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B sc cs i bo-de u-iv logic families

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B sc cs i bo-de u-iv logic families

  1. 1. Unit:4 Logic families & Semiconductors Basics of Digital Electronics Course: B.Sc.(CS) Sem.:1st
  2. 2. Logic FamiliesLogic Families  Logic Family :A collection of different IC’s that have similar circuit characteristics  The circuit design of the basic gate of each logic family is the same  The most important parameters for evaluating and comparing logic families include :  Logic Levels  Power Dissipation  Propagation delay  Noise margin  Fan-out ( loading )
  3. 3. Example Logic FamiliesExample Logic Families  General comparison or three commonly available logic families. the most important to understand
  4. 4. Implementing Logic CircuitsImplementing Logic Circuits  There are several varieties of transistors – the building blocks of logic gates – the most important are:  BJT (bipolar junction transistors)  one of the first to be invented  FET (field effect transistors)  especially Metal-Oxide Semiconductor types (MOSFET’s)  MOSFET’s are of two types: NMOS and PMOS
  5. 5. Transistor Size ScalingTransistor Size Scaling Performance improves as size is decreased: shorter switching time, lower power consumption. 2 orders of magnitude reduction in transistor size in 30 years.
  6. 6. Moore’s LawMoore’s Law  In 1965, Gordon Moore predicted that the number of transistors that can be integrated on a die would double every 18 to 14 months  i.e., grow exponentially with time  Considered a visionary – million transistor/chip barrier was crossed in the 1980’s  2300 transistors, 1 MHz clock (Intel 4004/4040) - 1971  42 Million transistors, 2 GHz clock (Intel P4) - 2001  140 Million transistors, (HP PA-8500)
  7. 7. Moore’s Law and IntelMoore’s Law and Intel From Intel’s 4040 (2300 transistors) to Pentium II (7,500,000 transistors) and beyond
  8. 8. TTL and CMOSTTL and CMOS  Connecting BJT’s together gives rise to a family of logic gates known as TTL  Connecting NMOS and PMOS transistors together gives rise to the CMOS family of logic gates BJT MOSFET (NMOS, PMOS) TTL CMOS transistor types logic gate families
  9. 9. Electrical Parameters And InterpretationElectrical Parameters And Interpretation Of Data SheetsOf Data Sheets  Voltages and Currents  Noise Margin  Power Dissipation  Propagation Delay  Speed-Power Product  Fan-In, Fan-Out  Comparison of Logic Families  Interpretation of Data Sheets
  10. 10. Electrical CharacteristicsElectrical Characteristics  TTL  faster (some versions)  strong drive capability  rugged  CMOS  lower power consumption  simpler to make  greater packing density  better noise immunity • Complex IC’s contain many millions of transistors • If constructed entirely from TTL type gates would melt • A combination of technologies (families) may be used • CMOS has become most popular and has had greatest development
  11. 11. Voltage & CurrentVoltage & Current  For a High-state gate driving a second gate, we define:  VOH (min), high-level output voltage, the minimum voltage level that a logic gate will produce as a logic 1 output.  VIH (min), high-level input voltage, the minimum voltage level that a logic gate will recognize as a logic 1 input. Voltage below this level will not be accepted as high.  IOH, high-level output current, current that flows from an output in the logic 1 state under specified load conditions.  IIH, high-level input current, current that flows into an input when a logic 1 voltage is applied to that input. Ground VIH VOH IOH IIH Test setup for measuring values
  12. 12. Voltage & CurrentVoltage & Current  For a Low-state gate driving a second gate, we define:  VOL (max), low-level output voltage, the maximum voltage level that a logic gate will produce as a logic 0 output.  VIL (max), low-level input voltage, the maximum voltage level that a logic gate will recognize as a logic 0 input. Voltage above this value will not be accepted as low.  IOL , low-level output current, current that flows from an output in the logic 0 state under specified load conditions.  IIL , low-level input current, current that flows into an input when a logic 0 voltage is applied to that input. Inputs are connected to Vcc instead of Ground Ground V IL VOL I OL I IL
  13. 13. Electrical CharacteristicsElectrical Characteristics  Important characteristics are:  VOHmin min value of output recognized as a ‘1’  VIHmin min value input recognized as a ‘1’  VILmax max value of input recognized as a ‘0’  VOLmax max value of output recognized as a ‘0’  Values outside the given range are not allowed. logic 0logic 0 logic 1logic 1 indeterminateindeterminate input voltageinput voltage
  14. 14. Logic Level & Voltage RangeLogic Level & Voltage Range  Typical acceptable voltage ranges for positive logic 1 and logic 0 are shown below  A logic gate with an input at a voltage level within the ‘indeterminate’ range will produce an unpredictable output level. Logic 1 Logic 0 5.0V 0V 2.5V Indeterminate 0.8V TTL Logic 1 Logic 0 5.0V Indeterminate 0V 1.5V CMOS 3.5V
  15. 15. Noise MarginNoise Margin  Manufacturers specify voltage limits to represent the logical 0 or 1.  These limits are not the same at the input and output sides.  For example, a particular Gate A may output a voltage of 4.8V when it is supposed to output a HIGH but, at its input side, it can take a voltage of 3V as HIGH.  In this way, if any noise should corrupt the signal, there is some margin for error.
