Page 1
EE1411
© Digital Integrated Circuits2nd Combinational Circuits
Digital Integrated Digital Integrated CircuitsCircuitsA Design PerspectiveA Design Perspective
Designing CombinationalDesigning CombinationalLogic CircuitsLogic Circuits
Jan M. RabaeyAnantha ChandrakasanBorivoje Nikolić
November 2002.
Page 2
EE1412
© Digital Integrated Circuits2nd Combinational Circuits
Combinational vs. Sequential LogicCombinational vs. Sequential Logic
Combinational Sequential
Output = f(In) Output = f(In, Previous In)
CombinationalLogicCircuit
OutInCombinational
LogicCircuit
OutIn
State
Page 3
EE1413
© Digital Integrated Circuits2nd Combinational Circuits
Static CMOS CircuitStatic CMOS Circuit
At every point in time (except during the switching transients) each gate output is connected to eitherVDD or Vss via a low-resistive path.
The outputs of the gates assume at all times the value of the Boolean function, implemented by the circuit (ignoring, once again, the transient effects during switching periods).
This is in contrast to the dynamic circuit class, which relies on temporary storage of signal values on the capacitance of high impedance circuit nodes.
Page 4
EE1414
© Digital Integrated Circuits2nd Combinational Circuits
Static Complementary CMOSStatic Complementary CMOSVDD
F(In1,In2,…InN)
In1In2
InN
In1In2InN
PUN
PDN
PMOS only
NMOS only
PUN and PDN are dual logic networks…
…
Page 5
EE1415
© Digital Integrated Circuits2nd Combinational Circuits
NMOS Transistors NMOS Transistors in Series/Parallel Connectionin Series/Parallel Connection
Transistors can be thought as a switch controlled by its gate signal
NMOS switch closes when switch control input is high
X Y
A B
Y = X if A and B
X Y
A
B Y = X if A OR B
NMOS Transistors pass a “strong” 0 but a “weak” 1
Page 6
EE1416
© Digital Integrated Circuits2nd Combinational Circuits
PMOS Transistors PMOS Transistors in Series/Parallel Connectionin Series/Parallel Connection
X Y
A B
Y = X if A AND B = A + B
X Y
A
B Y = X if A OR B = AB
PMOS Transistors pass a “strong” 1 but a “weak” 0
PMOS switch closes when switch control input is low
Page 7
EE1417
© Digital Integrated Circuits2nd Combinational Circuits
Threshold DropsThreshold DropsVDD
VDD → 0PDN
0 → VDD
CL
CL
PUN
VDD
0 → VDD - VTn
CL
VDD
VDD
VDD → |VTp|
CL
S
D S
D
VGS
S
SD
D
VGS
Page 8
EE1418
© Digital Integrated Circuits2nd Combinational Circuits
Complementary CMOS Logic StyleComplementary CMOS Logic Style
Page 9
EE1419
© Digital Integrated Circuits2nd Combinational Circuits
Example Gate: NANDExample Gate: NAND
Page 10
EE14110
© Digital Integrated Circuits2nd Combinational Circuits
Example Gate: NORExample Gate: NOR
Page 11
EE14111
© Digital Integrated Circuits2nd Combinational Circuits
Complex CMOS GateComplex CMOS Gate
OUT = D + A • (B + C)
DA
B C
D
AB
C
Page 12
EE14112
© Digital Integrated Circuits2nd Combinational Circuits
Constructing a Complex GateConstructing a Complex Gate
C
(a) pull-down network
SN1 SN4
SN2
SN3D
FF
A
DB
C
D
F
A
B
C
(b) Deriving the pull-up networkhierarchically by identifyingsub-nets
D
A
A
B
C
VDD VDD
B
(c) complete gate
Page 13
EE14113
© Digital Integrated Circuits2nd Combinational Circuits
Cell DesignCell Design
Standard CellsGeneral purpose logicCan be synthesizedSame height, varying width
Datapath CellsFor regular, structured designs (arithmetic)Includes some wiring in the cellFixed height and width
Page 14
EE14114
© Digital Integrated Circuits2nd Combinational Circuits
Standard Cell Layout Methodology Standard Cell Layout Methodology ––1980s1980s
signals
Routingchannel
VDD
GND
Page 15
EE14115
© Digital Integrated Circuits2nd Combinational Circuits
Standard Cell Layout Methodology Standard Cell Layout Methodology ––1990s1990s
M2
No Routingchannels VDD
GNDM3
VDD
GND
Mirrored Cell
Mirrored Cell
Page 16
EE14116
© Digital Integrated Circuits2nd Combinational Circuits
Standard CellsStandard Cells
Cell boundary
N WellCell height 12 metal tracksMetal track is approx. 3λ + 3λPitch = repetitive distance between objects
Cell height is “12 pitch”
2λ
Rails ~10λ
InOut
VDD
GND
Page 17
EE14117
© Digital Integrated Circuits2nd Combinational Circuits
