EE534
VLSI Design System
Summer 2004
Lecture 11:Chapter 7
Layout and
Combinational MOS logic circuits design approaches
Review: Construction of PDN
NMOS devices in
series
implement a NAND function
NMOS devices in
parallel
implement a NOR function
A
B
A
• B
A
B
A
+ B
Review: Equivalent inverter: effective width to length
ratios (model I)
For the
NOR gate
the
effective
width of the drivers transistors
doubles.
That means the effective
aspect ratio is increased.
For the
NAND gate
the
effective
length of the driver transistors
doubles
. That means the effective
aspect ratio is decreased
.
.
Parallel combination
Series combination
Review: CMOS NAND gate and its inverter equivalent
Can we estimate switching
threshold of the NAND gate by
using CMOS inverter
expression for the switching
threshold?
W
N
W
N
W
P
W
P
2W
P
½ W
N
2K
P
K
n
/2
If Kn=Kp, Vth=?
Equivalent inverter:
Worse case delay design consideration
Represent complex gate as inverter for delay
estimation
Use worse

case delays
Example: NAND gate
Worse

case (slowest) pull

up: only 1 PMOS “on”
Pull

down: both NMOS “on”
W
N
W
N
W
P
W
P
W
P
½ W
N
K
P
K
n
/2
Review: CMOS NOR gate: design consideration
Two input
For N inputs
One input
Equivalent inverter
Problems with equivalent inverter method:
Need to take into account load capacitance C
L

Depends on number of transistors connected
to output (junction capacitances)

Even transistors which are off (
not included
in equivalent inverter
) contribute to
capacitance
Need to include capacitance in intermediate
stack nodes
. Worse

case: need to
charge/discharge all nodes
Body effect
of stacked transistors
Transistor Sizing
NAND gate
NOR gate
Transistor sizing: an approach
If MOSFET serially connected in a current path, the
overall current path resistance will be
All serially connected MOSFET can be replace with a
single MOSFET as
If the MOSFET are connected in parallel combination then,
Graph

based dual network
Draw network for PUN or PDN
Circuit nodes are vertexes
Transistors are edges
A
B
F
gnd
A
B
F
Graph

based dual network (2)
To derive dual network:
Create new node in each enclosed region of
graph
Draw new edge intersecting each original edge
Edge is controlled by inverted input
A
B
A
B
A
B
F
Stick Diagrams
Contains no dimensions
Represents relative positions of transistors
In
Out
V
DD
GND
Inverter
A
Out
V
DD
GND
B
NAND2
Dual Graph concept
C
A
B
X = C • (A + B)
B
A
C
i
j
j
V
DD
X
X
i
GND
A
B
C
PUN
PDN
A
B
C
Logic Graph
CMOS gate layout
Goal:
minimum area
Method
Minimize diffusion breaks (reduces capacitance
on internal nodes)
Align transistors with common gates above each
other in layout (minimizes poly length)
Group PMOS and NMOS transistors together
Approach:
Use Euler path method to find ordering of
transistors
in layout
Layout: Euler path method
Goal:
layout without diffusion breaks
Method for finding ordering of transistors
in layout
→
Euler path
Euler path
→
path through a graph that
traverses each edge only once
Find
common Euler path
in pullup and
pulldown graph
This gives the ordering of inputs in the
layout
Complex CMOS logic gates
Complex CMOS logic gates:
Euler path method
Layout: Euler path method
A
B
C
D
E
C
E
D
B
A
Euler path: B→A→C→E→D
B
A
C
E
D
Vcc
Gnd
F
F
1.
Order transistors gates according to
Euler path
2.
Connect Vcc and Gnd
3.
Make other connections according
to circuit diagram
Fan

In Considerations
D
C
B
A
D
C
B
A
C
L
C
3
C
2
C
1
Distributed RC model
(Elmore delay)
T
pHL
=0.69[R
1
C
1
+(R
1
+R
2
)C
2
+(R
1
+R
2
+R
3
)C
3
+(R
1
+R
2
+R
3
+R
4
)C
L
]
t
pHL
= 0.69 R
eqn
(C
1
+2C
2
+3C
3
+4C
L
)
Propagation delay deteriorates rapidly as
a function of fan

in
–
quadratically
in the
worst case
.
t
p
as a Function of Fan

In
t
pH
L
t
pL
H
t
p
(psec)
fan

in
quadratic
function of
fan

in
linear
function of
fan

in
Gates with a fan

in greater than 4 should be avoided.
t
p
t
p
as a Function of Fan

Out
t
p
NOR2
t
p
(psec)
eff. fan

out
All gates
have the
same drive
current.
t
p
NAND2
t
p
INV
Slope is a
function of
“driving
strength”
Influence of Fan

