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ECE 555 Digital Circuits & Components
ECE555ECE555Lecture 5Lecture 5
Nam Sung KimUniversity of Wisconsin – Madison
Dept. of Electrical & Computer Engineering
1
RATIOED LOGICRATIOED LOGIC
2
ECE 555 Digital Circuits & Components
Ratioed LogicRatioed Logic
3
VDD
VSS
PDNIn1In2In3
F
RLLoad
VDD
VSS
In1In2In3
F
VDD
VSS
PDNIn1In2In3
F
VSS
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
ECE 555 Digital Circuits & Components
Ratioed LogicRatioed Logic
4
VDD
VSS
PDN
In1
In2
In3
F
RLLoad
ResistiveN transistors + Load
• VOH = VDD
• VOL = RPN
RPN + RL
• Assymetrical response
• Static power consumption
•
• tpL= 0.69 RLCL
ECE 555 Digital Circuits & Components
Active LoadsActive Loads
5
VDD
VSS
In1In2In3
F
VDD
VSS
PDN
In1In2In3
F
VSS
PDN
Depletion
LoadPMOSLoad
depletion load NMOS pseudo-NMOS
VT < 0
ECE 555 Digital Circuits & Components
Pseudo-NMOSPseudo-NMOS
6
VDD
A B C D
FCL
VOH = VDD (similar to complementary CMOS)
kn VDD VTn– VOL
VOL2
2-------------–
kp
2------ VDD VTp– 2=
VOL VDD VT– 1 1kpkn------–– (assuming that VT VTn VTp )= = =
SMALLER AREA & LOAD BUT STATIC POWER DISSIPATION!!!
ECE 555 Digital Circuits & Components
Revist: PLA SchematicRevist: PLA Schematic
7
ECE 555 Digital Circuits & Components
Pseudo-NMOS VTCPseudo-NMOS VTC
8
ECE 555 Digital Circuits & Components
Improved LoadsImproved Loads
9
A B C D
F
CL
M1M2 M1 >> M2Enable
VDD
Adaptive Load
ECE 555 Digital Circuits & Components
Improved Loads (2)Improved Loads (2)
10
VDD
VSS
PDN1
Out
VDD
VSS
PDN2
Out
AABB
M1 M2
Differential Cascode Voltage Switch Logic (DCVSL)
ECE 555 Digital Circuits & Components
DCVSL ExampleDCVSL Example
11
B
A A
B B B
Out
Out
XOR-NXOR gate
ECE 555 Digital Circuits & Components
DCVSL Transient ResponseDCVSL Transient Response
12
DYNAMIC CMOS DESIGNDYNAMIC CMOS DESIGN
13
ECE 555 Digital Circuits & Components
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
14
ECE 555 Digital Circuits & Components
Dynamic GateDynamic Gate
15
In1
In2 PDN
In3
Me
Mp
Clk
Clk
Out
CL
Out
Clk
Clk
A
BC
Mp
Me
Two phase operation
Precharge (CLK = 0)
Evaluate (CLK = 1)
ECE 555 Digital Circuits & Components
Dynamic GateDynamic Gate
16
In1
In2 PDN
In3
Me
Mp
Clk
Clk
Out
CL
Out
Clk
Clk
A
BC
Mp
Me
Two phase operation
Precharge (Clk = 0)
Evaluate (Clk = 1)
on
off
1
off
on
((AB)+C)
ECE 555 Digital Circuits & Components
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
17
ECE 555 Digital Circuits & Components
Properties of Dynamic GatesProperties 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
18
ECE 555 Digital Circuits & Components
Properties of Dynamic GatesProperties 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 clock19
ECE 555 Digital Circuits & Components
Issues in Dynamic Design 1:Issues in Dynamic Design 1: Charge Leakage
20
CL
Clk
Clk
Out
A
Mp
Me
Leakage sources
CLK
VOut
Precharge
Evaluate
