
0
0
0
1
0
0
1
1
0
0
1
0
1
1
0
1
1
1
0
0
1
0
1
1
1
0
1
1
1
0
11: Adders
Cout
S
CMOS VLSI Design
C
S
B
B
B
Cout
A
A
A
Cout S
Slide 3
Single-Bit Addition
A
Half Adder
S = A⊕B
Cout = Ag B
B
Full Adder
S = A⊕ B ⊕C
Cout
S
A
B
Cout
Cout = MAJ ( A , B ,C )
C
S
A
B
Cout
S
A
B
C
Cout S
0
0
0
0
0
0
0
0
0
0
1
0
1
0
0
1
0
1
1
0
0
1
0
1
0
0
1
1
1
1
0
0
1
1
1
0
1
0
0
0
1
1
0
1
1
0
1
1
0
1
0
1
1
1
1
1
11: Adders
CMOS VLSI Design
Slide 4
PGK
q For a full adder, define what happens to carries
– Generate: Cout = 1 independent of C
• G=
– Propagate: Cout = C
• P=
– Kill: Cout = 0 independent of C
• K=
11: Adders
CMOS VLSI Design
Slide 5
PGK
q For a full adder, define what happens to carries
– Generate: Cout = 1 independent of C
• G=A•B
– Propagate: Cout = C
• P=A⊕B
– Kill: Cout = 0 independent of C
• K = ~A • ~B
11: Adders
CMOS VLSI Design
Slide 6
Full Adder Design I
q Brute force implementation from eqns
S = A⊕ B ⊕C
Cout = MAJ ( A, B , C )
A
A
A
Cout
C
B
C
C
C
B
A
C
B
B
A
A
11: Adders
C
B
S
MAJ
A
B
C
S
B
A
B
A
B
C
B
B
B
C
A
C
A
B
Cout
B
A
CMOS VLSI Design
Slide 7
Full Adder Design II
q Factor S in terms of Cout
S = ABC + (A + B + C)(~Cout)
q Critical path is usually C to Cout in ripple adder
MINORITY
A
B
C
Cout
S
S
Cout
11: Adders
CMOS VLSI Design
Slide 8
Layout
q Clever layout circumvents usual line of diffusion
– Use wide transistors on critical path
– Eliminate output inverters
11: Adders
CMOS VLSI Design
Slide 9
Full Adder Design III
q Complementary Pass Transistor Logic (CPL)
– Slightly faster, but more area
B
B
B
C
B
C
B
C
B
C
A
S
B
C
B
C
B
C
B
C
Cout
A
A
S
A
Cout
B
B
11: Adders
CMOS VLSI Design
Slide 10
Full Adder Design IV
q Dual-rail domino
– Very fast, but large and power hungry
– Used in very fast multipliers
φ
C_h
A_h
B_h
A_h
C_l
B_h
A_l
C_h
B_h
A_h
C_l
B_l
Cout _l
A_l
B_l
B_l
φ
S_l
11: Adders
φ
Cout _h
S_h
C_h
B_h
A_l
CMOS VLSI Design
Slide 11
Carry Propagate Adders
q N-bit adder called CPA
– Each sum bit depends on all previous carries
– How do we compute all these carries quickly?