Standard CellsStandard Cells
InOut
VDD
GND
In Out
VDD
GND
With silicided diffusion
With minimaldiffusionrouting
OutIn
VDD
M2
M1
Page 18
EE14118
© Digital Integrated Circuits2nd Combinational Circuits
Standard CellsStandard Cells
A
Out
VDD
GND
B
2-input NAND gate
B
VDD
A
Page 19
EE14119
© Digital Integrated Circuits2nd Combinational Circuits
Stick DiagramsStick Diagrams
Contains no dimensionsRepresents relative positions of transistors
In
Out
VDD
GND
Inverter
A
Out
VDD
GNDB
NAND2
Page 20
EE14120
© Digital Integrated Circuits2nd Combinational Circuits
Stick DiagramsStick Diagrams
C
A B
X = C • (A + B)
B
AC
i
j
j
VDDX
X
i
GND
AB
C
PUN
PDNABC
Logic Graph
Page 21
EE14121
© Digital Integrated Circuits2nd Combinational Circuits
Two Versions of C Two Versions of C •• (A + B)(A + B)
X
CA B A B C
X
VDD
GND
VDD
GND
Page 22
EE14122
© Digital Integrated Circuits2nd Combinational Circuits
Consistent Euler PathConsistent Euler Path
j
VDDX
X
i
GND
AB
C
A B C
Page 23
EE14123
© Digital Integrated Circuits2nd Combinational Circuits
OAI22 Logic GraphOAI22 Logic Graph
C
A B
X = (A+B)•(C+D)
B
A
D
VDDX
X
GND
AB
C
PUN
PDN
C
D
D
ABCD
Page 24
EE14124
© Digital Integrated Circuits2nd Combinational Circuits
Example: x = Example: x = abab++cdcd
GND
x
a
b c
d
VDDx
GND
x
a
b c
d
VDDx
(a) Logic graphs for (ab+cd) (b) Euler Paths {a b c d}
a c d
x
VDD
GND
(c) stick diagram for ordering {a b c d}b
Page 25
EE14125
© Digital Integrated Circuits2nd Combinational Circuits
MultiMulti--Fingered TransistorsFingered TransistorsOne finger Two fingers (folded)
Less diffusion capacitance
Page 26
EE14126
© Digital Integrated Circuits2nd Combinational Circuits
Properties of Complementary CMOS Gates Properties of Complementary CMOS Gates SnapshotSnapshot
High noise margins: VOH and VOL are at VDD and GND, respectively.
No static power consumption:There never exists a direct path between VDD and VSS (GND) in steady-state mode.
Comparable rise and fall times:(under appropriate sizing conditions)
Page 27
EE14127
© Digital Integrated Circuits2nd Combinational Circuits
CMOS PropertiesCMOS PropertiesFull rail-to-rail swing; high noise marginsLogic levels not dependent upon the relative device sizes; ratiolessAlways a path to Vdd or Gnd in steady state; low output impedanceExtremely high input resistance; nearly zero steady-state input currentNo direct path steady state between power and ground; no static power dissipationPropagation delay function of load capacitance and resistance of transistors
Page 28
EE14128
© Digital Integrated Circuits2nd Combinational Circuits
Switch Delay ModelSwitch Delay Model
A
Req
A
Rp
A
Rp
A
Rn CL
A
CL
B
Rn
A
Rp
B
Rp
A
Rn Cint
B
Rp
A
Rp
A
Rn
B
Rn CL
Cint
NAND2 INV NOR2
Page 29
EE14129
© Digital Integrated Circuits2nd Combinational Circuits
Input Pattern Effects on DelayInput Pattern Effects on Delay
Delay is dependent on the pattern of inputsLow 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 transitionboth inputs go high
– delay is 0.69 2Rn CL
CL
B
Rn
ARp
BRp
A
Rn Cint
Page 30
EE14130
© Digital Integrated Circuits2nd Combinational Circuits
Delay Dependence on Input PatternsDelay Dependence on Input Patterns
-0.5
0
0.5
1
1.5
2
2.5
3
0 100 200 300 400
A=B=1→0
A=1, B=1→0
A=1 →0, B=1
time [ps]
Vol
tage
[V]
81A= 1→0, B=1
80A=1, B=1→0
45A=B=1→0
61A= 0→1, B=1
64A=1, B=0→1
67A=B=0→1
Delay(psec)
Input DataPattern
NMOS = 0.5µm/0.25 µmPMOS = 0.75µm/0.25 µmCL = 100 fF
Page 31
EE14131
© Digital Integrated Circuits2nd Combinational Circuits
Transistor SizingTransistor Sizing
CL
B
Rn
A
Rp
B
Rp
A
Rn Cint
B
Rp
A
Rp
A
Rn
B
Rn CL
Cint
2
2
2 2
11
4
4
Page 32
EE14132
© Digital Integrated Circuits2nd Combinational Circuits
Transistor Sizing a Complex Transistor Sizing a Complex CMOS GateCMOS Gate
OUT = D + A • (B + C)
DA
B C
D
AB
C
1
2
2 2
4
48
8
6
36
6
Page 33
EE14133
© Digital Integrated Circuits2nd Combinational Circuits
FanFan--In ConsiderationsIn Considerations
DCBA
D
C
B
A CL
C3
C2
C1
Distributed RC model(Elmore delay)
tpHL = 0.69 Reqn(C1+2C2+3C3+4CL)
Propagation delay deteriorates rapidly as a function of fan-in –quadratically in the worst case.