  16. 16. Noise MarginNoise Margin  If noise in the circuit is high enough it can push a logic 0 up or drop a logic 1 down into the indeterminate or “illegal” region  The magnitude of the voltage required to reach this level is the noise margin  Noise margin for logic high is:  NMH =VOHmin –VIHmin VOHmin VIHmin VILmax VOLmax logic 0logic 0 logic 1logic 1 indeterminateindeterminate input voltageinput voltage
  17. 17. Noise MarginNoise Margin  Difference between the worst case output voltage of one stage and worst case input voltage of next stage  Greater the difference, the more unwanted signal that can be added without causing incorrect gate operation NMNMhighhigh = V= VOHminOHmin - V- VIHminIHmin NMNMlowlow = V= VILmaxILmax - V- VOLmaxOLmax
  18. 18. Worked ExampleWorked Example  Given the following parameters, calculate the noise margin of 74LS series. Parameter 74LS VIH(min) 2V VIL(max) 0.8V VOH(min) 2.7V VOL(max) 0.4V Solution: High Level Noise Margin, VNH = VOH (min) - VIH (min)=2.7V-2.0V=0.7V Low Level Noise Margin, VNL = VIL (max) - VOL (max)=0.8V-0.4V=0.4V
  19. 19. Noise Margin & Noise ImmunityNoise Margin & Noise Immunity  Noise immunity of a logic circuit refers to the circuit’s ability to tolerate noise voltages on its inputs.  A quantitative measure of noise immunity is called noise margin  High Level Noise Margin,VNH =VOH (min) -VIH (min)  Low Level Noise Margin,VNL =VIL (max) -VOL (max) Logic 1 Logic 0 Logic 0 Logic 1 VOH (min) VOL (max) VIH (min) VIL (max) VNH VNL Output Voltage Ranges Input Voltage Ranges
  20. 20. Further Important CharacteristicsFurther Important Characteristics  The propagation delay (tpd) which is the time taken for a change at the input to appear at the output  The fan-out, which is the maximum number of inputs that can be driven successfully to either logic level before the output becomes invalid
  21. 21. Speed: Rise & Fall TimesSpeed: Rise & Fall Times  Rise Time  Time from 10% to 90% of signal, Low to High  Fall Time  Time from 90% to 10% of signal, High to Low rise time 10% 90% 90% 10% fall time
  22. 22. Speed: Propagation DelaySpeed: Propagation Delay  A logic gate always takes some time to change states  tPLH is the delay time before output changes from low to high  tPHL is the delay time before output changes from high to low  both tPLH & tPHL are measured between the 50% points on the input and output transitions 50% Input Output 0 0 tPHL tPLH
  23. 23. Power DissipationPower Dissipation  Static  I2 R losses due to passive components, no input signal  Dynamic  I2 R losses due to charging and discharging capacitances through resistances, due to input signal
  24. 24. Speed-Power ProductSpeed-Power Product  Speed (propagation delay) and power consumption are the two most important performance parameters of a digital IC.  A simple means for measuring and comparing the overall performance of an IC family is the speed-power product (the smaller, the better).  For example, an IC has  an average propagation delay of 10 ns  an average power dissipation of 5 mW  the speed-power product = (10 ns) x (5 mW) = 50 picoJoules (pJ)
  25. 25. Logic Family TradeoffsLogic Family Tradeoffs  Looking for the best speed/power product  tp and Pd are normally included in the data sheet for each device  Older logic families are the worst  CMOS is one of the best  FPGAs use CMOS
  26. 26. Comparison of Logic FamiliesComparison of Logic Families
  27. 27. TTL -TTL - ExampleExample SN74LS00SN74LS00  Recommended operating conditions  Vcc supply voltage 5V ± 0.5V  input voltages VIH = 2V VIL = 0.8V  Electrical Characteristics  output voltage VOH = 2.7V (worst case) VOL = 0.5V  max input currents IIH = 20µA IIL = -0.4mA  propagation delay tpd = 15 nS  noise margins for a logic 0 = 0.3V for a logic 1 = 0.7V  Fan-out 20 TTL loads 5 Volt 0 Volt 0.8 0.5 2.0 2.7 Input Range for 1 Input Range for 0 Output Range for 0 Output Range for 1
  28. 28. Fan-InFan-In  Number of input signals to a gate  Not an electrical property  Function of the manufacturing process NAND gate with a Fan-in of 8
