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7_DVD_Combinational_MOS_Logic_Circuits.pdf

CMOS, Digital VLSI Design, Combinational Circuits, Pass Transistor, Pass Gate

1 of 133
11/22/2023 CMOS Combination Design 1
CMOS
Combinational Logic Design
Prof. (Dr.) Usha Mehta
usha.mehta@nirmauni.ac.in
11/22/2023 CMOS Combination Design 2
Parameters:
• Depending on the application, the emphasis will be on different metrics
(e.g.,). In addition to these
metrics, robustness to noise is also a very important consideration.
11/22/2023 CMOS Combination Design 3
Parameters/Specifications
• Boolean Function : combinational logic (or non-regenerative)
circuits that have the property that at any point in time, the output
of the circuit is related to its current input signals by some Boolean
expression
• Speed: in high performance processor, the switching speed of
digital circuits is the primary metric
• Power : In a battery operated circuit, the energy and power
dissipation is major concern
• Area
• Noise Margin
11/22/2023 CMOS Combination Design 4
Static CMOS Design
• CMOS advantages:
• Full swing output: Robustness to noise
• No steady state power
• High performance
• Static Design: static circuits in which at every point in
time (except during the switching transients), each gate
output is connected to either VDD or Vss via a low-
resistance path. Also, 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)
11/22/2023 CMOS Combination Design 5
Before we start……
• Logic with Switching Circuits….
• Logic can be done with switches
as well as gates.
• A parallel connection implements OR. A series connection
implements AND. Series and parallel combinations can do
complex logic.
• Loop Analysis
• Construct all paths between a logic “1” and the output. Each
path is a string of ANDs. which are ORed together. The
expression comes out as a Sum-of-Products (Σ of Π)
• Cut-Set Analysis
• Make all the cuts that completely separate the output and
Vcc.The cuts must only pass through switches. The switches
in the cut are ORed together. The expression comes out as a
Product-of-Sums (Π of Σ)
11/22/2023 CMOS Combination Design 6
In case you forgot…..

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7_DVD_Combinational_MOS_Logic_Circuits.pdf

  • 1. 11/22/2023 CMOS Combination Design 1 CMOS Combinational Logic Design Prof. (Dr.) Usha Mehta usha.mehta@nirmauni.ac.in
  • 2. 11/22/2023 CMOS Combination Design 2 Parameters: • Depending on the application, the emphasis will be on different metrics (e.g.,). In addition to these metrics, robustness to noise is also a very important consideration.
  • 3. 11/22/2023 CMOS Combination Design 3 Parameters/Specifications • Boolean Function : combinational logic (or non-regenerative) circuits that have the property that at any point in time, the output of the circuit is related to its current input signals by some Boolean expression • Speed: in high performance processor, the switching speed of digital circuits is the primary metric • Power : In a battery operated circuit, the energy and power dissipation is major concern • Area • Noise Margin
  • 4. 11/22/2023 CMOS Combination Design 4 Static CMOS Design • CMOS advantages: • Full swing output: Robustness to noise • No steady state power • High performance • Static Design: static circuits in which at every point in time (except during the switching transients), each gate output is connected to either VDD or Vss via a low- resistance path. Also, 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)
  • 5. 11/22/2023 CMOS Combination Design 5 Before we start…… • Logic with Switching Circuits…. • Logic can be done with switches as well as gates. • A parallel connection implements OR. A series connection implements AND. Series and parallel combinations can do complex logic. • Loop Analysis • Construct all paths between a logic “1” and the output. Each path is a string of ANDs. which are ORed together. The expression comes out as a Sum-of-Products (Σ of Π) • Cut-Set Analysis • Make all the cuts that completely separate the output and Vcc.The cuts must only pass through switches. The switches in the cut are ORed together. The expression comes out as a Product-of-Sums (Π of Σ)
  • 6. 11/22/2023 CMOS Combination Design 6 In case you forgot…..
