Introduction to
CMOS VLSI
Design
Circuits & Layout
Outline





CMOS Gate Design
Pass Transistors
CMOS Latches & Flip-Flops
Standard Cell Layouts
Stick Diagrams
Circuits and Layout
CMOS VLSI Design
Slide 2
CMOS Gate Design
 Activity:
– Sketch a 4-input CMOS NAND gate
Circuits and Layout
CMOS VLSI Design
Slide 3
CMOS Gate Design
 Activity:
– Sketch a 4-input CMOS NOR gate
A
B
C
D
Y
Circuits and Layout
CMOS VLSI Design
Slide 4
Complementary CMOS
 Complementary CMOS logic gates
– nMOS pull-down network
– pMOS pull-up network
inputs
– a.k.a. static CMOS
Pull-up OFF
Pull-up ON
Pull-down OFF Z (float)
1
Pull-down ON
X (crowbar)
Circuits and Layout
0
CMOS VLSI Design
pMOS
pull-up
network
output
nMOS
pull-down
network
Slide 5
Series and Parallel




nMOS: 1 = ON
pMOS: 0 = ON
Series: both must be ON
Parallel: either can be ON
a
a
0
g1
g2
(a)
(b)
a
g1
g2
(c)
a
g1
g2
b
Circuits and Layout
CMOS VLSI Design
0
1
b
b
OFF
OFF
OFF
ON
a
a
a
a
0
1
1
1
0
1
b
b
b
b
ON
OFF
OFF
OFF
a
a
a
a
0
0
b
1
b
0
b
1
1
0
g2
a
b
a
g1
a
0
0
b
(d)
a
0
1
1
0
1
1
b
b
b
b
OFF
ON
ON
ON
a
a
a
a
0
0
0
1
1
0
1
1
b
b
b
b
ON
ON
ON
OFF
Slide 6
Conduction Complement
 Complementary CMOS gates always produce 0 or 1
 Ex: NAND gate
– Series nMOS: Y=0 when both inputs are 1
– Thus Y=1 when either input is 0
Y
– Requires parallel pMOS
A
B
 Rule of Conduction Complements
– Pull-up network is complement of pull-down
– Parallel -> series, series -> parallel
Circuits and Layout
CMOS VLSI Design
Slide 7
Compound Gates
 Compound gates can do any inverting function
 Ex: Y = (A.B + C.D)’
A
C
A
C
B
D
B
D
(a)
A
(b)
B C
D
(c)
C
D
A
B
(d)
C
D
A
B
A
B
C
D
Y
A
C
B
D
Y
(f)
(e)
Circuits and Layout
CMOS VLSI Design
Slide 8
Example: O3AI
 Y = ((A+B+C).D)’
Circuits and Layout
CMOS VLSI Design
Slide 9
Example: O3AI
 Y = ((A+B+C).D)’
A
B
C
D
Y
D
A
Circuits and Layout
B
C
CMOS VLSI Design
Slide 10
Signal Strength
 Strength of signal
– How close it approximates ideal voltage source
 VDD and GND rails are strongest 1 and 0
 nMOS pass strong 0
– But degraded or weak 1
 pMOS pass strong 1
– But degraded or weak 0
 Thus nMOS are best for pull-down network
Circuits and Layout
CMOS VLSI Design
Slide 11
Pass Transistors
 Transistors can be used as switches
g
s
d
g
s
d
Circuits and Layout
CMOS VLSI Design
Slide 12
Pass Transistors
 Transistors can be used as switches
g=0
g
s
d
s
d
Input g = 1 Output
0
strong 0
g=1
s
d
g=0
g
s
s
g=1
Input
d
d
g=1
s
Circuits and Layout
1
d
CMOS VLSI Design
degraded 1
g=0
0
Output
degraded 0
g=0
strong 1
Slide 13
Transmission Gates
 Pass transistors produce degraded outputs
 Transmission gates pass both 0 and 1 well
Circuits and Layout
CMOS VLSI Design
Slide 14
Transmission Gates
 Pass transistors produce degraded outputs
 Transmission gates pass both 0 and 1 well
Input
g
a
b
gb
a
g = 0, gb = 1
a
b
g = 1, gb = 0
0
strong 0
g = 1, gb = 0
a
b
g = 1, gb = 0
strong 1
