Digital Integrated Circuits A Design Perspective

1
Digital Integrated CircuitsA Design Perspective
Jan M. Rabaey Anantha Chandrakasan Borivoje
Nikolic
Designing CombinationalLogic Circuits
November 2002.
2
Combinational vs. Sequential Logic
Combinational
Sequential
Output
(
)
f
In, Previous In
Output
(
)
f
In
3
Static CMOS Circuit
4
Static Complementary CMOS
VDD
In1
PMOS only
In2
PUN

InN
F(In1,In2,InN)
In1
In2
PDN

NMOS only
InN
PUN and PDN are dual logic networks
5
NMOS Transistors in Series/Parallel Connection
Transistors can be thought as a switch controlled
by its gate signal NMOS switch closes when switch
control input is high
6
PMOS Transistors in Series/Parallel Connection
7
Threshold Drops
VDD
VDD
PUN
S
D
VDD
D
S
0 ? VDD
0 ? VDD - VTn
VGS
VDD ? 0
PDN
VDD ? VTp
VGS
S
D
VDD
S
D
8
Complementary CMOS Logic Style
9
Example Gate NAND
10
Example Gate NOR
11
Complex CMOS Gate
OUT D A (B C)
A
D
B
C
12
Constructing a Complex Gate
13
Cell Design

• Standard Cells
• General purpose logic
• Can be synthesized
• Same height, varying width
• Datapath Cells
• For regular, structured designs (arithmetic)
• Includes some wiring in the cell
• Fixed height and width

14
Standard Cell Layout Methodology 1980s
Routing channel
VDD
signals
GND
15
Standard Cell Layout Methodology 1990s
Mirrored Cell
No Routing channels
VDD
VDD
M2
M3
GND
GND
Mirrored Cell
16
Standard Cells
N Well
Cell height 12 metal tracks Metal track is
approx. 3? 3? Pitch repetitive distance
between objects Cell height is 12 pitch
Out
In
2?
Rails 10?
GND
Cell boundary
17
Standard Cells
With silicided diffusion
With minimaldiffusionrouting
Out
In
Out
In
GND
GND
18
Standard Cells
2-input NAND gate
A
B
Out
GND
19
Two Versions of C (A B)
C
A
B
A
B
C
VDD
VDD
X
X
GND
GND
20
Multi-Fingered Transistors
One finger
Two fingers (folded)
Less diffusion capacitance
21
Properties of Complementary CMOS Gates Snapshot
High noise margins

V
and
V
are at
V
and
GND
, respectively.
OH
OL
DD
No static power consumption

There never exists a direct path between
V
and
DD
V
(
GND
) in steady-state mode
.
SS
Comparable rise and fall times
(under appropriate sizing conditions)
22
CMOS Properties

• Full rail-to-rail swing high noise margins
• Logic levels not dependent upon the relative
  device sizes ratioless
• Always a path to Vdd or Gnd in steady state low
  output impedance
• Extremely high input resistance nearly zero
  steady-state input current
• No direct path steady state between power and
  ground no static power dissipation
• Propagation delay function of load capacitance
  and resistance of transistors

23
Switch Delay Model
Req
A
A
NOR2
INV
NAND2
24
Input Pattern Effects on Delay

• Delay is dependent on the pattern of inputs
• Low to high transition
• both inputs go low
• delay is 0.69 Rp/2 CL
• one input goes low
• delay is 0.69 Rp CL
• High to low transition
• both inputs go high
• delay is 0.69 2Rn CL

Rn
B
25
Delay Dependence on Input Patterns
Input Data Pattern Delay (psec)
AB0?1 67
A1, B0?1 64
A 0?1, B1 61
AB1?0 45
A1, B1?0 80
A 1?0, B1 81
AB1?0
A1 ?0, B1
A1, B1?0
Voltage V
time ps
NMOS 0.5?m/0.25 ?m PMOS 0.75?m/0.25 ?m CL
100 fF
26
Transistor Sizing

4 4
2 2
27
Transistor Sizing a Complex CMOS Gate
B
8
6
4
3
C
8
6
4
6
OUT D A (B C)
A
2
D
1
B
C
2
2
28
Fan-In Considerations
A
Distributed RC model
(Elmore delay) tpHL 0.69 Reqn(C12C23C34CL)
Propagation delay deteriorates rapidly as a
function of fan-in quadratically in the worst
case.
B
C
D
29
tp as a Function of Fan-In
Gates with a fan-in greater than 4 should be
avoided.
tp (psec)
tpLH
fan-in
30
tp as a Function of Fan-Out
All gates have the same drive current.
tpNOR2
tpNAND2
tpINV
tp (psec)
Slope is a function of driving strength
eff. fan-out
31
tp 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 CL
• tp a1FI a2FI2 a3FO