  • 8. 11/22/2023 CMOS Combination Design 8 Switch Model
  • 9. 11/22/2023 CMOS Combination Design 9 MOS as Switch
  • 10. 11/22/2023 CMOS Combination Design 10 Pull Up / Pull Down
  • 11. 11/22/2023 CMOS Combination Design 11 nMOS-Pull Down pMOS-Pull Up
  • 12. 11/22/2023 CMOS Combination Design 12 Pull Up / Pull Down….. • For nMOS • Vgs >= Vtn • Vg-Vs>=Vtn • Vsmax = Vg-Vtn i.e. ( 0 to Vg-Vtn only, never Vg=VDD full is coming to Vs) • For pMOS • Vgs =< Vtp • Vg-Vs =< Vtp • Vsmin = Vg-Vtp i.e. ( Vtp to Vg only, never Vg=0 is coming to Vs)
  • 13. 11/22/2023 CMOS Combination Design 13 CMOS Inverter
  • 14. 11/22/2023 CMOS Combination Design 14 Determine the logic….
  • 15. 11/22/2023 CMOS Combination Design 15 Logic to Voltages
  • 16. 11/22/2023 CMOS Combination Design 16 Source and Drain Terminals
  • 17. 11/22/2023 CMOS Combination Design 17 nMOS-pMOS/Series-Parallel
  • 18. 11/22/2023 CMOS Combination Design 18 NAND2
  • 19. 11/22/2023 CMOS Combination Design 19 NAND2…
  • 20. 11/22/2023 CMOS Combination Design 20 NAND2 & NOR2
  • 21. 11/22/2023 CMOS Combination Design 21 Comparisons of CMOS NOR/NAND with inverter • Similarity • Rail-to-rail output swing • No static power dissipation • Differences • DC voltage transfer characteristic • Noise Margin • These two are complicated as it depends on data input patterns
  • 22. 11/22/2023 CMOS Combination Design 22 VTC of CMOS NAND2 1. A=0, B=0 A=0->1, B=0->1 A=1, B=1 2. A=0, B=1 A=0->1, B=1 A=1, B=1 3. A=1, B=0 A=1, B=0->1 A=1, B=1
  • 23. 11/22/2023 CMOS Combination Design 23 Let’s understand the difference in VTC • The difference between VTC1 and VTC2 or VTC3 is because of strong pull up network of two pMOS conducting together.
  • 24. 11/22/2023 CMOS Combination Design 24 Let’s understand the difference in VTC • The difference between VTC2 and VTC3 is because of the location difference M1 and M2 • When M1and M2 both are on, the M2 has a body effect because of VB2=0V, VS2=VDS1 • Thus switching characteristic of M1 is better than
  • 25. 11/22/2023 CMOS Combination Design 25 Which is better? NAND or NOR? Why? • Considering propagation delay…. • Do remember • For pMOS and nMOS : • Hence nMOS is speedier than pMOS. • So overall delay of CMOS is delay of pMOS • So to reduce the delay of pMOS, reduce the Ron of pMOS and hence widen the pMOS i.e. double size. • R 1/IDD , IDD W/L => R 1/(W/L)
  • 26. 11/22/2023 CMOS Combination Design 26 Which is better? NAND or NOR? Why? • Now using inverter model, • Delay • For NAND • Worst case rise time delay is • Worst case ( and only) fall time delay is • For NOR • Worst case (and only) rise time delay is : • Worst case fall time delay is: • Consider Rn = 1 Unit, analyse delay for NOR and NAND • in case of pMOS and nMOS of equal size • Consider widening of pMOS
  • 27. 11/22/2023 CMOS Combination Design 27 Gates with large Fan-ins • The earlier slide’s analysis draws the attentions towards the deficiency of CMOS for large Fan-in gates • Larger Fan-ins means • Either the series connected pMOSes or nMOSes • Large difference in rise time and fall time • Need to find out solutions..