1
g
g
b
gb
Circuits and Layout
a
g
b
gb
Output
a
b
gb
CMOS VLSI Design
Slide 15
Tristates
 Tristate buffer produces Z when not enabled
EN
A
0
0
0
1
1
0
1
1
EN
Y
Y
A
EN
Y
A
EN
Circuits and Layout
CMOS VLSI Design
Slide 16
Tristates
 Tristate buffer produces Z when not enabled
EN
A
Y
0
0
Z
0
1
Z
1
0
0
1
1
1
EN
Y
A
EN
Y
A
EN
Circuits and Layout
CMOS VLSI Design
Slide 17
Nonrestoring Tristate
 Transmission gate acts as tristate buffer
– Only two transistors
– But nonrestoring
• Noise on A is passed on to Y
EN
A
Y
EN
Circuits and Layout
CMOS VLSI Design
Slide 18
Tristate Inverter
 Tristate inverter produces restored output
– Violates conduction complement rule
– Because we want a Z output
A
EN
Y
EN
Circuits and Layout
CMOS VLSI Design
Slide 19
Tristate Inverter
 Tristate inverter produces restored output
– Violates conduction complement rule
– Because we want a Z output
A
A
A
EN
Y
Y
Y
EN = 0
Y = 'Z'
EN = 1
Y=A
EN
Circuits and Layout
CMOS VLSI Design
Slide 20
Multiplexers
 2:1 multiplexer chooses between two inputs
S
S
D1
D0
0
X
0
0
X
1
1
0
X
1
1
X
Circuits and Layout
Y
D0
0
Y
D1
CMOS VLSI Design
1
Slide 21
Multiplexers
 2:1 multiplexer chooses between two inputs
S
S
D1
D0
Y
0
X
0
0
0
X
1
1
1
0
X
0
1
1
X
1
Circuits and Layout
CMOS VLSI Design
D0
0
Y
D1
1
Slide 22
Gate-Level Mux Design
 Y  SD1  SD0 (too many transistors)
 How many transistors are needed?
Circuits and Layout
CMOS VLSI Design
Slide 23
Gate-Level Mux Design
 Y  SD1  SD0 (too many transistors)
 How many transistors are needed? 20
D1
S
D0
D1
S
D0
Circuits and Layout
Y
4
2
4
2
4
2
Y
2
CMOS VLSI Design
Slide 24
Transmission Gate Mux
 Nonrestoring mux uses two transmission gates
Circuits and Layout
CMOS VLSI Design
Slide 25
Transmission Gate Mux
 Nonrestoring mux uses two transmission gates
– Only 4 transistors
S
D0
Y
S
D1
S
Circuits and Layout
CMOS VLSI Design
Slide 26
Inverting Mux
 Inverting multiplexer
– Use compound AOI22
– Or pair of tristate inverters
– Essentially the same thing
 Noninverting multiplexer adds an inverter
D0
S
S
D1
D0
D1
S
S
Y
S
S
S
Y
S
D0
Y
S
D1
Circuits and Layout
CMOS VLSI Design
0
1
Slide 27
4:1 Multiplexer
 4:1 mux chooses one of 4 inputs using two selects
Circuits and Layout
CMOS VLSI Design
Slide 28
4:1 Multiplexer
 4:1 mux chooses one of 4 inputs using two selects
– Two levels of 2:1 muxes
S1S0 S1S0 S1S0 S1S0
– Or four tristates
D0
S0
D0
0
D1
1
S1
D1
0
Y
Y
D2
0
D3
1
1
D2
D3
Circuits and Layout
CMOS VLSI Design
Slide 29
D Latch
 When CLK = 1, latch is transparent
– D flows through to Q like a buffer
 When CLK = 0, the latch is opaque
– Q holds its old value independent of D
 a.k.a. transparent latch or level-sensitive latch
D
Latch
CLK
Circuits and Layout
CLK
D
Q
Q
CMOS VLSI Design
Slide 30
D Latch Design
 Multiplexer chooses D or old Q
CLK
D
1
CLK
Q
Q
Q
D
Q
0
CLK
CLK
CLK
Circuits and Layout
CMOS VLSI Design
Slide 31
D Latch Operation