  29. 29. Fan-OutFan-Out  A measure of the ability of the output of one gate to drive the input(s) of subsequent gates  Usually specified as standard loads within a single family  e.g., an input to an inverter in the same family  May have to compute based on current drive requirements when mixing families  Although mixing families is not usually recommended
  30. 30. VOH IIH Low VOL IIL High Current Sourcing and SinkingCurrent Sourcing and Sinking  Current-source : the driving gate produces a outgoing current  Current-sinking : the driving gate receives an incoming current
  31. 31. Fan-OutFan-Out  An illustration of fan-out and the associated source and sink currents
  32. 32. Worked ExampleWorked Example  How many 74LS00 NAND gate inputs can be driven by a 74LS00 NAND gate outputs ? Solution: Refer to data sheet of 74LS00, the maximum values of IOH = 0.4mA, IOL = 8mA, IIH = 20uA, and IIL = 0.4mA Hence, fan-out(high) = IOH(max) / IIH (max)=0.4mA/20uA=20 fan-out(low) = IOL(max) / IIL(max)=8mA/0.4mA=20, the overall fan-out = fan-out(high) or fan-out(low) whichever is lower. Hence, overall fan-out = 20
  33. 33. Gate Drive Capability: Fan-OutGate Drive Capability: Fan-Out  A logic gate can supply a maximum output current  IOH(max), in the high state or  IOL(max), in the low state  A logic gate requires a maximum input current  IIH(max), in the high state or  IIL(max), in the low state  Ratio of output and input current decide how many logic gates can be driven by a logic gate  fan-out(high) = IOH(max) / IIH (max)  fan-out(low) = IOL(max) / IIL(max)  overall fan-out = fan-out(high) or fan-out(low) whichever is lower  A typical figure of fan-out is ten (10)
  34. 34. Wired-ANDWired-AND  Open collector outputs connected together to a common pull-up resistor  Any collector can pull the signal line low  Logically an AND gate
  35. 35. Tri-State LogicTri-State Logic  Both output transistors of totem-pole output are turned off  Usually used to bus multiple signals on the same wire  Gates not enabled present high-Z to bus and therefore do not interfere with other gates putting signals on the bus
  36. 36. Tri-State LogicTri-State Logic  Tri-state logic includes a switch at the output  In the figure below, the three states are illustrated: a) Logic High output b) Logic Low output c) High impedance (Hi-Z) output
  37. 37. Electronic Combinational LogicElectronic Combinational Logic  Within each of these families there is a large variety of different devices  We can break these into groups based on the number gates per device AcronymAcronym DescriptionDescription No GatesNo Gates ExampleExample SSISSI Small-scale integrationSmall-scale integration <12<12 4 NAND gates4 NAND gates MSIMSI Medium-scale integrationMedium-scale integration 12 – 10012 – 100 AdderAdder LSILSI Large-scale integrationLarge-scale integration 100 – 1000100 – 1000 68006800 VLSIVLSI Very large-scale integrationVery large-scale integration 1000 – 1M1000 – 1M 6800068000 ULSIULSI Ultra large scale integrationUltra large scale integration > 1M> 1M 80486/8058680486/80586
  38. 38. SSI DevicesSSI Devices  Each package contains a code identifying the package N74LS00 Manufacturers Code N = National Semiconductors SN = Signetics Specification Family L LS H Member 00 = Quad 2 input NAND 02 = Quad 2 input Nor 04 = Hex Invertors 20 = Dual 4 Input NAND
  39. 39. 7400 Series History7400 Series History  1960s space program drove development of 7400 series  Consumed all available devices for internal flight computer  $1000 / device (1960 dollars)  10:1 integration improvement over discrete transistors  1963 Minuteman missile forced 7400 into mass production  Drove pricing down to $25 / circuit (1963 dollars)
  40. 40. 7400 Series Evolution7400 Series Evolution  BJT storage time reduction by using a BC Schottky diode.  Schottky diode has aVfw=0.25V.When BC junction becomes forward biased Schottky diode will bypass base current. B C