AN...1 BN...1
Cout
Cout
+
SN...1
11: Adders
Cin
Cin
00000
1111
+0000
1111
CMOS VLSI Design
Cout
11111
1111
+0000
0000
Cin
carries
A4...1
B4...1
S4...1
Slide 12
Carry-Ripple Adder
q Simplest design: cascade full adders
– Critical path goes from Cin to Cout
– Design full adder to have fast carry delay
A4
B4
Cout
B3
C3
S4
11: Adders
A3
A2
B2
C2
S3
A1
B1
Cin
C1
S2
CMOS VLSI Design
S1
Slide 13
Inversions
q Critical path passes through majority gate
– Built from minority + inverter
– Eliminate inverter and use inverting full adder
A4
B4
Cout
B3
C3
S4
11: Adders
A3
A2
B2
C2
S3
A1
B1
Cin
C1
S2
CMOS VLSI Design
S1
Slide 14
Generate / Propagate
q Equations often factored into G and P
q Generate and propagate for groups spanning i:j
Gi: j =
Pi: j =
q Base case
Gi:i ≡ Gi =
Pi:i ≡ Pi =
0 GCP
0:00:0 in
G0:0 ≡ G0 =
P0:0 ≡ P0 =
q Sum:
Si =
11: Adders
CMOS VLSI Design
Slide 15
Generate / Propagate
q Equations often factored into G and P
q Generate and propagate for groups spanning i:j
Gi: j = Gi:k + Pi:k g Gk −1: j
Pi: j = Pi:k g Pk −1: j
q Base case
Gi:i ≡ Gi = Ai g Bi
Pi:i ≡ Pi = Ai ⊕ Bi
0 GCP
0:00:0 in
G0:0 ≡ G0 = Cin
P0:0 ≡ P0 = 0
q Sum:
Si = Pi ⊕ Gi −1:0
11: Adders
CMOS VLSI Design
Slide 16
PG Logic
A4
B4
A3
B3
A2
B2
A1
B1
Cin
1: Bitwise PG logic
G4
P4
G3
P3
G2
P2
G1
P1
G0
P0
2: Group PG logic
G3:0
G2:0
G1:0
G0:0
C3
C2
C1
C0
3: Sum logic
C4
Cout
11: Adders
S4
S3
S2
S1
CMOS VLSI Design
Slide 17
Carry-Ripple Revisited
Gi:0 = Gi + Pi g Gi −1:0
A4
B4
G4
P4
A3
B3
G3
P3
A2
B2
G2
P2
A1
B1
G1
P1
Cin
G0
G 3:0
G2:0
G1:0
G0:0
C3
C2
C1
C0
P0
C4
Cout
11: Adders
S4
S3
S2
S1
CMOS VLSI Design
Slide 18
Carry-Ripple PG Diagram
Bit Position
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
tripple =
Delay
15:0 14:0 13:0 12:0 11:0 10:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
11: Adders
CMOS VLSI Design
Slide 19
Carry-Ripple PG Diagram
Bit Position
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
tripple = t pg + ( N − 1)t AO + txor
Delay
15:0 14:0 13:0 12:0 11:0 10:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
11: Adders
CMOS VLSI Design
Slide 20
PG Diagram Notation
Black cell
i:k
Gray cell
k-1:j
i:k
i:j
Gi:k
Pi:k
Gk-1:j
Pk-1:j
11: Adders
Buffer
k-1:j
i:j
i:j
i:j
Gi:j
Gi:k
Pi:k
Gk-1:j
Pi:j
CMOS VLSI Design
Gi:j
Gi:j
Gi:j
Pi:j
Pi:j
Slide 21
Carry-Skip Adder
q Carry-ripple is slow through all N stages
q Carry-skip allows carry to skip over groups of n bits
– Decision based on n-bit propagate signal
Cout
A16:13 B16:13
A12:9 B12:9
A8:5 B8:5
A4:1
P16:13
P12:9
P8:5
P4:1
1
0
C12
+
S16:13
11: Adders
1
0
C8
+
1
0
S12:9
CMOS VLSI Design
C4
+
S8:5
B4:1
1
0
+
Cin
S4:1
Slide 22
Carry-Skip PG Diagram
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
16:0 15:0 14:0 13:0 12:0 11:0 10:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
For k n-bit groups (N = nk)
tskip =
11: Adders
CMOS VLSI Design
Slide 23
Carry-Skip PG Diagram
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
16:0 15:0 14:0 13:0 12:0 11:0 10:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
For k n-bit groups (N = nk)
tskip = t pg + 2 ( n − 1) + (k − 1) t AO + txor
11: Adders
CMOS VLSI Design
Slide 24
Variable Group Size
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
16:0 15:0 14:0 13:0 12:0 11:0 10:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
Delay grows as O(sqrt(N))
11: Adders
CMOS VLSI Design
Slide 25
Carry-Lookahead Adder
q Carry-lookahead adder computes Gi:0 for many bits
in parallel.
q Uses higher-valency cells with more than two inputs.
