Digital - TTL

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					01000110 01000001 01010111 01001110 01001001 01011010 01010101 01000001 01011010 01001101 01000001 01000100 01001001
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                                 Digital Electronics II

                              Integrated Circuit Technology
                                Tocci-Widmer, Chapter 8.
                                       Part I : TTL



01001000 01010101 01010011 01010011 01001001 01001110 01000000 01010101 01010100 01010000 00101111 00110000 00111001
01000110 01000001 01010111 01001110 01001001 01011010 01010101 01000001 01011010 01001101 01000001 01000100 01001001
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                                             Objectives
      • Read and understand digital IC terminology.
      • Compare characteristics of standard logic and logic
        families.
      • Interpret IC’s data sheets.
      • Determine IC characteristics such as fan-out, noise
        margins, etc.
      • As a start for us to the digital systems design.




                                                                                                            IC/TTL -   2
01001000 01010101 01010011 01010011 01001001 01001110 01000000 01010101 01010100 01010000 00101111 00110000 00111001
01000110 01000001 01010111 01001110 01001001 01011010 01010101 01000001 01011010 01001101 01000001 01000100 01001001
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                                      How BJT Works
      • Please refer to BJT Magic presentation.




                            click here

                                                                                                            IC/TTL -   3
01001000 01010101 01010011 01010011 01001001 01001110 01000000 01010101 01010100 01010000 00101111 00110000 00111001
01000110 01000001 01010111 01001110 01001001 01011010 01010101 01000001 01011010 01001101 01000001 01000100 01001001
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                                    TTL Logic Family
      • TTL – Transistor-Transistor Logic
      • Composed of Bipolar Junction                                                             VCC
        Transistors (BJT).                                              1                                  14

      • E.g. 7400 NAND gate.                                            2                                  13

                                                                        3                                  12

                                                        Totem pole      4                                  11

                                                                        5                                  10

                                                                        6                                  9

                                                                        7                                  8
                                                                             GND


                                                                                                               IC/TTL -   4
01001000 01010101 01010011 01010011 01001001 01001110 01000000 01010101 01010100 01010000 00101111 00110000 00111001
01000110 01000001 01010111 01001110 01001001 01011010 01010101 01000001 01011010 01001101 01000001 01000100 01001001
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                                     TTL NAND Gate
      • HIGH OUTPUT.
      • When either or both of the inputs are LOW.




                                                                                                            IC/TTL -   5
01001000 01010101 01010011 01010011 01001001 01001110 01000000 01010101 01010100 01010000 00101111 00110000 00111001
01000110 01000001 01010111 01001110 01001001 01011010 01010101 01000001 01011010 01001101 01000001 01000100 01001001
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                                     TTL NAND Gate
      • The basic TTL logic circuit. The simplest form in the family.
      • Let’s analyse. LOW OUTPUT




                                                                                    Note:
                                                                                    Review diode characteristics
                                                                                    if necessary.

                                                                                                                   IC/TTL -   6
01001000 01010101 01010011 01010011 01001001 01001110 01000000 01010101 01010100 01010000 00101111 00110000 00111001
01000110 01000001 01010111 01001110 01001001 01011010 01010101 01000001 01011010 01001101 01000001 01000100 01001001
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                   TTL in Action : Current-Sinking
      • Figure 8-9(a) shows conditions for
        the driving gate in the low state.
      • Transistor Q3 is an open circuit,
        while Q4 provides a very low
        resistance for the current flowing
        out of the driven gate’s input.
      • The input circuit is actually a
        forward biased diode and several
        mA of sink current flows.
      • Current flow in the base-emitter
        (diode) causes collector current to
        flow in Q1.




                                                                                                            IC/TTL -   7
01001000 01010101 01010011 01010011 01001001 01001110 01000000 01010101 01010100 01010000 00101111 00110000 00111001
01000110 01000001 01010111 01001110 01001001 01011010 01010101 01000001 01011010 01001101 01000001 01000100 01001001
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                  TTL in Action : Current-Sourcing
      • Figure 8-9(b) shows conditions for
        the driving gate in the high state.
      • Transistor Q4 is an open circuit,
        while Q3 provides a low resistance
        for the current flowing into the
        driven gate’s input.
      • The input circuit is now a reverse
        biased diode, and microamps of
        source current flows.
      • The source current is simply the
        reverse biased diode’s leakage
        current. Transistor Q1 is turned off




                                                                                                            IC/TTL -   8
01001000 01010101 01010011 01010011 01001001 01001110 01000000 01010101 01010100 01010000 00101111 00110000 00111001
01000110 01000001 01010111 01001110 01001001 01011010 01010101 01000001 01011010 01001101 01000001 01000100 01001001
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                        Other TTL Family Members
      • What about other logic gates?
      • TTL NOR. Read on your own (pp. 427-428)
      • TTL NOT. Any intelligent guess?
            – Basic structure is similar to NAND gate.
            – Difference in the input structure.
      • What about TTL AND and OR gates?
            – Not in our scope of study.
            – Once you understand the principles behind TTL NAND
              gate, everything comes in place.