  41. 41. Too Much of a Good Thing?Too Much of a Good Thing? Families Packages Reliability options Speed grades Features Functions An availability nightmare! >> 500K unique devices
  42. 42. The World of TTLThe World of TTL
  43. 43. Success Drives ProliferationSuccess Drives Proliferation  New families introduced based on  Higher performance  Lower power  New features  New signaling threshold  Spawned over 32 unique families! 19602003
  44. 44. Success Drives ProliferationSuccess Drives Proliferation  Products introduced in the 1960 are near the end of their life cycle  Decreasing supplier base  Increasing prices  Not recommended for new designs  Products considered to be “mature” are about 2 decades into their life cycle  High-volume production  Multiple suppliers  Low prices  Newer products are only a few years into their life cycle  High performance  High level of vendor and supplier support  Newest technologies  Higher prices
  45. 45. Characteristics: TTL and MOSCharacteristics: TTL and MOS  TTL stands for Transistor-Transistor Logic  uses BJTs  MOS stands for Metal Oxide Semiconductor  uses FETs  MOS can be classified into three sub-families:  PMOS (P-channel)  NMOS (N-channel)  CMOS (Complementary MOS, most common) Remember:Remember:
  46. 46. TTL Circuit OperationTTL Circuit Operation A B Y O/P +Vcc Q 1 Q 2 Q 3 Q 4 4K 1.6K 130 R1 R2 R3 R4 1K I CQ1 D 3 D 1 D2 A B I CQ1 Q 1 Q 2 Q 3 Q 4 Y O/P 0 0 + ON OFF OFF ON 1 0 1 + ON OFF OFF ON 1 1 0 + ON OFF OFF ON 1 1 1 - OFF ON ON OFF 0 A standard TTL NAND gate circuit Table explaining the operation of the TTL NAND gate circuit
  47. 47. Transistor-Transistor Logic FamiliesTransistor-Transistor Logic Families  Transistor-Transistor Logic Families:  74L Low power  74H High speed  74S Schottky  74LS Low power Schottky  74AS Advanced Schottky  74ALS Advance Low power Schottky
  48. 48. MOS Circuit OperationMOS Circuit Operation +VDD O/P I/P S D D S Q Q 1 2 I/P Q1 Q2 O/P 0 ON OFF 1 1 OFF ON 0 Table explaining the operation of the CMOS inverter circuitA CMOS inverter circuit
  49. 49. CMOS Logic FamiliesCMOS Logic Families  CMOS Logic Families  40xx/45xx Metal-gate CMOS  74C TTL-compatible CMOS  74HC High speed CMOS  74ACT Advanced CMOS -TTL compatible
  50. 50. CMOS Family EvolutionCMOS Family Evolution  CMOS LogicTrend: Reduction of dynamic losses (cross-conduction, capacitive charge/discharge cycles) by decreasing supply voltages:  12V 5V 3.3V 2.5V 1.8V 1.5V …→ → → → →  Reduction of IC power dissipation is the key to:  lower cost (packaging)  higher integration  improved reliability
  51. 51. Comparison of Logic FamiliesComparison of Logic Families vi vo
  52. 52. Comparison Logic FamiliesComparison Logic Families
  53. 53. Comparison of Logic FamiliesComparison of Logic Families speed power product = a constant
  54. 54. References  Digital Logic and Computer Design – M. Morris Mano – Pearson  Fundamentals of Digital Circuits – A.Anand Kumar – PHI  Digital Electronics - Gothmen - PHI  Digital Electronics Principles - Malvino & Leech - MGH  Digital fundamentals - Thomes L.Floyd and Jain - Pearson  Modern Digital Electronics - R.P. Jain - TMH  Digital Electronics -Tokneinh - MGH
  55. 55. Web References  hyperphysics.phy-astr.gsu.edu/hbase/electronic/logfam.html  is.iiita.ac.in/study/Digital%20Systems %20Design/KBab9Martarizal.pdf  nptel.ac.in/courses/106108099//Digital%20Systems.pdf  www.owlnet.rice.edu/~dodds/Files331/digi_notes.pdf

Editor's Notes

  • So Far have been looking at how to specify digital circuits
    Now going to look at practical implementation
    CMOS - Complementary MOSFET
    MOSFET - Metal-Oxide Semiconductor Field Effect Transistor
    TTL - Transistor-Transistor Logic
  • for logic low is Vilmax - Volmax
  • for logic low is Vilmax - Volmax
  • So Far have been looking at how to specify digital circuits
    Now going to look at practical implementation
    CMOS - Complementary MOSFET
    MOSFET - Metal-Oxide Semiconductor Field Effect Transistor
    TTL - Transistor-Transistor Logic

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