Dominant component is subthreshold current
ECE 555 Digital Circuits & Components
Solution to Charge LeakageSolution to Charge Leakage
21
CL
Clk
Clk
Me
Mp
A
B
Out
Mkp
Same approach as level restorer for pass-transistor logic
Keeper
ECE 555 Digital Circuits & Components
Issues in Dynamic Design 2:Issues in Dynamic Design 2: Charge Sharing
22
CL
Clk
Clk
CA
CB
B=0
A
OutMp
Me
Charge stored originally on CL is
redistributed (shared) over CL and
CA leading to reduced robustness
ECE 555 Digital Circuits & Components
Charge Sharing ExampleCharge Sharing Example
23
CL=50fF
Clk
Clk
A A
B B B !B
CC
Out
Ca=15fF
Cc=15fF
Cb=15fF
Cd=10fF
ECE 555 Digital Circuits & Components
Charge SharingCharge Sharing
24
Mp
Me
VDD
Out
A
B = 0
CL
Ca
Cb
Ma
Mb
X
CLVDD CLVout t Ca VDD VTn VX – +=
or
Vout Vout t VDD–CaCL-------- VDD VTn VX
– –= =
Vout VDD
CaCa 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
ECE 555 Digital Circuits & Components
Solution to Charge RedistributionSolution to Charge Redistribution
25
Clk
Clk
Me
Mp
A
B
OutMkp
Clk
Precharge internal nodes using a clock-driven transistor (at the
cost of increased area and power)
ECE 555 Digital Circuits & Components
Issues in Dynamic Design 3:Issues in Dynamic Design 3: Backgate Coupling
26
CL1
Clk
Clk
B=0
A=0
Out1Mp
Me
Out2
CL2
In
Dynamic NAND Static NAND
=1=0
ECE 555 Digital Circuits & Components
Backgate Coupling EffectBackgate Coupling Effect
27
ECE 555 Digital Circuits & Components
Issues in Dynamic Design 4Issues in Dynamic Design 4
28
CL
Clk
Clk
B
A
OutMp
Me
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.
ECE 555 Digital Circuits & Components
Clock FeedthroughClock Feedthrough
29
-0.5
0.5
1.5
2.5
0 0.5 1
Clk
Clk
In1
In2
In3
In4
Out
In &
ClkOut
Time, ns
Vol
tage
Clock feedthrough
Clock feedthrough
ECE 555 Digital Circuits & Components
Cascading Dynamic GatesCascading Dynamic Gates
30
Clk
Clk
Out1
In
Mp
Me
Mp
Me
Clk
Clk
Out2
V
t
Clk
In
Out1
Out2V
VTn
Only 0 1 transitions allowed at inputs!
ECE 555 Digital Circuits & Components
Domino LogicDomino Logic
31
In1
In2 PDN
In3
Me
Mp
Clk
ClkOut1
In4 PDN
In5
Me
Mp
Clk
ClkOut2
Mkp
1 1
1 0 0 0
0 1
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
ECE 555 Digital Circuits & Components
Why Domino?Why Domino?
32
Like falling dominos!
ECE 555 Digital Circuits & Components
Designing with Domino LogicDesigning with Domino Logic
33
Mp
Me
VDD
PDN
Clk
In1
In2
In3
Out1
Clk
Mp
Me
VDD
PDN
Clk
In4
Clk
Out2
Mr
VDD
Inputs = 0
during precharge
Can be eliminated!
ECE 555 Digital Circuits & Components
Differential (Dual Rail) DominoDifferential (Dual Rail) Domino
34
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
ECE 555 Digital Circuits & Components
NP-CMOSNP-CMOS
35
In1
In2 PDN
In3
Me
Mp
Clk
ClkOut1
In4 PUN
In5
Me
MpClk
Clk
Out2
(to PDN)
1 1
1 0
0 0
0 1
Only 0 1 transitions allowed at inputs of PDN Only 1
0 transitions allowed at inputs of PUN