Chapter 1 Introduction to CMOS Circuit Design
Transcript of Chapter 1 Introduction to CMOS Circuit Design
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Chapter 1 Chapter 1 ppIntroduction to CMOS Circuit Introduction to CMOS Circuit
DesignDesign
Jin-Fu LiAdvanced Reliable Systems (ARES) Lab.
Department of Electrical EngineeringN l C l UNational Central University
Jhongli, Taiwan
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OutlineIntroductionMOS Transistor SwitchesMOS Transistor SwitchesCMOS LogicCi i d R iCircuit and System Representation
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Introduction
AA
C
B
C=AxB
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C=AxB
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Switch: MOSFETMOSFETs are basic electronic devices used to direct and control logic signals in IC designto direct and control logic signals in IC design
MOSFET: Metal-Oxide-Semiconductor Field-Effect TransistorN-type MOS (NMOS) and P-type MOS (PMOS) Voltage-controlled switchesg
A MOSFET has four terminals: gate, source, drain and substrate (body)drain, and substrate (body)Complementary MOS (CMOS)
Usin t t p s f MOSFETs t t l i Using two types of MOSFETs to create logic networksNMOS & PMOS
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NMOS & PMOS
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Silicon Lattice and Dopant AtomsPure silicon consists of a 3D lattice of atoms
Silicon is a Group IV element and it forms covalent pbonds with four adjacent atomsIt is a poor conductor
N-type (P-type) semiconductorBy introducing small amounts of Group V-As (Group By ntroduc ng small amounts of Group V As (Group III-B) into the silicon lattice
Si SiSi
Si SiSi +
Si SiSi
As SiSi
--
Si SiSi
B SiSi
+
Si SiSi Si SiSi Si SiSi
L tti f L tti f N t L tti f P t
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Lattice of pure Silicon
Lattice of N-type Semiconductor
Lattice of P-type Semiconductor
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P-N JunctionsA junction between p-type and n-type semiconductor forms a diode.semiconductor forms a diode.Current flows only in one direction
p-type n-typep yp yp
anode cathodeanode cathode
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NMOS TransistorFour terminals: gate, source, drain, bodyGate–oxide–body stack looks like a capacitorGate oxide body stack looks like a capacitor
Gate and body are conductorsSiO2 (oxide) is a very good insulator2 ( ) y gCalled metal–oxide–semiconductor (MOS) capacitorEven though gate is no longer made of metal
GateSource DrainPolysilicon
SiO2
n+
p bulk Si
n+
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NMOS OperationsBody is commonly tied to ground (0 V)When the gate is at a low voltage:When the gate is at a low voltage:
P-type body is at low voltageSource body and drain body diodes are OFFSource-body and drain-body diodes are OFFNo current flows, transistor is OFF
GateSource DrainPolysilicon
SiO2
0n+
p bulk Si
n+D
0
S
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NMOS Operations (Cont.)When the gate is at a high voltage:
Positive charge on gate of MOS capacitorPositive charge on gate of MOS capacitorNegative charge attracted to bodyInverts a channel under gate to n-typeInverts a channel under gate to n typeNow current can flow through n-type silicon from source through channel to drain, transistor is ONsource through channel to dra n, trans stor s ON
GateSource DrainPolysilicon
SiO2
Polysilicon
n+
p bulk Si
n+D
1
S
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p bu S
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PMOS OperationsSimilar, but doping and voltages reversed
Body tied to high voltage (VDD)Body tied to high voltage (VDD)Gate low: transistor ONGate high: transistor OFFGate high: transistor OFFBubble indicates inverted behavior
SiO