32
Fast Complex GatesDesign Technique 1

• Transistor sizing
• as long as fan-out capacitance dominates
• Progressive sizing

Distributed RC line M1 gt M2 gt M3 gt gt MN (the
fet closest to the output is the smallest)
InN
MN
In3
M3
In2
M2
Can reduce delay by more than 20 decreasing
gains as technology shrinks
In1
M1
33
Fast Complex GatesDesign Technique 2

• Transistor ordering

critical path
critical path
0?1
charged
charged
In1
1
In3
M3
M3
1
In2
1
In2
M2
discharged
M2
charged
1
In3
discharged
In1
M1
charged
M1
0?1
delay determined by time to discharge CL, C1 and
C2
delay determined by time to discharge CL
34
Fast Complex GatesDesign Technique 3

• Alternative logic structures

F ABCDEFGH
35
Fast Complex GatesDesign Technique 4

• Isolating fan-in from fan-out using buffer
  insertion

36
Fast Complex GatesDesign Technique 5

• Reducing the voltage swing
• linear reduction in delay
• also reduces power consumption
• But the following gate is much slower!
• Or requires use of sense amplifiers on the
  receiving end to restore the signal level (memory
  design)

tpHL 0.69 (3/4 (CL VDD)/ IDSATn )
0.69 (3/4 (CL Vswing)/ IDSATn )
37
Pass-TransistorLogic
38
Pass-Transistor Logic
39
Example AND Gate
40
NMOS-Only Logic
3.0
In
Out
2.0
V
x

e
g
a
t
l
o
V
1.0
0.0
0
0.5
1
1.5
2
Time ns
41
NMOS-only Switch
V
C
2.5 V
C
2.5

M
2
A
2.5 V
B
A
2.5 V
M
n
B
M
C
1
L
does not pull up to 2.5V, but 2.5V -
V
V
TN
B
Threshold voltage loss causes
static power consumption
NMOS has higher threshold than PMOS (body effect)
42
NMOS Only Logic Level Restoring Transistor
V
DD
V
DD
Level Restorer
M
r
B
M
2
X
M
A
Out
n
M
1
Advantage Full Swing
Restorer adds capacitance, takes away pull down
current at X
Ratio problem
43
Restorer Sizing
3.0

• Upper limit on restorer size
• Pass-transistor pull-downcan have several
  transistors in stack

W
/
L
1.75/0.25
V
r
e
W
/
L
1.50/0.25
g
r
a
t
l
o
V
W
/
L
1.25/0.25
W
/
L
1.0/0.25
r
r
Time ps
44
Solution 2 Single Transistor Pass Gate with VT0
V
DD
V
DD
0V
2.5V
Out
0V
V
DD
2.5V
WATCH OUT FOR LEAKAGE CURRENTS
45
Complementary Pass Transistor Logic
46
Solution 3 Transmission Gate
C
C
A
A
B
B
C
C
C
2.5 V
A
2.5 V
B
C
L
C

0 V
47
Resistance of Transmission Gate
48
Pass-Transistor Based Multiplexer
S
VDD
GND
In1
In2
S
49
Transmission Gate XOR
B
B
M2
A
A
F
M1
M3/M4
B
B
50
Delay in Transmission Gate Networks
m
R
R
R
eq
eq
eq
In
C
C
C
C
(c)
51
Delay Optimization
52
Transmission Gate Full Adder
Similar delays for sum and carry
53
Dynamic Logic
54
Dynamic CMOS

• In static circuits at every point in time (except
  when switching) the output is connected to either
  GND or VDD via a low resistance path.
• fan-in of n requires 2n (n N-type n P-type)
  devices
• Dynamic circuits rely on the temporary storage of
  signal values on the capacitance of high
  impedance nodes.
• requires on n 2 (n1 N-type 1 P-type)
  transistors

55
Dynamic Gate
Mp
Clk
Out
In1
In2
PDN
In3
Me
Clk
Two phase operation Precharge (CLK 0)
Evaluate (CLK 1)
56
Dynamic Gate
off
Mp
Clk
on
1
Out
In1
In2
PDN
In3
Me
Clk
off
on
Two phase operation Precharge (Clk 0)
Evaluate (Clk 1)
57
Conditions on Output