  • 29. 11/22/2023 CMOS Combination Design 29 Solution 1 Transistor Sizing
  • 30. 11/22/2023 CMOS Combination Design 30 Solution 2 Progressive Transistor Sizing
  • 31. 11/22/2023 CMOS Combination Design 31 Solution 3 Transistor Ordering
  • 32. 11/22/2023 CMOS Combination Design 32 Primitive Gates • INVERTER, NAND, NOR • AND, OR • What about XOR, XNOR? • Draw transistor level Schematic
  • 33. 11/22/2023 CMOS Combination Design 33 Six Transistors XOR-XNOR
  • 34. 11/22/2023 CMOS Combination Design 34 Tristate
  • 35. 11/22/2023 CMOS Combination Design 35 Tristate Inverter
  • 36. 11/22/2023 CMOS Combination Design 36 COMPLEX Boolean FUNCTIONS AND ITS TRANSISTOR LEVEL SCHEMATIC
  • 37. 11/22/2023 CMOS Combination Design 37 Complimentary Static CMOS Gates • At any time, the output is connected to either power supply or ground with low resistance path. • Conduction of Pull-up network (PUN)and Pull- down network (PDN) should be mutually exclusive. (Why?) • PDN and PUN are dual. • Complimentary static CMOS are inverting.
  • 38. 11/22/2023 CMOS Combination Design 38 Implementation of Combinational Logic
  • 39. 11/22/2023 CMOS Combination Design 39 Implementation of Combinational Logic
  • 40. 11/22/2023 CMOS Combination Design 40 Implementation of Combinational Logic
  • 41. 11/22/2023 CMOS Combination Design 41 Implementation of Combinational Logic
  • 42. 11/22/2023 CMOS Combination Design 42 Implementation of Combinational Logic
  • 43. 11/22/2023 CMOS Combination Design 43 CMOS Topology AOI/OAI
  • 44. 11/22/2023 CMOS Combination Design 44 DO REMEBER • Find and simplify F’ • Make sure that complements are down to the literal level. • Implement F’ as nMOS net and connect it between ground and output • OR operations by parallel connected nMOS • AND operations are series connected nMOS • Find dual of F’, implement it as pMOS net and connect it between output and power supply. • AND operations by parallel connected pMOS • OR operations are series connected pMOS
  • 45. 11/22/2023 CMOS Combination Design 45 Optimize the transistor level schematic for # of transistor • F’ • F • Literals are available in normal form only • Literals are available in normal form and complement form.
  • 46. 11/22/2023 CMOS Combination Design 46 Stick Diagram for CMOS Combinational Circuits
  • 47. 11/22/2023 CMOS Combination Design 47 CMOS Structure
  • 48. 11/22/2023 CMOS Combination Design 48 gate drain source nMOS Layout polysilicon metal Contact holes diffusion (active region)
  • 49. 11/22/2023 CMOS Combination Design 49 Stick Diagram • Stick diagrams help plan layout quickly • Need not be to scale • Draw with color pencils or dry-erase markers • Estimate area by counting wiring tracks and other areas Vin Vout VDD GND
  • 50. 11/22/2023 CMOS Combination Design 50 Stick Diagram Colour Notation Silicon layers are typically colour coded as follows : This colour representation is used during mask layer definition Translation from circuit format to a mask layout (and vice-versa) is relatively straightforward diffusion (device well, local interconnect) polysilicon (gate electrode, interconnect) metal (contact, interconnect) contact windows depletion implant P well (CMOS devices)
  • 51. 11/22/2023 CMOS Combination Design 51 Layer contact mask layout representation A transistor is formed when device well is crossed by polysilicon. Device well oxide : thin gate oxide Metal contacting diffusion Metal contacting polysilicon Metal contacting diffusion (no contact, electricall isolated with thick oxide) Metal crossing polysilicon (no contact, electrically isolated with thick oxide and so can carry separate voltages) diffusion polysilicon metal contact windows depletion implant P well