GateSource DrainPolysilicon
SiO2
n bulk Si
p+ p+
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Threshold VoltageEvery MOS transistor has a characterizing parameter called the threshold voltage VTparameter called the threshold voltage VT
The specific value of VT is established during the manufacturing process the manufacturing process Threshold voltage of an NMOS and a PMOS
V
VADrainV
VA
NMOS PMOS
Source+ V
VA=1Mn On
VDD
VA
VGS
MnGate+
VA=1Mp Off
VDD
VDD-|VTp|VA
VGSp
MpGate
+
-
VDD
VA=0 Mn Off
VTn
0
VGSn
Source-
VA=0 Mp On0Drain
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Logic translationGate-source voltage Logic translationGate-source voltage
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MOS Transistor is Like a Tap…
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Source: Prof. Banerjee, ECE, UCSB
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MOS SwitchesNMOS symbol and characteristics
Vth5v
0v5v
0v5v-Vth
th
PMOS symbol and characteristicsy0v
0v5v
Vth
Vth
5v0v Vth
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CMOS SwitchA complementary CMOS switch
Transmission gateTransmission gate
-s -s
Ca b a b a b
-s
Symbols
s s s
0v5v
0v
5v0v
0v
Characteristics5v
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CMOS Logic-InverterThe NOT or INVERT function is often considered the simplest Boolean operationp p
F(x)=NOT(x)=x’ Vdd
Vin Vout Vin Vout
Vdd Vdd Vdd
0
Vdd Vdd Vdd
1 1 Vdd/2 Indeterminate0 1 1 0 Vdd/2 Indeterminatelogic level
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Combinational LogicSerial structure
S1=0S2 0
S1=0S2 1
S1=1S2 0
S1=1S2 1a
S1
S2=0 S2=1 S2=0 S2=1aS1
0
0 1
a!=b a!=b
S2S2
0
1
a!=b a!=b
a!=b a=b
S1=0S2=0
S1=0S2=1
S1=1S2=0
S1=1S2=1
b
a S1
S1
S2 0 S2 1 S2 0 S2 1a S1
0
0 1
a=b a!=b
S2
S21 a!=b a!=b
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b
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Combinational LogicParallel structure
S1=0S2=0
S1=0S2=1
S1=1S2=0
S1=1S2=1a S1
0 1
S1 S2 S20
1
a!=b a=b
a=b a=b
S1=0S2=0
S1=0S2=1
S1=1S2=0
S1=1S2=1
b
a S1
S1 S2
S2 0 S2 1 S2 0 S2 1
S20
0 1
a=b a=b
b
1 a=b a!=b
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b
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NAND Gate
Output AA
Output A
0
10
1 1B B
1
0 1 1
1 0
AB
OutputB
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NOR Gate
A
B
Output
A
0
10
1 0B
1
0 1 0
0 0
AB
OutputB
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Compound Gate))()(( CDABF +=
A BA B
C DAB
FF
C D B
CD
A
B
C
DB D
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Structured Logic DesignCMOS logic gates are intrinsically inverting
The output always produces a NOT operation The output always produces a NOT operation acting on the input variables
For example the inverter shown below For example, the inverter shown below illustrates this property
VDD1
f=0a=1
0
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Structured Logic DesignThe inverting nature of CMOS logic circuits allows us to construct logic circuits for AOI allows us to construct logic circuits for AOI and OAI expressions using a structured approachapproachAOI logic function
Implements the operations in the order AND then Implements the operations in the order AND then OR then NOTE g dcbadcbag )( +=E.g.,
OAI logic functionImplements the operations in the order OR then
dcbadcbag ..),,,( +
Implements the operations in the order OR then AND then NOTE g )()()( dcbadcbag +⋅+=
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E.g., )()(),,,( dcbadcbag ++=
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Structured Logic DesignBehaviors of nMOS and pMOS groups
Parallel-connected nMOS Parallel connected nMOS OR-NOT operations
Parallel-connected pMOSpAND-NOT operations
Series-connected nMOSAND-NOT operations
Series-connected pMOS OR-NOT operations