Page 34
EE14134
© Digital Integrated Circuits2nd Combinational Circuits
ttpp as a Function of Fanas a Function of Fan--InIn
tpLH
t p(p
sec)
fan-in
Gates with a fan-in greater than 4 should be avoided.
0
250
500
750
1000
1250
2 4 6 8 10 12 14 16
tpHL
quadratic
linear
tp
Page 35
EE14135
© Digital Integrated Circuits2nd Combinational Circuits
ttpp as a Function of Fanas a Function of Fan--OutOut
2 4 6 8 10 12 14 16
tpNOR2
t p(p
sec)
eff. fan-out
All gates have the same drive current.
tpNAND2
tpINV
Slope is a function of “driving strength”
Page 36
EE14136
© Digital Integrated Circuits2nd Combinational Circuits
ttpp as a Function of Fanas a Function of Fan--In and FanIn and Fan--OutOut
Fan-in: quadratic due to increasing resistance and capacitanceFan-out: each additional fan-out gate adds two gate capacitances to CL
tp = a1FI + a2FI2 + a3FO
Page 37
EE14137
© Digital Integrated Circuits2nd Combinational Circuits
Fast Complex Gates:Fast Complex Gates:Design Technique 1Design Technique 1
Transistor sizingas long as fan-out capacitance dominates
Progressive sizing
InN CL
C3
C2
C1In1
In2
In3
M1
M2
M3
MNDistributed RC line
M1 > M2 > M3 > … > MN(the fet closest to theoutput is the smallest)
Can reduce delay by more than 20%; decreasing gains as technology shrinks
Page 38
EE14138
© Digital Integrated Circuits2nd Combinational Circuits
Fast Complex Gates:Fast Complex Gates:Design Technique 2Design Technique 2
Transistor ordering
C2
C1In1
In2
In3
M1
M2
M3 CL
C2
C1In3
In2
In1
M1
M2
M3 CL
critical path critical path
charged1
0→1charged
charged1
delay determined by time to discharge CL, C1 and C2
delay determined by time to discharge CL
1
1
0→1 charged
discharged
discharged
Page 39
EE14139
© Digital Integrated Circuits2nd Combinational Circuits
Fast Complex Gates:Fast Complex Gates:Design Technique 3Design Technique 3Alternative logic structures
F = ABCDEFGH
Page 40
EE14140
© Digital Integrated Circuits2nd Combinational Circuits
Fast Complex Gates:Fast Complex Gates:Design Technique 4Design Technique 4
Isolating fan-in from fan-out using buffer insertion
CLCL
Page 41
EE14141
© Digital Integrated Circuits2nd Combinational Circuits
Fast Complex Gates:Fast Complex Gates:Design Technique 5Design Technique 5
Reducing the voltage swing
linear reduction in delayalso 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 )
Page 42
EE14142
© Digital Integrated Circuits2nd Combinational Circuits
Sizing Logic Paths for SpeedSizing Logic Paths for Speed
Frequently, input capacitance of a logic path is constrainedLogic also has to drive some capacitanceExample: ALU load in an Intel’s microprocessor is 0.5pFHow do we size the ALU datapath to achieve maximum speed?We have already solved this for the inverter chain – can we generalize it for any type of logic?
Page 43
EE14143
© Digital Integrated Circuits2nd Combinational Circuits
Buffer ExampleBuffer Example
( )∑=
⋅+=N
iiii fgpDelay
1
For given N: Ci+1/Ci = Ci/Ci-1To find N: Ci+1/Ci ~ 4How to generalize this to any logic path?