In and Fan

Out on Delay
V
DD
A
B
A
B
C
D
C
D
Fan

out
: Number of Gates
connected to the output
in static CMOS, there are
two gate capacitances per
Fan

out
Fan

in
: Number of independent
variables for the logic function,
which has a
quadratic
effect on
t
p
due to:
resistance increasing
capacitance increasing
t
p
as a Function of Fan

In and Fan

Out
Fan

in:
quadratic
due to increasing resistance and
capacitance
Fan

out: each additional fan

out gate adds
two
gate
capacitances to C
L
t
p
= a
1
FI + a
2
FI
2
+ a
3
FO
Fast Complex Gates: Design Technique 1
Transistor sizing
as long as fan

out capacitance dominates
Progressive sizing
In
N
C
L
C
3
C
2
C
1
In
1
In
2
In
3
M1
M2
M3
MN
Distributed RC line
M1 > M2 > M3 > … > MN
(the MOSFET closest to the
output
should be the
smallest)
Can reduce delay by more
than 20%; decreasing gains
as technology shrinks
Resistance of M
1
(R
1
) N times in the delay
Equation. The resistance of M
2
(R
2
) appears N

1 times etc.
Fast Complex Gates: Design Technique 2
Input re

ordering
when not all inputs arrive at the same time
C
2
C
1
In
1
In
2
In
3
M1
M2
M3
C
L
C
2
C
1
In
3
In
2
In
1
M1
M2
M3
C
L
critical path
critical path
charged
1
0
1
charged
charged
1
delay determined by time to
discharge C
L
, C
1
and C
2
delay determined by time to
discharge C
L
1
1
0
1
charged
discharged
discharged
Sizing and Ordering Effects
D
C
B
A
D
C
B
A
C
L
C
3
C
2
C
1
Progressive sizing
in pull

down
chain gives up to a 23%
improvement.
Input ordering saves 5%
critical path A
–
23%
3
3
3
3
4
4
4
4
4
5
6
7
= 100 fF
Fast Complex

Gate Design Techniques: Stages and Fan

in
Considerations
Fast Complex Gates: Design Technique 3
Alternative logic structures
F = ABCDEFGH
Fast Complex Gates: Design Technique 4
Reducing the voltage swing
linear reduction in delay
also reduces power consumption
t
pHL
= 0.69 (
C
L
V
DD
)/ I
DSATn
)
= 0.69
(
C
L
V
swing
)/ I
DSATn
)
CMOS disadvantages
For N

input CMOS gate, 2N transistors required
Each input connects to an NMOS and PMOS transistor
Large input capacitance: limits fanout
Large fan

in gates: always have long transistor stack
in PUN or PDN
Limits pullup or pulldown delay
Requires very large transistors
Ratioed Logic
Ratioed logic is an attempt to reduce
The number of transistors required to
implant a given logic function, often at the
cost of reduced robustness and extra
power dissipation
Ratioed Logic
Ratioed Logic
Active Loads
Pseudo

NMOS logic
Pseudo

NMOS: replace PMOS PUN with single
“always

on” PMOS device
Some problems as pseudo

NMOS inverter:
V
OL
larger than 0
static power when PDN is on
Advantages
Replace large PMOS stacks with single device
Reduces overall gate size, input capacitance
Especially useful for wide

NOR structures
Pseudo

NMOS
0
Overall functionality of
the gates depend on the
NMOS and PMOS size
Pseudo

NMOS VTC
0.0
0.5
1.0
1.5
2.0
2.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
V
in
[V]
V
o
u
t
[V]
W/L
p
= 4
W/L
p
= 2
W/L
p
= 1
W/L
p
= 0.25
W/L
p
= 0.5
Load Lines of Ratioed Gates
Comments 0
Log in to post a comment