  • 52. 11/22/2023 CMOS Combination Design 52 Transistor Mask Layout Preparation A transistor is formed when device well is crossed by polysilicon. Device well oxide : thin gate oxide Depletion mode transistor (extra well implant to provide Vth  -0.6Vdd ) Enhancement mode transistor (Vth  0.2Vdd ) diffusion polysilicon metal contact windows depletion implant P well
  • 53. 11/22/2023 CMOS Combination Design 53 nMOS transistor coloured stick diagram
  • 54. 11/22/2023 CMOS Combination Design 54 nMOS Inverter Stick Diagram
  • 55. 11/22/2023 CMOS Combination Design 55 CMOS Inverter
  • 56. 11/22/2023 CMOS Combination Design 56 Static CMOS NAND2 Gate Stick Diagram
  • 57. 11/22/2023 CMOS Combination Design 57 Static NOR2 Gate Stick Diagram
  • 58. 11/22/2023 CMOS Combination Design 58 Static CMOS Design Example Layout
  • 59. 11/22/2023 CMOS Combination Design 59 Layout 2 (Different layout style to previous but same function being implemented)
  • 60. 11/22/2023 CMOS Combination Design 60 Two Stick Diagrams of F = (C*(A+B))’ A B C X VDD GND X C A B VDD GND uninterrupted diffusion strip crossover requiring vias
  • 61. 11/22/2023 CMOS Combination Design 61 To construct a minimum area layout… • For a complex logic function, if we choose arbitrary ordering of polysilicon gates column: • The separation between the polysilicon columns must allow • Diffusion to diffusion separation • Diffusion to metal separation • Hence, to reduce the number of diffusion area break, the ordering of gate column should be properly planned.
  • 62. 11/22/2023 CMOS Combination Design 62 Let’s try….. • F=(A(D+E)+BC)’
  • 63. 11/22/2023 CMOS Combination Design 63 Stick Diagram
  • 64. 11/22/2023 CMOS Combination Design 64 Euler Path • Find a common Euler path for both p and n graphs • Euler Path: • Uninterrupted path which traverse each edge of the graph exactly once.
  • 65. 11/22/2023 CMOS Combination Design 65 p-NET and n-NET
  • 66. 11/22/2023 CMOS Combination Design 66 Euler’s Path
  • 67. 11/22/2023 CMOS Combination Design 67 Stick Diagram as per Euler’s path
  • 68. 11/22/2023 CMOS Combination Design 68 Euler’s Theorem
  • 69. 11/22/2023 CMOS Combination Design 69 Euler’s Theorem B A D VDD GND C X
  • 70. 11/22/2023 CMOS Combination Design 70 Let’s have one magic!!!!! • Pl. find the Euler path for x = !(a + bc + de) • Ok….. • Now try x = !(bc + a + de)
  • 71. 11/22/2023 CMOS Combination Design 71 Duality of Function • Pl. find Euler’s path for • F=!(AB+BC+CA) • Now try with duality concept…. • F = dual of F • Now pl. check the n and p graph and find the Euler’s path • So easy to draw the stick diagram…
  • 72. 11/22/2023 CMOS Combination Design 72 Transistor level Schematic of Full Adder
  • 73. 11/22/2023 CMOS Combination Design 73 LAYOUT (W/L is a big concern here!)
  • 74. 11/22/2023 CMOS Combination Design 74 CMOS Inverter Mask Layout (using Microwind) diffusion polysilicon metal contact windows depletion implant P well
  • 75. 11/22/2023 CMOS Combination Design 75 Static CMOS NOR2 gate
  • 76. 11/22/2023 CMOS Combination Design 76 Static CMOS NOR2 gate
  • 77. 11/22/2023 CMOS Combination Design 77 CMOS NAND2 Mask Layout diffusion polysilicon metal contact windows depletion implant P well
  • 78. 11/22/2023 CMOS Combination Design 78 Layout Design rules & Lambda () • Lambda () : distance by which a geometrical feature or any one layer may stay from any other geometrical feature on the same layer or any other layer. • All processing factors are included plus a safety margin. •  used to prevent IC manufacturing problems due to mask misalignment • or exposure & development variations on every feature, which otherwise could lead to : • over-diffusion • over-etching • inadvertent transistor creation etc •  is the minimum dimension which can be accurately re-produced on the silicon wafer for a particular technology.