CL
In Out
1 2 N
(in units of τinv)
Page 44
EE14144
© Digital Integrated Circuits2nd Combinational Circuits
Logical EffortLogical Effort
( )fgpCCCRkDelay
in
Lunitunit
⋅+=
+⋅=
τγ
1
p – intrinsic delay (3kRunitCunitγ) - gate parameter ≠ f(W)g – logical effort (kRunitCunit) – gate parameter ≠ f(W)f – effective fanout
Normalize everything to an inverter:ginv =1, pinv = 1
Divide everything by τinv(everything is measured in unit delays τinv)Assume γ = 1.
Page 45
EE14145
© Digital Integrated Circuits2nd Combinational Circuits
Delay in a Logic GateDelay in a Logic Gate
Gate delay:d = h + p
effort delay intrinsic delay
Effort delay:
h = g f
logical effort
effective fanout = Cout/Cin
Logical effort is a function of topology, independent of sizingEffective fanout (electrical effort) is a function of load/gate size
Page 46
EE14146
© Digital Integrated Circuits2nd Combinational Circuits
Logical EffortLogical Effort
Inverter has the smallest logical effort and intrinsic delay of all static CMOS gatesLogical effort of a gate presents the ratio of its input capacitance to the inverter capacitance when sized to deliver the same currentLogical effort increases with the gate complexity
Page 47
EE14147
© Digital Integrated Circuits2nd Combinational Circuits
Logical EffortLogical EffortLogical effort is the ratio of input capacitance of a gate to the inputcapacitance of an inverter with the same output current
g = 1 g = 4/3 g = 5/3
B
A
A B
F
VDDVDD
A B
A
B
F
VDD
A
A
F
1
2 2 2
2
21 1
4
4
Inverter 2-input NAND 2-input NOR
Page 48
EE14148
© Digital Integrated Circuits2nd Combinational Circuits
Logical Effort of GatesLogical Effort of Gates
Fan-out (h)
Nor
mal
ized
del
ay (d
)
t
1 2 3 4 5 6 7
pINVtpNAND
F(Fan-in)
g =p =d =
g =p =d =
Page 49
EE14149
© Digital Integrated Circuits2nd Combinational Circuits
Logical Effort of GatesLogical Effort of Gates
Fan-out (h)
Nor
mal
ized
del
ay (d
)
t
1 2 3 4 5 6 7
pINVtpNAND
F(Fan-in)
g = 1p = 1d = h+1
g = 4/3p = 2d = (4/3)h+2
Page 50
EE14150
© Digital Integrated Circuits2nd Combinational Circuits
Logical Effort of GatesLogical Effort of Gates
Intrinsic�Delay
EffortDelay
1 2 3 4 5Fanout f
1
2
3
4
5
Inverter:
g = 1; p = 12-i
nput
NAND: g = 4/
3;p =
2
Nor
mal
ized
Del
ay
Page 51
EE14151
© Digital Integrated Circuits2nd Combinational Circuits
Add Branching EffortAdd Branching Effort
Branching effort:
pathon
pathoffpathonC
CCb
−
−− +=
Page 52
EE14152
© Digital Integrated Circuits2nd Combinational Circuits
Multistage NetworksMultistage Networks
Stage effort: hi = gifiPath electrical effort: F = Cout/Cin
Path logical effort: G = g1g2…gN
Branching effort: B = b1b2…bN
Path effort: H = GFB
Path delay D = Σdi = Σpi + Σhi
( )∑=
⋅+=N
iiii fgpDelay
1
Page 53
EE14153
© Digital Integrated Circuits2nd Combinational Circuits
Optimum Effort per StageOptimum Effort per Stage
HhN =
When each stage bears the same effort:
N Hh =
( ) PNHpfgD Niii +=+= ∑ /1ˆ
Minimum path delay
Effective fanout of each stage: ii ghf =
Stage efforts: g1f1 = g2f2 = … = gNfN
Page 54
EE14154
© Digital Integrated Circuits2nd Combinational Circuits