Consequently, wired groups of nMOS and pMOS are logical duals of another
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Dual PropertyIf an NMOS group yields a function of the form
)(b
then an identically wired PMOS array gives the
)( cbag +⋅=
then an identically wired PMOS array gives the dual function
h h AND d OR i h b
)( cbaG ⋅+=
where the AND and OR operations have been interchanged
h f NM PM This is an interesting property of NMOS-PMOS logic that can be exploited in some CMOS designs
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An Example of Structured Design)( dcbaX +⋅+=
VDD
cb
d
a
X
Group 1 Group 2
Group 3
ab
dc
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An Example of XOR GateBoolean equation of the two input XOR gate
this is not in AOI formbababa ⋅+⋅=⊕ , this is not in AOI formBut, , this is in AOI form Therefore
bababa +=⊕bababa ⋅+⋅=⊕
bbbb ⊕⊕ )(Therefore, babababa ⋅+⋅=⊕=⊕ )(
VDDb
VDDba
b
b
a
a
b a
b• •••
a aba ⊕
a aba ⊕• •••
b b b b
XOR Gate XNOR Gate
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XOR Gate XNOR Gate
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Multiplexer
1110
AB
AB
Y10
100100
BCD
Y
S S
-SS
A
S1 S0
AY
S YB
CB
-S
C
D
S1 -S1 S0 -S0
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S
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Static CMOS SummaryIn static circuits at every point in time (except when switching), the output is connected to g), peither Vdd or Gnd through a low resistance path
Fan-in of n (or n inputs) requires 2n (n N-type and n P-type) devices
Non-ratioed logic: gates operate independent of PMO NMO iPMOS or NMOS sizesNo path ever exists between Vdd and Gnd: low
static powerFully-restored logic (NMOS passes “0” only and PMOS passes “1” onlyGates must be inverting
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Latches
D -Q
CLKQ
Q -QD D
CLK
Q
-Q Q
Q Q
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Flip Flop
QMaster Slave
QD-Q
CLK
D D QQ-Q-Q
CLKCLK
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Circuit and System RepresentationsBehavioral representation
Functional high levelFunctional, high levelFor documentation, simulation, verification
Structural representationStructural representationSystem level – CPU, RAM, I/OFunctional level ALU Multiplier AdderFunctional level – ALU, Multiplier, AdderGate level – AND, OR, XORCi it l l T sist s R L CCircuit level – Transistors, R, L, CFor design & simulation
Ph i l iPhysical representationFor fabrication
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Behavior RepresentationA one-bit full adder (Verilog)
module fadder(sum,cout,a,b,ci);output sum, cout;i b iinput a, b, ci;reg sum, cout;
a b
always @(a or b or ci) beginsum = a^b^ci;
ci coutfadder
sum = a^b^ci;cout = (a&b)|(b&ci)|(ci&a);
end
sum
endendmodule
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Structure RepresentationA four-bit full adder (Verilog)
module adder4(s,c4,a,b,ci); a b( )output[3:0] sum;output c4;input[3:0] a b;
a[0] b[0] a[1] b[1] a[2] b[2] a[3] b[3]input[3:0] a, b;input ci;reg[3:0] s;
ci
s[0]
a0 a3a1 a2
s[1] s[2] s3]
co[0] co[1] co[2]
reg c4;wire[2:0] co;
fadder a0(s[0] co[0] a[0] b[0] ci);
s[0] s[1] s[2] s3]
s adder4fadder a0(s[0],co[0],a[0],b[0],ci);fadder a1(s[1],co[1],a[1],b[1],co[0]);fadder a2(s[2],co[2],a[2],b[2],co[1]);
adde
fadder a3(s[3],c4,a[3],b[3],co[2]);endmodule
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Physical RepresentationLayout of a 4-bit NAND gate
in1 in2 in3 in4
Vdd Vdd
in1 in2 in3 in4
in1Out
in2Out
in3
in4
in1 in2 in3 in4
Gnd
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in1 in2 in3 in4
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Design Flow for a VLSI Chip
S ifi tiSpecification
Function
Behavioral Design
Structural Design
Function
Function
Physical Design
FunctionTimingPower
Physical Design
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