  • 79. 11/22/2023 CMOS Combination Design 79 Layout Design rules & Lambda () • Minimum photolithographic dimension (width, not separation) is 2. • Hence, the minimum channel length dimension is 2. • Where a 0.25m gate length is quoted,  is 0.125 microns (m). • Minimum distance rules between device layers are used, e.g., • polysilicon  metal • metal  metal • diffusion  diffusion and • minimum layer overlaps • Layout design rule checker (DRC) automatically verifies that no design rules have been broken • Note however, the use of Lambda is not optimal but supports design reuse
  • 80. 11/22/2023 CMOS Combination Design 80 Layout Design rules & Lambda () Lambda based design: half of technology since 1985. As technology changes with smaller dimensions, a simple change in the value of  can be used to produce a new mask set. 6 2 6  4 Hcmos6 technology : =0.2µm Hcmos8 technology : =0.1µm All device mask dimensions are based on multiples of , e.g., polysilicon minimum width = 2. Minimum metal to metal spacing = 3
  • 81. 11/22/2023 CMOS Combination Design 81 Basic Design Rules • Minimize spared diffusion • Use minimum poly width (2) •1 contact = 1mA •Multiply contacts 2mA
  • 82. 11/22/2023 CMOS Combination Design 82 Basic Design Rules • Same N and P alters symmetry • L min • Wpmos=2 Wnmos Width of pMOS should be twice the width of nMOS
  • 83. 11/22/2023 CMOS Combination Design 83 Calculation of Size Basically calculation of W/L…….. all other size is with reference to W/L
  • 84. 11/22/2023 CMOS Combination Design 84 Design Strategy • Fan-out, Rise time, Fall time… • Load Current, VOL, VOH..… • Depends on W/L and other parameters… • Designable parameter is (W/L). • Find the worst case of above parameters and design for that (W/L)
  • 85. 11/22/2023 CMOS Combination Design 85 • Performing a manual analysis of the dynamic behavior of complex gates is only tractable via a switch model. • Here, the transistor is modelled as a switch with an infinite off-resistance and a finite on resistance, Ron. • Ron is chosen so that the equivalent RC-circuit has a propagation delay identical to the original transistor- capacitor model. • Ron is inversely proportional to the W/L ratio but varies during the switching transient. • Deriving propagation delay can be done by analysing the RC network.
  • 87. 11/22/2023 CMOS Combination Design 87 W/L Equivalent (Resistive Load Model) 𝑊 𝐿 𝑒𝑞𝑖 = ෍ 𝑘(𝑜𝑛) 𝑊 𝐿 𝑘 𝑊 𝐿 𝑒𝑞𝑢𝑖 = 1 σ𝐾(𝑜𝑛) 1 (𝑊/𝐿)𝑘
  • 88. 11/22/2023 CMOS Combination Design 88 Calculate (W/L)equivalent for resistive load implementation of F=!(A(D+E)+BC)
  • 89. 11/22/2023 CMOS Combination Design 89 Let’s try….. • Calculate (W/L)equivalent for CMOS implementation of F=!(A+D+E)(B+C) assuming (W/L)n = 10 and (W/L)p=15
  • 90. 11/22/2023 CMOS Combination Design 90 Various VOL Values
  • 91. 11/22/2023 CMOS Combination Design 91 To calculate W/L…. • Specify maximum allowable VOL value • Calculate equivalent (W/L)driver using for that VOL • Determine worst case path (class-1) • Determine worst case path transistor size to give the equivalent worst path VOL same as equivalent (W/L)driver
  • 92. 11/22/2023 CMOS Combination Design 92 Layout Optimization Device Folding / Fingering & Sharing • When we have to make the devices for large current (like drivers), the width of the devices should be very high compared to the other devices • Large transistors can be split into smaller ones and then shorting the corresponding terminals making up the required W/L • In such an arrangement we can share the diffusion drain and source of adjacent transistors and then we can short the terminals
  • 93. 11/22/2023 CMOS Combination Design 93 Folding & Sharing (Cont...) Why to have folding / fingering? ➢Poly resistance is reduced as single poly is divided in multiple parts of poly ➢To maintain the uniformity in diffusion area ➢Process variations are less Why to have sharing? ➢To save the diffusion area ➢To reduce the parasitic associated with the devices
  • 94. 11/22/2023 CMOS Combination Design 94 Transistor Folding
  • 95. 11/22/2023 CMOS Combination Design 95 Folding
  • 96. 11/22/2023 CMOS Combination Design 96 Sharing…..