Optimal Number of StagesOptimal Number of StagesFor a given load, and given input capacitance of the first gateFind optimal number of stages and optimal sizing
invN NpNHD += /1
( ) 0ln /1/1/1 =++−=∂∂
invNNN pHHH
ND
NHh ˆ/1=Substitute ‘best stage effort’
Page 55
EE14155
© Digital Integrated Circuits2nd Combinational Circuits
Logical EffortLogical Effort
From Sutherland, Sproull
Page 56
EE14156
© Digital Integrated Circuits2nd Combinational Circuits
Example: Optimize PathExample: Optimize Path
Effective fanout, F =G = H =h =a =b =
1a
b c
5
g = 1f = a
g = 5/3f = b/a
g = 5/3f = c/b
g = 1f = 5/c
Page 57
EE14157
© Digital Integrated Circuits2nd Combinational Circuits
Example: Optimize PathExample: Optimize Path1
ab c
5
g = 1f = a
g = 5/3f = b/a
g = 5/3f = c/b
g = 1f = 5/c
Effective fanout, F = 5G = 25/9H = 125/9 = 13.9h = 1.93a = 1.93b = ha/g2 = 2.23c = hb/g3 = 5g4/f = 2.59
Page 58
EE14158
© Digital Integrated Circuits2nd Combinational Circuits
Example: Optimize PathExample: Optimize Path
1 a
b c 5
Effective fanout, H = 5G = 25/9F = 125/9 = 13.9f = 1.93a = 1.93b = fa/g2 = 2.23c = fb/g3 = 5g4/f = 2.59
g1 = 1 g2 = 5/3 g3 = 5/3 g4 = 1
Page 59
EE14159
© Digital Integrated Circuits2nd Combinational Circuits
Example Example –– 88--input ANDinput AND
Page 60
EE14160
© Digital Integrated Circuits2nd Combinational Circuits
Method of Logical EffortMethod of Logical Effort
Compute the path effort: F = GBHFind the best number of stages N ~ log4FCompute the stage effort f = F1/N
Sketch the path with this number of stagesWork either from either end, find sizes: Cin = Cout*g/f
Reference: Sutherland, Sproull, Harris, “Logical Effort, Morgan-Kaufmann 1999.
Page 61
EE14161
© Digital Integrated Circuits2nd Combinational Circuits
SummarySummary
Sutherland,SproullHarris
Page 62
EE14162
© Digital Integrated Circuits2nd Combinational Circuits
Ratioed Ratioed LogicLogic
Page 63
EE14163
© Digital Integrated Circuits2nd Combinational Circuits
RatioedRatioed LogicLogic
VDD
VSS
PDNIn1In2In3
F
RLLoad
VDD
VSS
In1In2In3
F
VDD
VSS
PDNIn1In2In3
FVSS
PDN
Resistive DepletionLoad
PMOSLoad
(a) resistive load (b) depletion load NMOS (c) pseudo-NMOS
VT < 0
Goal: to reduce the number of devices over complementary CMOS
Page 64
EE14164
© Digital Integrated Circuits2nd Combinational Circuits
RatioedRatioed LogicLogicVDD
VSS
PDNIn1In2In3
F
RLLoadResistive
N transistors + Load
• VOH = VDD
• VOL = RPN
RPN + RL
• Assymetrical response
• Static power consumption
•
• tpL= 0.69 RLCL
Page 65
EE14165
© Digital Integrated Circuits2nd Combinational Circuits
Active LoadsActive LoadsVDD
VSS
In1In2In3
F
VDD
VSS
PDNIn1In2In3
F
VSS
PDN
DepletionLoad
PMOSLoad
depletion load NMOS pseudo-NMOS
VT < 0
Page 66
EE14166
© Digital Integrated Circuits2nd Combinational Circuits
PseudoPseudo--NMOSNMOS
VDD
A B C D
FCL
VOH = VDD (similar to complementary CMOS)
kn VDD VTn–( )VOLVOL
2
2-------------–
kp
2------ VDD VTp–( )
2=
VOL VDD VT–( ) 1 1kpkn------–– (assuming that VT VTn VTp )= = =
SMALLER AREA & LOAD BUT STATIC POWER DISSIPATION!!!