Q
D
CLK = 1
Q
Q
D
Q
CLK = 0
CLK
D
Q
Circuits and Layout
CMOS VLSI Design
Slide 32
D Flip-flop
 When CLK rises, D is copied to Q
 At all other times, Q holds its value
 a.k.a. positive edge-triggered flip-flop, master-slave
flip-flop
CLK
CLK
D
Flop
D
Q
Q
Circuits and Layout
CMOS VLSI Design
Slide 33
D Flip-flop Design
 Built from master and slave D latches
CLK
CLK
CLK
QM
D
CLK
QM
Latch
D
Latch
CLK
CLK
CLK
CLK
Q
CLK
Circuits and Layout
Q
CMOS VLSI Design
CLK
Slide 34
D Flip-flop Operation
D
QM
Q
CLK = 0
D
QM
Q
CLK = 1
CLK
D
Q
Circuits and Layout
CMOS VLSI Design
Slide 35
Race Condition
 Back-to-back flops can malfunction from clock skew
– Second flip-flop fires late
– Sees first flip-flop change and captures its result
– Called hold-time failure or race condition
CLK1
CLK2
Q1
Flop
D
Flop
CLK1
CLK2
Q2
Q1
Q2
Circuits and Layout
CMOS VLSI Design
Slide 36
Nonoverlapping Clocks
 Nonoverlapping clocks can prevent races
– As long as nonoverlap exceeds clock skew
 We will use them in this class for safe design
– Industry manages skew more carefully instead
2
1
QM
D
2
2
2
Q
1
1
1
1
2
Circuits and Layout
CMOS VLSI Design
Slide 37
Gate Layout
 Layout can be very time consuming
– Design gates to fit together nicely
– Build a library of standard cells
 Standard cell design methodology
– VDD and GND should abut (standard height)
– Adjacent gates should satisfy design rules
– nMOS at bottom and pMOS at top
– All gates include well and substrate contacts
Circuits and Layout
CMOS VLSI Design
Slide 38
Example: Inverter
Circuits and Layout
CMOS VLSI Design
Slide 39
Example: NAND3





Horizontal N-diffusion and p-diffusion strips
Vertical polysilicon gates
Metal1 VDD rail at top
Metal1 GND rail at bottom
32 l by 40 l
Circuits and Layout
CMOS VLSI Design
Slide 40
Stick Diagrams
 Stick diagrams help plan layout quickly
– Need not be to scale
– Draw with color pencils or dry-erase markers
Circuits and Layout
CMOS VLSI Design
Slide 41
Wiring Tracks
 A wiring track is the space required for a wire
– 4 l width, 4 l spacing from neighbor = 8 l pitch
 Transistors also consume one wiring track
Circuits and Layout
CMOS VLSI Design
Slide 42
Well spacing
 Wells must surround transistors by 6 l
– Implies 12 l between opposite transistor flavors
– Leaves room for one wire track
Circuits and Layout
CMOS VLSI Design
Slide 43
Area Estimation
 Estimate area by counting wiring tracks
– Multiply by 8 to express in l
Circuits and Layout
CMOS VLSI Design
Slide 44
Example: O3AI
 Sketch a stick diagram for O3AI and estimate area
– Y = ((A+B+C).D)’
Circuits and Layout
CMOS VLSI Design
Slide 45
Example: O3AI
 Sketch a stick diagram for O3AI and estimate area
– Y = ((A+B+C).D)’
Circuits and Layout
CMOS VLSI Design
Slide 46
Example: O3AI
 Sketch a stick diagram for O3AI and estimate area
– Y = ((A+B+C).D)’
Circuits and Layout
CMOS VLSI Design
Slide 47