  • 97. 11/22/2023 CMOS Combination Design 97 Static CMOS Do remember… • Every point in time, gate output is connected to either supply or ground via low resistance path • Rail to rail output voltage • Ratioless design • Low output and Extremely High input impedance • No static power dissipation • Good Noise Margin • BUT what about Rise and Fall Time????
  • 98. 11/22/2023 CMOS Combination Design 98 • The complementary CMOS circuit style falls under a broad class of logic circuits called static circuits in which at every point in time (except during the switching transients), each gate output is connected to either VDD or Vss via a low-resistance path. • Also, 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, that relies on temporary storage of signal values on the capacitance of high-impedance circuit nodes. The latt r approach has the advantage that the resulting gate is simpler and faster. On the other hand, its design and operation are more involved than those of its static counterpart, due to an increased sensitivity to noise.
  • 99. 11/22/2023 CMOS Combination Design 99 Static CMOS Do remember………
  • 100. 11/22/2023 CMOS Combination Design 100 Gates with a fan-in greater than 4 become excessively slow and must be avoided. Solutions????
  • 102. 11/22/2023 CMOS Combination Design 102 Pseudo nMOS Gates • Only single pMOS in load, permanently gate of pMOS connected to ground • Disadvantages: • Always on load i.e. steady state current, static power • o/p voltage < Vdd • Ratioed logic
  • 103. 11/22/2023 CMOS Combination Design 103 Ratioed Logic
  • 105. 11/22/2023 CMOS Combination Design 105 Pass Transistor • Strength of signal • How close it approximates ideal voltage source • VDD and GND rails are strongest 1 and 0 • What if source > 0, gate >0 ? • e.g. pass transistor passing VDD • Let Vg = VDD • Now if Vs > VDD-Vt, Vgs < Vt • Hence transistor would turn itself off • nMOS pass strong 0 • But degraded or weak 1 • pMOS pass strong 1 • But degraded or weak 0 • NMOS pass transistors pull-up no higher than VDD-Vtn • Called a degraded “1” • Approach degraded value slowly (low Ids) • PMOS pass transistors pull-down no lower than Vtp • Called a degraded “0” VDD VDD 22-11-2023 Dynamic Logic Usha Mehta 105
  • 106. 11/22/2023 CMOS Combination Design 106 Pass Transistors • Transistors can be used as switches g s d g = 0 s d g = 1 s d 0 strong 0 Input Output 1 degraded 1 g s d g = 0 s d g = 1 s d 0 degraded 0 Input Output strong 1 g = 1 g = 1 g = 0 g = 0 22-11-2023 Dynamic Logic Usha Mehta 106
  • 107. 11/22/2023 CMOS Combination Design 107 But do remember…. VDD VDD VSS VDD VDD VDD VDD VDD VDD VDD VDD Vs = VDD -Vtn VSS Vs = |Vtp | VDD VDD -Vtn VDD -Vtn VDD -Vtn VDD VDD VDD VDD VDD VDD -Vtn VDD -2Vtn 22-11-2023 Dynamic Logic Usha Mehta 107
  • 108. 11/22/2023 CMOS Combination Design 108 Logic Functions using Pass Transistors • As nMOS (or even pMOS) acts as a switch, we can implement any Boolean Function using them. • Primary inputs drive the gate terminals + source/drain terminals. • In contrast to static CMOS –primary inputs drive gate terminals. • With one nMOS only….What is the function? A B Z = ? 22-11-2023 Dynamic Logic Usha Mehta 108