Page 67
EE14167
© Digital Integrated Circuits2nd Combinational Circuits
PseudoPseudo--NMOS VTCNMOS VTC
0.0 0.5 1.0 1.5 2.0 2.50.0
0.5
1.0
1.5
2.0
2.5
3.0
Vin [V]
Vou
t[V
]
W/Lp = 4
W/Lp = 2
W/Lp = 1
W/Lp = 0.25
W/Lp = 0.5
Page 68
EE14168
© Digital Integrated Circuits2nd Combinational Circuits
Improved LoadsImproved Loads
A B C D
F
CL
M1M2 M1 >> M2Enable
VDD
Adaptive Load
Page 69
EE14169
© Digital Integrated Circuits2nd Combinational Circuits
Improved Loads (2)Improved Loads (2)VDD
VSS
PDN1
Out
VDD
VSS
PDN2
Out
AABB
M1 M2
Differential Cascode Voltage Switch Logic (DCVSL)
Page 70
EE14170
© Digital Integrated Circuits2nd Combinational Circuits
DCVSL ExampleDCVSL Example
B
A A
B B B
Out
Out
XOR-NXOR gate
Page 71
EE14171
© Digital Integrated Circuits2nd Combinational Circuits
DCVSL Transient ResponseDCVSL Transient Response
0 0.2 0.4 0.6 0.8 1.0-0.5
0.5
1.5
2.5
Time [ns]
Vol
tag e
[V] A B
A B
A,BA,B
Page 72
EE14172
© Digital Integrated Circuits2nd Combinational Circuits
PassPass--TransistorTransistorLogicLogic
Page 73
EE14173
© Digital Integrated Circuits2nd Combinational Circuits
PassPass--Transistor LogicTransistor LogicIn
puts Switch
Network
OutOut
A
B
B
B
• N transistors• No static consumption
Page 74
EE14174
© Digital Integrated Circuits2nd Combinational Circuits
Example: AND GateExample: AND Gate
B
B
A
F = AB
0
Page 75
EE14175
© Digital Integrated Circuits2nd Combinational Circuits
NMOSNMOS--Only LogicOnly Logic
VDD
In
Outx
0.5µm/0.25µm0.5µm/0.25µm
1.5µm/0.25µm
0 0.5 1 1.5 20.0
1.0
2.0
3.0
Time [ns]
Volt a
ge[V
]
xOut
In
Page 76
EE14176
© Digital Integrated Circuits2nd Combinational Circuits
NMOSNMOS--only Switchonly Switch
A = 2.5 V
B
C = 2.5V
CL
A = 2.5 V
C = 2.5 V
BM2
M1
Mn
Threshold voltage loss causesstatic power consumption
VB does not pull up to 2.5V, but 2.5V -VTN
NMOS has higher threshold than PMOS (body effect)
Page 77
EE14177
© Digital Integrated Circuits2nd Combinational Circuits
NMOS Only Logic: NMOS Only Logic: Level Restoring TransistorLevel Restoring Transistor
M2
M1
Mn
Mr
OutA
B
VDDVDDLevel Restorer
X
• Advantage: Full Swing• Restorer adds capacitance, takes away pull down current at X• Ratio problem
Page 78
EE14178
© Digital Integrated Circuits2nd Combinational Circuits
Restorer SizingRestorer Sizing
0 100 200 300 400 5000.0
1.0
2.0
W/Lr =1.0/0.25 W/Lr =1.25/0.25
W/Lr =1.50/0.25
W/Lr =1.75/0.25
Vol
t ag e
[V]
Time [ps]
3.0•Upper limit on restorer size•Pass-transistor pull-downcan have several transistors in stack
Page 79
EE14179
© Digital Integrated Circuits2nd Combinational Circuits
Solution 2: Single Transistor Pass Gate with Solution 2: Single Transistor Pass Gate with VVTT=0=0
Out
VDD
VDD
2.5V
VDD
0V 2.5V
0V
WATCH OUT FOR LEAKAGE CURRENTS
Page 80
EE14180
© Digital Integrated Circuits2nd Combinational Circuits
Complementary Pass Transistor LogicComplementary Pass Transistor Logic
A
B
A
B
B B B B
A
B
A
B
F=AB
F=AB
F=A+B
F=A+B
B B
A
A
A
A
F=A⊕ΒÝ
F=A⊕ΒÝ
OR/NOR EXOR/NEXORAND/NAND
F
F
Pass-TransistorNetwork
Pass-TransistorNetwork
AABB
AABB
Inverse
(a)
(b)
Page 81
EE14181
© Digital Integrated Circuits2nd Combinational Circuits
Solution 3: Transmission GateSolution 3: Transmission Gate
A B
C
C
A B
C
C
BCL
C = 0 V
A = 2.5 V
C = 2.5 V
Page 82
EE14182
© Digital Integrated Circuits2nd Combinational Circuits
Resistance of Transmission GateResistance of Transmission Gate
Vout
0 V
2.5 V
2.5 VRn
Rp
0.0 1.0 2.00
10
20
30
Vout, V
Res
ista
nce,
ohm
s
Rn
Rp
Rn || Rp
Page 83
EE14183
© Digital Integrated Circuits2nd Combinational Circuits
PassPass--Transistor Based MultiplexerTransistor Based Multiplexer
AM2
M1
B
S
S
S F
VDD
GND
VDD
In1 In2S S
S S
Page 84
EE14184
© Digital Integrated Circuits2nd Combinational Circuits
Transmission Gate XORTransmission Gate XOR
A
B
F
B
A
B
BM1
M2
M3/M4
Page 85
EE14185
© Digital Integrated Circuits2nd Combinational Circuits
Delay in Transmission Gate NetworksDelay in Transmission Gate Networks
V1 Vi-1
C
2.5 2.5
0 0
Vi Vi+1
CC
2.5
0
Vn-1 Vn
CC
2.5
0
In
V1 Vi Vi+1
C
Vn-1 Vn
CC
InReqReq Req Req
CC
(a)
(b)
C
Req Req
C C
Req
C C
Req Req
C C
Req
CIn
m
(c)
Page 86
EE14186
© Digital Integrated Circuits2nd Combinational Circuits