• Once the output of a dynamic gate is discharged,
  it cannot be charged again until the next
  precharge operation.
• Inputs to the gate can make at most one
  transition during evaluation.
• Output can be in the high impedance state during
  and after evaluation (PDN off), state is stored
  on CL

58
Properties of Dynamic Gates

• Logic function is implemented by the PDN only
• number of transistors is N 2 (versus 2N for
  static complementary CMOS)
• Full swing outputs (VOL GND and VOH VDD)
• Non-ratioed - sizing of the devices does not
  affect the logic levels
• Faster switching speeds
• reduced load capacitance due to lower input
  capacitance (Cin)
• reduced load capacitance due to smaller output
  loading (Cout)
• no Isc, so all the current provided by PDN goes
  into discharging CL

59
Properties of Dynamic Gates

• Overall power dissipation usually higher than
  static CMOS
• no static current path ever exists between VDD
  and GND (including Psc)
• no glitching
• higher transition probabilities
• extra load on Clk
• PDN starts to work as soon as the input signals
  exceed VTn, so VM, VIH and VIL equal to VTn
• low noise margin (NML)
• Needs a precharge/evaluate clock

60
Issues in Dynamic Design 1 Charge Leakage
CLK
Clk
Mp
Out
A
Evaluate
VOut
Clk
Me
Precharge
Leakage sources
Dominant component is subthreshold current
61
Solution to Charge Leakage
Keeper
Clk
Mp
Mkp
A
Out
B
Clk
Me
Same approach as level restorer for
pass-transistor logic
62
Issues in Dynamic Design 2 Charge Sharing
Charge stored originally on CL is redistributed
(shared) over CL and CA leading to reduced
robustness
Clk
Mp
Out
A
B0
Clk
Me
63
Charge Sharing Example
Clk
Out
A
A
B
B
B
!B
C
C
Clk
64
Charge Sharing
V
DD
(Vx reaches Vdd-Vt i.e. Ca /CL small)
M
Clk
p
Out
C
L
A
M
a
X
C
a
(Vout and Vx reach the same value)

M
B
0
b
C
b
M
Clk
e
65
Solution to Charge Redistribution
Clk
Clk
Mp
Mkp
Out
A
B
Clk
Me
Precharge internal nodes using a clock-driven
transistor (at the cost of increased area and
power)
66
Issues in Dynamic Design 3 Backgate Coupling
Clk
Mp
Out1
1
Out2
0
In
A0
B0
Clk
Me
Dynamic NAND
Static NAND
67
Backgate Coupling Effect
Out1
Voltage
Clk
Out2
In
Time, ns
68
Issues in Dynamic Design 4 Clock Feedthrough
Coupling between Out and Clk input of the
precharge device due to the gate to drain
capacitance. So voltage of Out can rise above
VDD. The fast rising (and falling edges) of the
clock couple to Out.
Clk
Mp
Out
A
B
Clk
Me
69
Clock Feedthrough
Clock feedthrough
Clk
Out
In1
In2
In3
In Clk
Voltage
In4
Out
Clk
Time, ns
Clock feedthrough
70
Other Effects

• Capacitive coupling
• Substrate coupling
• Minority charge injection
• Supply noise (ground bounce)

71
Cascading Dynamic Gates
V
Clk
Clk
Mp
Mp
Out2
Out1
In
Clk
Clk
Me
Me
t
Only 0 ? 1 transitions allowed at inputs!
72
Domino Logic
Mp
Clk
Mkp
Mp
Clk
Out1
Out2
1 ? 1 1 ? 0
0 ? 0 0 ? 1
In1
In4
PDN
In2
PDN
In5
In3
Me
Clk
Me
Clk
73
Why Domino?
Clk
Clk
Like falling dominos!
74
Properties of Domino Logic

• Only non-inverting logic can be implemented
• Very high speed
• static inverter can be skewed, only L-H
  transition
• Input capacitance reduced smaller logical
  effort

75
Designing with Domino Logic
V
V
DD
DD
V
DD
Clk
M
Clk
M
p
p
M
r
Out1
Out2
In
1
PDN
In
PDN
In
2
4
In
3
Can be eliminated!
M
Clk
M
Clk
e
e
Inputs 0 during precharge