  • 109. 11/22/2023 CMOS Combination Design 109 Examples…. • 2:1 MUX • Using gate level implementation in CMOS: • 20 • Using transistor level implementation in CMOS: • 10 • Using pass transistor: • 04 4 4 D1 D0 S Y 4 2 2 2 Y 2 D1 D0 S 22-11-2023 Dynamic Logic Usha Mehta 109
  • 110. 11/22/2023 CMOS Combination Design 110 Shannon’s Expansion to implement Boolean Function • Theorem: An arbitrary logic function f (x1, x2, …, xn) is expanded as follows: f = xi ’f0 + xi f1 f0 = f(x1, x2, …, xi-1, 0, xi+1, …, xn-1, xn) f1 = f(x1, x2, …, xi-1, 1, xi+1, …, xn-1, xn) f0 and f1 are “cofactors” of f NOTE: Although this theorem is named after Claude Shannon who made the extremely insightful observation that Boolean algebra could be used to model switching circuits, the theorem was actually presented by mathematician George Boole in 1854. 22-11-2023 Dynamic Logic Usha Mehta 110
  • 111. 11/22/2023 CMOS Combination Design 111 Complimentary Pass Transistor Logic • To accept and produce true and complimentary inputs and outputs 22-11-2023 Dynamic Logic Usha Mehta 111
  • 112. 11/22/2023 CMOS Combination Design 112 CPL Gates 22-11-2023 Dynamic Logic Usha Mehta 112
  • 113. 11/22/2023 CMOS Combination Design 113 CPL…. • Since circuit is differential, complimentary inputs and outputs are available. Although generating differential signals require extra circuitry, complex gates such as XORs, MUXs and adders can be realized efficiently. • CPL is a static gate, because outputs are connected to Vdd or GND through a low-resistance path (high noise resilience). • Design is modular – all gates use same topology; only inputs are permuted. This facilitates the design of a library of gates. 22-11-2023 Dynamic Logic Usha Mehta 113
  • 114. 11/22/2023 CMOS Combination Design 114 NAND 4 Lay out • Design is modular…. 114 Pass Transistor Usha Mehta 22-11-2023
  • 115. 11/22/2023 CMOS Combination Design 115 Comparison of Pass Transistor with CMOS • The presence of the switch driven by B is essential to ensure that the gate is static – a low-impedance path must exist to supply rails. Adv.: • Fewer devices to implement some functions. • Example: AND2 requires 4 devices (including inverter to invert B) vs. 6 for complementary CMOS (lower total capacitance). Disadv.: • NMOS is effective at passing a 0, but poor at pulling a node to Vdd. When the pass transistor a node high, the output only charges up to Vdd- Vtn. This becomes worse due to the body effect. The node will be charged up to Vdd – Vtn (Vs) 22-11-2023 Dynamic Logic Usha Mehta 115
  • 116. 11/22/2023 CMOS Combination Design 116 Main Problem on NMOS only Switches • VB does not pull up to 2.5V, but 2.5V -VTN • Threshold voltage loss causes • static power consumption • slower transition 22-11-2023 Dynamic Logic Usha Mehta 116
  • 117. 11/22/2023 CMOS Combination Design 117 Solution:1 Level Restoring Transistor • Advantage: Full Swing. Eliminates static power in inverter + static power through level restorer and pass transistor, since restorer is only active when A is high. • Restorer adds capacitance, takes away pull down current at X – contention between Mn and Mr (slower switching). Hence Mr must be sized small. Mn and Mr must be sized such that the voltage at node X drops below the threshold of the inverter VM, which is a function in the sizes of M1 and M2. 22-11-2023 Dynamic Logic Usha Mehta 117