A16:13 B16:13
Cout
G16:13
P16:13
+
S16:13
11: Adders
C12
A12:9 B12:9
G12:9
P12:9
A8:5 B8:5
C8
+
S12:9
CMOS VLSI Design
A4:1
C4
G8:5
P8:5
B4:1
G4:1
P4:1
+
+
S8:5
S4:1
Cin
Slide 26
CLA PG Diagram
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
16:0 15:0 14:0 13:0 12:0 11:0 10:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
11: Adders
CMOS VLSI Design
Slide 27
Higher-Valency Cells
i:k k-1:l l-1:m m-1:j
i:j
Gi:k
Pi:k
Gk-1:l
Pk-1:l
Gl-1:m
Pl-1:m
Gm-1:j
Gi:j
Pi:j
Pm-1:j
11: Adders
CMOS VLSI Design
Slide 28
Carry-Select Adder
q Trick for critical paths dependent on late input X
– Precompute two possible outputs for X = 0, 1
– Select proper output when X arrives
q Carry-select adder precomputes n-bit sums
– For both possible carries into n-bit group
A16:13 B16:13
0
+
Cout
B4:1
1
C4
+
Cin
0
CMOS VLSI Design
A4:1
0
+
1
0
S12:9
B8:5
+
C8
1
+
1
0
1
S16:13
A8:5
0
+
C12
1
+
11: Adders
A12:9 B12:9
S8:5
S4:1
Slide 29
Carry-Increment Adder
q Factor initial PG and final XOR out of carry-select
15
14
13
12
11
10
13:12
8
7
6
9:8
14:12
15:12
9
4
3
2
1
0
5:4
10:8
11:8
5
6:4
7:4
15:0 14:0 13:0 12:0 11:0 10:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
tincrement =
11: Adders
CMOS VLSI Design
Slide 30
Carry-Increment Adder
q Factor initial PG and final XOR out of carry-select
15
14
13
12
11
10
13:12
8
7
6
9:8
14:12
15:12
9
4
3
2
1
0
5:4
10:8
11:8
5
6:4
7:4
15:0 14:0 13:0 12:0 11:0 10:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
tincrement = t pg +  ( n − 1) + (k − 1) t AO + txor
11: Adders
CMOS VLSI Design
Slide 31
Variable Group Size
q Also buffer
noncritical
signals
15
14
13
12
11
10
9
12:11
8
7
8:7
13:11
5
4
5:4
9:7
14:11
6
3
2
1
0
3:2
6:4
10:7
15:11
15:0 14:0 13:0 12:0 11:0 10:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
15
14
13
12
11
10
9
12:11
7
6
8:7
13:11
14:11
8
9:7
10:7
5
5:4
6:4
4
3
3:2
2
1
0
1:0
3:0
6:0
15:11
15:0 14:0 13:0 12:0 11:0 10:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
11: Adders
CMOS VLSI Design
Slide 32
Tree Adder
q If lookahead is good, lookahead across lookahead!