Delay OptimizationDelay Optimization
Page 87
EE14187
© Digital Integrated Circuits2nd Combinational Circuits
Transmission Gate Full AdderTransmission Gate Full Adder
A
B
P
Ci
VDDA
A A
VDD
Ci
A
P
AB
VDD
VDD
Ci
Ci
Co
S
Ci
P
P
P
P
P
Sum Generation
Carry Generation
Setup
Similar delays for sum and carry
Page 88
EE14188
© Digital Integrated Circuits2nd Combinational Circuits
Dynamic LogicDynamic Logic
Page 89
EE14189
© Digital Integrated Circuits2nd Combinational Circuits
Dynamic CMOSDynamic 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 (n+1 N-type + 1 P-type) transistors
Page 90
EE14190
© Digital Integrated Circuits2nd Combinational Circuits
Dynamic GateDynamic Gate
In1
In2 PDNIn3
Me
Mp
Clk
ClkOut
CL
Out
Clk
Clk
A
BC
Mp
Me
Two phase operationPrecharge (CLK = 0)Evaluate (CLK = 1)
Page 91
EE14191
© Digital Integrated Circuits2nd Combinational Circuits
Dynamic GateDynamic Gate
In1
In2 PDNIn3
Me
Mp
Clk
ClkOut
CL
Out
Clk
Clk
A
BC
Mp
Me
Two phase operationPrecharge (Clk = 0)Evaluate (Clk = 1)
on
off
1off
on
((AB)+C)
Page 92
EE14192
© Digital Integrated Circuits2nd Combinational Circuits
Conditions on OutputConditions 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
Page 93
EE14193
© Digital Integrated Circuits2nd Combinational Circuits
Properties of Dynamic GatesProperties of Dynamic GatesLogic 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 levelsFaster 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
Page 94
EE14194
© Digital Integrated Circuits2nd Combinational Circuits
Properties of Dynamic GatesProperties of Dynamic GatesOverall power dissipation usually higher than static CMOS
no static current path ever exists between VDD and GND (including Psc)no glitchinghigher transition probabilitiesextra 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
Page 95
EE14195
© Digital Integrated Circuits2nd Combinational Circuits
Issues in Dynamic Design 1: Issues in Dynamic Design 1: Charge LeakageCharge Leakage
CL
Clk
ClkOut
A
Mp
Me
Leakage sources
CLK
VOut
Precharge
Evaluate
Dominant component is subthreshold current
Page 96
EE14196
© Digital Integrated Circuits2nd Combinational Circuits
Solution to Charge LeakageSolution to Charge Leakage
CL
Clk
Clk
Me
Mp
A
B
Out
Mkp
Same approach as level restorer for pass-transistor logic
Keeper
Page 97
EE14197
© Digital Integrated Circuits2nd Combinational Circuits
Issues in Dynamic Design 2: Issues in Dynamic Design 2: Charge SharingCharge Sharing
CL
Clk
Clk
CA
CB
B=0
AOut
Mp
Me
Charge stored originally on CL is redistributed (shared) over CL and CA leading to reduced robustness
Page 98
EE14198
© Digital Integrated Circuits2nd Combinational Circuits
Charge Sharing ExampleCharge Sharing Example
CL=50fF
Clk
Clk
A A
B B B !B
CC
Out
Ca=15fF
Cc=15fF
Cb=15fF
Cd=10fF
Page 99
EE14199
© Digital Integrated Circuits2nd Combinational Circuits
Charge SharingCharge Sharing
CLVDD CLVout t( ) Ca VDD VTn VX( )–( )+=
or
∆Vout Vout t( ) VDD–CaCL-------- VDD VTn VX( )–( )–= =
∆Vout VDDCa
Ca CL+----------------------
–=
case 1) if ∆Vout < VTn
case 2) if ∆Vout > VTnB = 0
Clk
X
CL
Ca
Cb
A
Out
Mp
Ma
VDD
Mb
Clk Me
Page 100
EE141100
© Digital Integrated Circuits2nd Combinational Circuits
Solution to Charge RedistributionSolution to Charge Redistribution
Clk
Clk
Me
Mp
A
B
OutMkp
Clk
Precharge internal nodes using a clock-driven transistor (at the cost of increased area and power)
Page 101
EE141101
© Digital Integrated Circuits2nd Combinational Circuits
Issues in Dynamic Design 3: Issues in Dynamic Design 3: BackgateBackgate CouplingCoupling
CL1
Clk
Clk
B=0
A=0
Out1Mp
Me
Out2
CL2In
Dynamic NAND Static NAND
=1 =0
Page 102
EE141102
© Digital Integrated Circuits2nd Combinational Circuits
BackgateBackgate Coupling EffectCoupling Effect
-1
0
1
2
3
0 2 4 6
Vol
tage
Time, ns
Clk
In
Out1
Out2
Page 103
EE141103
© Digital Integrated Circuits2nd Combinational Circuits
Issues in Dynamic Design 4: Clock Issues in Dynamic Design 4: Clock FeedthroughFeedthrough
CL
Clk
Clk
B
AOut
Mp
Me
Coupling between Out and Clk input of the prechargedevice 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.