  • 118. 11/22/2023 CMOS Combination Design 118 Solution:2 Pass Transistor with Lower VT • Use very low threshold values for NMOS pass transistors, and standard high threshold devices for inverters. DisAdv.: Leakage Current • While these leakage paths are not critical when the device is switching constantly, they do pose a large energy overhead when the circuit is in the ideal state. 22-11-2023 Dynamic Logic Usha Mehta 118
  • 119. 11/22/2023 CMOS Combination Design 119 Solution: 3 Voltage Bootstrapping • This technique is being used to overcome the thresold voltage drop in Pass Transistor gates or enhancement load logic gates • Let’s consider the enhancement load inverter, • if Vx is increased to Vdd + Vth then Vout will be Vdd. 22-11-2023 Dynamic Logic Usha Mehta 119
  • 120. 11/22/2023 CMOS Combination Design 120 Voltage Bootstrapping • Do you remember voltage double or voltage trippler using diodes and capacitors? • i.e. Diode and capacitors can increase the voltage level!!!! 22-11-2023 Dynamic Logic Usha Mehta 120
  • 121. 11/22/2023 CMOS Combination Design 121 Solution: 4 Pass Gate/Transmission Gate • NMOS passes a strong “0” but degraded “1” • PMOS passes a strong “1” but degraded “0” • Transmission gates enable rail-to-rail swing • These gates are particularly efficient in implementing MUXs 22-11-2023 Dynamic Logic Usha Mehta 121
  • 122. 11/22/2023 CMOS Combination Design 122 Pass Gate 22-11-2023 Dynamic Logic Usha Mehta 122
  • 123. 11/22/2023 CMOS Combination Design 123 Equivalent Resistance of Pass Gate 22-11-2023 Dynamic Logic Usha Mehta 123
  • 124. 11/22/2023 CMOS Combination Design 124 Equivalent Resistance of Pass Gate 22-11-2023 Dynamic Logic Usha Mehta 124
  • 125. 11/22/2023 CMOS Combination Design 125 Equivalent Resistance of Pass Gate 22-11-2023 Dynamic Logic Usha Mehta 125
  • 126. 11/22/2023 CMOS Combination Design 126 Transmission Gate 2:1 MUX • NonRestoring: • Noise on A is passed on to Y (after several stages, the noise may degrade the signal beyond recognition) • Nonrestoring mux uses two transmission gates • Only 4 transistors 22-11-2023 Dynamic Logic Usha Mehta 126
  • 127. 11/22/2023 CMOS Combination Design 127 Transmission Gate 4:1 MUX • 4:1 mux chooses one of 4 inputs using two selects • Two levels of 2:1 muxes 22-11-2023 Dynamic Logic Usha Mehta 127
  • 128. 11/22/2023 CMOS Combination Design 128 Inverting MUX • Inverting multiplexer • Use compound AOI22 • Or pair of tristate inverters • Essentially the same thing • Noninverting multiplexer adds an inverter S D0 D1 Y S D0 D1 Y 0 1 S Y D0 D1 S S S S S S 22-11-2023 Dynamic Logic Usha Mehta 128
  • 129. 11/22/2023 CMOS Combination Design 129 XOR Gate using Pass Gate Topology 22-11-2023 Dynamic Logic Usha Mehta 129
  • 130. 11/22/2023 CMOS Combination Design 130 Complimentary Pass Gate Logic 22-11-2023 Dynamic Logic Usha Mehta 130
  • 131. 11/22/2023 CMOS Combination Design 131 Which function is implemented here? 22-11-2023 Dynamic Logic Usha Mehta 131