– Recursive lookahead gives O(log N) delay
q Many variations on tree adders
11: Adders
CMOS VLSI Design
Slide 33
Brent-Kung
15 14 13
15:14
13:12
15:12
12 11 10
11:10
9
9:8
11:8
8
7
6
7:6
5
5:4
7:4
15:8
4
3
3:2
2
1
0
1:0
3:0
7:0
11:0
13:0
9:0
5:0
15:0 14:0 13:0 12:0 11:0 10:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
11: Adders
CMOS VLSI Design
Slide 34
Sklansky
15 14 13 12 11 10
15:14
13:12
11:10
15:12 14:12
15:8
14:8
11:8 10:8
13:8
9
9:8
8
7
6
7:6
7:4
5
5:4
6:4
4
3
2
3:2
3:0
1
0
1:0
2:0
12:8
15:0 14:0 13:0 12:0 11:0 10:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
11: Adders
CMOS VLSI Design
Slide 35
Kogge-Stone
15 14 13 12 11 10
9
8
7
6
5
4
3
2
15:14 14:13 13:12 12:11 11:10 10:9
9:8
8:7
7:6
6:5
5:4
4:3
3:2
2:1
15:12 14:11 13:10
3:0
2:0
15:8
14:7
13:6
12:9
11:8 10:7
9:6
8:5
7:4
6:3
5:2
4:1
12:5
11:4 10:3
9:2
8:1
7:0
6:0
5:0
4:0
1
0
1:0
15:0 14:0 13:0 12:0 11:0 10:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
11: Adders
CMOS VLSI Design
Slide 36
Tree Adder Taxonomy
q Ideal N-bit tree adder would have
– L = log N logic levels
– Fanout never exceeding 2
– No more than one wiring track between levels
q Describe adder with 3-D taxonomy (l, f, t)
– Logic levels:
L+l
– Fanout:
2f + 1
– Wiring tracks:
2t
q Known tree adders sit on plane defined by
l + f + t = L-1
11: Adders
CMOS VLSI Design
Slide 37
Tree Adder Taxonomy
l (Logic Levels)
3 (7)
f (Fanout)
2 (6)
3 (9)
1 (5)
2 (5)
1 (3)
0 (2)
0 (4)
0 (1)
1 (2)
2 (4)
3 (8)
t (Wire Tracks)
11: Adders
CMOS VLSI Design
Slide 38
Tree Adder Taxonomy
l (Logic Levels)
3 (7)
Brent-Kung
f (Fanout)
2 (6)
Sklansky
3 (9)
1 (5)
2 (5)
1 (3)
0 (2)
0 (4)
0 (1)
1 (2)
2 (4)
Kogge-Stone
3 (8)
t (Wire Tracks)
11: Adders
CMOS VLSI Design
Slide 39
Han-Carlson
15 14 13 12 11 10
9
8
7
6
5
4
3
15:14
13:12
11:10
9:8
7:6
5:4
3:2
15:12
13:10
11:8
9:6
7:4
5:2
3:0
15:8
13:6
11:4
9:2
7:0
5:0
2
1
0
1:0
15:0 14:0 13:0 12:0 11:0 10:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
11: Adders
CMOS VLSI Design
Slide 40
Knowles [2, 1, 1, 1]
15 14 13 12 11 10
9
8
7
6
5
4
3
2
15:14 14:13 13:12 12:11 11:10 10:9
9:8
8:7
7:6
6:5
5:4
4:3
3:2
2:1
15:12 14:11 13:10
3:0
2:0
15:8
14:7
13:6
12:9
11:8 10:7
9:6
8:5
7:4
6:3
5:2
4:1
12:5
11:4 10:3
9:2
8:1
7:0
6:0
5:0
4:0
1
0
1:0
15:0 14:0 13:0 12:0 11:0 10:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
11: Adders
CMOS VLSI Design
Slide 41
Ladner-Fischer
15 14 13 12 11 10
15:14
13:12
15:12
11:10
9
9:8
11:8
15:8
13:8
15:8
13:0