Page 104
EE141104
© Digital Integrated Circuits2nd Combinational Circuits
Clock Clock FeedthroughFeedthrough
-0.5
0.5
1.5
2.5
0 0.5 1
Clk
Clk
In1
In2
In3
In4
Out
In &Clk
Out
Time, ns
Vol
tage
Clock feedthrough
Clock feedthrough
Page 105
EE141105
© Digital Integrated Circuits2nd Combinational Circuits
Other EffectsOther Effects
Capacitive couplingSubstrate couplingMinority charge injectionSupply noise (ground bounce)
Page 106
EE141106
© Digital Integrated Circuits2nd Combinational Circuits
Cascading Dynamic GatesCascading Dynamic Gates
Clk
Clk
Out1In
Mp
Me
Mp
Me
Clk
Clk
Out2
V
t
Clk
In
Out1
Out2 ∆V
VTn
Only 0 → 1 transitions allowed at inputs!
Page 107
EE141107
© Digital Integrated Circuits2nd Combinational Circuits
Domino LogicDomino Logic
In1
In2 PDNIn3
Me
Mp
Clk
Clk Out1
In4 PDNIn5
Me
Mp
Clk
ClkOut2
Mkp
1 → 11 → 0
0 → 00 → 1
Page 108
EE141108
© Digital Integrated Circuits2nd Combinational Circuits
Why Domino?Why Domino?
Clk
Clk
Ini PDNInj
IniInj
PDN Ini PDNInj
Ini PDNInj
Like falling dominos!
Page 109
EE141109
© Digital Integrated Circuits2nd Combinational Circuits
Properties of Domino LogicProperties of Domino Logic
Only non-inverting logic can be implementedVery high speed
static inverter can be skewed, only L-H transitionInput capacitance reduced – smaller logical effort
Page 110
EE141110
© Digital Integrated Circuits2nd Combinational Circuits
Designing with Domino LogicDesigning with Domino Logic
Mp
Me
VDD
PDN
Clk
In1In2
In3
Out1
Clk
Mp
Me
VDD
PDN
Clk
In4
Clk
Out2
Mr
VDD
Inputs = 0during precharge
Can be eliminated!
Page 111
EE141111
© Digital Integrated Circuits2nd Combinational Circuits
Footless DominoFootless Domino
The first gate in the chain needs a foot switchPrecharge is rippling – short-circuit currentA solution is to delay the clock for each stage
VDD
Clk Mp
Out1
In1
1 0
VDD
Clk Mp
Out2
In2
VDD
Clk Mp
Outn
InnIn3
1 0
0 1 0 1 0 1
1 0 1 0
Page 112
EE141112
© Digital Integrated Circuits2nd Combinational Circuits
Differential (Dual Rail) DominoDifferential (Dual Rail) Domino
A
B
Me
Mp
Clk
ClkOut = AB
!A !B
MkpClk
Out = ABMkp Mp
Solves the problem of non-inverting logic
1 0 1 0
onoff
Page 113
EE141113
© Digital Integrated Circuits2nd Combinational Circuits
npnp--CMOSCMOS
In1
In2 PDNIn3
Me
Mp
Clk
Clk Out1
In4 PUNIn5
Me
MpClk
Clk
Out2(to PDN)
1 → 11 → 0
0 → 00 → 1
Only 0 → 1 transitions allowed at inputs of PDN Only 1 → 0 transitions allowed at inputs of PUN
Page 114
EE141114
© Digital Integrated Circuits2nd Combinational Circuits
NORA LogicNORA Logic
In1
In2 PDNIn3
Me
Mp
Clk
Clk Out1
In4 PUNIn5
Me
MpClk
Clk
Out2(to PDN)
1 → 11 → 0
0 → 00 → 1
to otherPDN’s
to otherPUN’s
WARNING: Very sensitive to noise!