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ECE4121 Static CMOS Logic.1 ZALAM
VLSI Design
Static CMOS Logic
[Adapted from Rabaey’s Digital Integrated Circuits , ©2002, J. Rabaey et al.]
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ECE4121 Static CMOS Logic.2 ZALAM
Review: CMOS Process at a Glance
Define active areasEtch and fill trenches
Implant well regions
Deposit and patternpolysilicon layer
Implant source and drain
regions and substrate contacts
Create contact and via windowsDeposit and pattern metal layers
One full photolithographysequence per layer(mask)
Built (roughly) from thebottom up
4 metal2 polysilicon3 source and drain diffusions
1 tubs (aka wells, active areas)
exception!
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ECE4121 Static CMOS Logic.3 ZALAM
CMOS Circuit StylesStatic complementary CMOS - except during switching,output connected to either V DD or GND via a low-resistance path
high noise margins- full rail to rail swing- VOH and V OL are at V DD and GND, respectively
low output impedance, high input impedanceno steady state path between V DD and GND ( no static power
consumption)delay a function of load capacitance and transistor resistancecomparable rise and fall times (under the appropriate transistorsizing conditions)
Dynamic CMOS - relies on temporary storage of signalvalues on the capacitance of high-impedance circuitnodes
simpler, faster gatesincreased sensitivity to noise
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ECE4121 Static CMOS Logic.4 ZALAM
Static Complementary CMOS
VDD
F(In 1,In2,…In N)
In1In2
InN
In1In2
InN
PUN
PDN
PUN and PDN are dual logic networks
Pull-up network (PUN) and pull-down network (PDN)
PMOS transistors only
pull-up: make a connection from V DD to Fwhen F(In 1,In2,…In N) = 1
NMOS transistors only
pull-down: make a connection from F toGND when F(In 1,In2,…In N) = 0
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ECE4121 Static CMOS Logic.6 ZALAM
Threshold Drops
VDD
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
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Construction of PDN
NMOS devices in series implement a NAND function
NMOS devices in parallel implement a NOR function
A
B
A • B
A BA + B
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Dual PUN and PDN
PUN and PDN are dual networksDeMorgan’s theorems
A + B = A • B [!(A + B) = !A • !B or !(A | B) = !A & !B]
A • B = A + B [!(A • B) = !A + !B or !(A & B) = !A | !B]
a parallel connection of transistors in the PUN corresponds to aseries connection of the PDN
Complementary gate is naturally inverting (NAND,
NOR, AOI, OAI)Number of transistors for an N-input logic gate is 2N
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CMOS NAND
A
B
A • B
A B
A B F
0 0 1
0 1 1
1 0 1
1 1 0
AB
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Complex CMOS Gate
OUT = !(D + A • (B + C))
D
A
B C
D
A
B
C
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Standard Cell Layout Methodology
signals
Routingchannel
VDD
GND
What logic function is this?
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OAI21 Logic Graph
C
A B
X = !(C • (A + B))
B
AC
i
j
j
VDDX
X
i
GND
AB
C
PUN
PDNABC
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ECE4121 Static CMOS Logic.18 ZALAM
OAI22 Logic Graph
C
A B
X = !((A+B)•(C+D))
B
A
D
VDDX
X
GND
AB
C
PUN
PDN
C
D
D
ABCD
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ECE4121 Static CMOS Logic.19 ZALAM
OAI22 Layout
BA D
VDD
GND
C
X
Some functions have no consistent Euler path like
x = !(a + bc + de) (but x = !(bc + a + de) does!)
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ECE4121 Static CMOS Logic.20 ZALAM
Stick Diagrams
Contains no dimensionsRepresents relative positions of transistors
In
Out
V DD
GND
Inverter
A
Out
V DD
GND B
NAND2
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ECE4121 Static CMOS Logic.21 ZALAM
OAI22 Logic Graph
C
A B
X = (A+B)•(C+D)
B
A
D
C
DX
VDD
X
GND
AB
C
PUN
PDN
D
ABCD
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ECE4121 Static CMOS Logic.24 ZALAM
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ECE4121 Static CMOS Logic.25 ZALAM
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ECE4121 Static CMOS Logic.26 ZALAM
A simple method for finding the optimum gate ordering is the Euler-path method: Simply
find a Euler path in the pull-down network graph and a Euler path in the pull-up networkgraph with the identical ordering of input labels, i.e., find a common Euler path for bothgraphs. The Euler path is defined as an uninterrupted path that traverses each edge(branch) of the graph exactly once. Figure 3.12 shows the construction of a commonEuler path for both graphs in our example.
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ECE4121 Static CMOS Logic.27 ZALAM
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ECE4121 Static CMOS Logic.28 ZALAM
VTC is Data-Dependent
A
B
F= A • B
A B
M1
M2
M3 M4
Cint
VGS1 = VB
VGS2 = VA –VDS1
0
1
2
3
0 1 2
A,B: 0 -> 1B=1, A:0 -> 1A=1, B:0->1
0.5 μ /0.25 μ NMOS
0.75 μ /0.25 μ PMOS
The threshold voltage of M 2 is higher than M 1 due to thebody effect ( γ)
VTn2 = VTn0 + γ(√(|2 φF| + V int) - √|2 φF|)since V SB of M2 is not zero (when V B = 0) due to the presence of C int
VTn1 = VTn0
D
DS
S
weakerPUN
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ECE4121 Static CMOS Logic.30 ZALAM
Static CMOS Full Adder Circuit
B
B B
B B
B
B
B
A
A
A
A
A
A A
A
C in
C in
C in
Cin
C in
!Cout !Sum
!Cout = !C in & (!A | !B) | (!A & !B)
Cout = C in & (A | B) | (A & B)
!Sum = C out & (!A | !B | !C in) | (!A & !B & !C in)
Sum = !C out & (A | B | C in) | (A & B & C in)
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ECE4121 Static CMOS Logic.31 ZALAM
Next Time: Pass Transistor Circuits
B
A A B
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