8
7
6
7:6
5:4
7:4
7:0
11:0
5
4
3
3:2
2
1
0
1:0
3:0
5:0
9:0
15:0 14:0 13:0 12:0 11:0 10:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
11: Adders
CMOS VLSI Design
Slide 42
Taxonomy Revisited
(f)Ladner-Fischer
(b) Sklansky
15
15 14 13 12 1 1 10
9
8
7
6
5
4
3
2
1
14
15:14
15:14
13:12
11:10
9:8
7:6
5:4
3:2
13
12
15:8
14:8
11:8 10:8
13:8
7:4
6:4
3:0
BrentKung
LadnerFischer
15:014:013:012:011:010:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
LadnerFischer
f (Fanout)
13:12
11:10
4
3
2
15:14 14:13 13:12 12:11 11:10 10:9
9:8
8:7
7:6
6:5
5:4
4:3
3:2
2:1
15:12 14:11 13:10
9:6
8:5
7:4
6:3
5:2
4:1
3:0
2:0
12:9
11:8 10:7
1
15:8
13:8
15:8
13:0
15:14
0 (2)
0
1:0
0 (4)
0 (1)
13:12
11:10
15:12
14:7
13:6
12:5
11:4 10:3
9:2
8:1
7:0
6:0
5:0
Knowles
[4,2,1,1]
HanCarlson
4:0
3:2
0
1:0
0:0
1:0
5:0
12:0 11:0 10:0
9:0
8:0
7:0
6:0
5:0
4:0
7
6
5
4
3
2
3:0
2:0
9
8
1
0
9:8
7:6
5:4
3:2
7:4
1:0
3:0
7:0
9:0
5:0
(d) Han-Carlson
9
8
7
6
5
4
3
2
10:9
9:8
8:7
7:6
6:5
5:4
4:3
3:2
2:1
12:9
11:8
10:7
9:6
8:5
7:4
6:3
5:2
4:1
3:0
2:0
12:5
11:4
10:3
9:2
8:1
7:0
6:0
5:0
4:0
1
12 11
10
9
8
7
6
5
4
3
2
1
0
0
1:0
Kogge3 (8)
Stone
15:14
13:12
11:10
9:8
7:6
5:4
3:2
15:12
13:10
11:8
9:6
7:4
5:2
3:0
15:8
13:6
11:4
9:2
7:0
5:0
1:0
t (Wire Tracks)
15:014:013:0 12:011:0 10:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
15:0 14:0 13:0 12:0 11:010:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
11: Adders
1
3:0
15:0 14:0 13:0 12:0 11:0 10:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
15 14 13
15 14 13 12 11 10
13:6
5:4
7:0
15:8
HanCarlson
(c) Kogge-Stone
14:7
7:6
1 (2)
2 (4)
15:8
2
11:0
Knowles
[2,1,1,1]
15:12 14:11 13:10
3
New
(1,1,1)
15:014:013:012:011:0 10:0 9:0 8:0 7:0 6:0 5:0 4:0 3:0 2:0 1:0 0:0
15:14 14:13 13:12 12:11 11:10
4
9:0
11:8
13:0
15:8
5
7:4
11:0
15 14 13 12 11 10
1 (3)
5
9:8
11:8
15:0 14:0 13:0
2 (5)
6
6
1 (5)
(e) Knowles [2,1,1,1]
7
7
(a) Brent-Kung
2 (6)
3 (9)
8
8
3 (7)
Sklansky
9
9
l (Logic Levels)
2:0
12:8
15 14 13 1 2 11 10
10
1:0
15:12
15:12 14:12
11
0
CMOS VLSI Design
Slide 43
Summary
Adder architectures offer area / power / delay tradeoffs.
Choose the best one for your application.
Architecture
Classification Logic
Levels
Max
Tracks
Fanout
Cells
Carry-Ripple
N-1
1
1
N
Carry-Skip n=4
N/4 + 5
2
1
1.25N
Carry-Inc. n=4
N/4 + 2
4
1
2N
1
2N
Brent-Kung
(L-1, 0, 0)
2log2N – 1 2
Sklansky
(0, L-1, 0)
log2N
N/2 + 1 1
0.5 Nlog2N
Kogge-Stone
(0, 0, L-1)
log2N
2
Nlog2N
11: Adders
CMOS VLSI Design
N/2
Slide 44