                                                                                                            IC/TTL -   9
01001000 01010101 01010011 01010011 01001001 01001110 01000000 01010101 01010100 01010000 00101111 00110000 00111001
01000110 01000001 01010111 01001110 01001001 01011010 01010101 01000001 01011010 01001101 01000001 01000100 01001001
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                                           TTL Family
      • Standard TTL (74)
      • Schottky TTL (74S)
            – Schottky-clamped transistor, which allows faster ON OFF switching.
      • Low-Power Schottky TTL (74LS)
            – Low power, slower speed version
            – Schottky-clamped transistor, with larger resistor values than 74S
      • Advanced Schottky TTL (74AS)
            – Improvement in power and speed over 74LS
      • Advanced Low-Power Schottky TTL (74ALS)
            – Lowest speed-power product and the lowest gate power dissipation of
              all the TTL series
      • Fast TTL (74F)
            – New IC fabrication technique to reduce interdevice capacitance

                                                                                                           IC/TTL -   10
01001000 01010101 01010011 01010011 01001001 01001110 01000000 01010101 01010100 01010000 00101111 00110000 00111001
01000110 01000001 01010111 01001110 01001001 01011010 01010101 01000001 01011010 01001101 01000001 01000100 01001001
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                                    What is Schottky?
      • RC time constant contributes to propagation delay.
      • Another speed reducing mechanism is “storage time” delay.
            – When a transistor is fully turned ON (saturation), the collector-base
              depletion zone is literally saturated with carriers (electrons/holes).
            – The transistor can’t switch to OFF until those carriers can be cleared
              from the depletion zone. It takes time to sweep the carriers out.
      • That’s where Schottky diode comes into play
            – Schottky diode is on the next page.
            – The trick is to clamp the collector to the base with a Schottky diode.
            – The Schottky diode turns ON and bypasses much of the current
              (carriers) that would otherwise be stored in the depletion zone.
      • 74S is six times faster than 74 series, due to Schottky diode.


                                                                                                           IC/TTL -   11
01001000 01010101 01010011 01010011 01001001 01001110 01000000 01010101 01010100 01010000 00101111 00110000 00111001
01000110 01000001 01010111 01001110 01001001 01011010 01010101 01000001 01011010 01001101 01000001 01000100 01001001
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                                       Schottky Diode
      • Walter Schottly: Physicist – solid-state physics and electronics.
      • Schottky diode prevents the transistor from going into
        saturation.
      • Analogy: Prevents deep-sleep. Only “tidur ayam”.




                                                                                                           IC/TTL -   12
01001000 01010101 01010011 01010011 01001001 01001110 01000000 01010101 01010100 01010000 00101111 00110000 00111001
01000110 01000001 01010111 01001110 01001001 01011010 01010101 01000001 01011010 01001101 01000001 01000100 01001001
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                             TTL Loading & Fan-out
      • Concept behind loading and fan-out limitation.
            – When output is LOW, Q4 ON resistance is small but not zero.
            – Current IOL produces voltage drop VOL across transistor Q4.
            – VOL must not exceed VOL (max) limit of the IC.
            – If it does, we cannot guarantee a
              valid logic output.
            – IOL is the sum of all IIL currents
              from load gates’ inputs.
            – This limits the maximum value of
              IOL and thus the number of loads
              that can be driven.
            – In datasheet, max. current is a value
              that will not cause VOL to exceed
              VOL(max).

                                                                                                           IC/TTL -   13
01001000 01010101 01010011 01010011 01001001 01001110 01000000 01010101 01010100 01010000 00101111 00110000 00111001
01000110 01000001 01010111 01001110 01001001 01011010 01010101 01000001 01011010 01001101 01000001 01000100 01001001
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                                   Example (Fan-out)
      •      A standard TTL gate can deliver a maximum output current
             of 0.4 mA (400 µA) in the high state.
      •      Each driven gate input requires a current of 40µA when the
                                                            µ
             driving gate output is high.
      •      Choose the correct answer:
            a)    IOL (max) = 400µA IIL(max) = 40µA
            b)    IOL (max) = 40µA IIL(max) = 400µA
            c)    IOH (max) = 400µA IIH(max) = 40µA
            d)    IOH (max) = 40µA IIH(max) = 400µA


      •      Refer to page 7 if you’re not sure
             what the current terminologies mean.
      •      Calculate the fan-out.

                                                                                                           IC/TTL -   14
01001000 01010101 01010011 01010011 01001001 01001110 01000000 01010101 01010100 01010000 00101111 00110000 00111001
01000110 01000001 01010111 01001110 01001001 01011010 01010101 01000001 01011010 01001101 01000001 01000100 01001001
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                            The Truth About Fan-out
      • The driving gate’s maximum output current in the low state is very different
        from its maximum output current in the high state.
      • In some cases the fan-out in the two states is a little different. In that case,
        whichever state has the smaller fan-out number rules.
      • In each case the output current of
        the driving gate is equal to the sum
        of all of the input currents.
      • The fan-out number is the same
        for all gates within a given sub-
        family; for example the fan-out
        of all 74LSXX gates is the same.
        If you try to mix gates such as
        74XX and 74LSXX in the same
        circuit, you can run into fan-out
        problems.

                                                                                                           IC/TTL -   15
01001000 01010101 01010011 01010011 01001001 01001110 01000000 01010101 01010100 01010000 00101111 00110000 00111001
01000110 01000001 01010111 01001110 01001001 01011010 01010101 01000001 01011010 01001101 01000001 01000100 01001001
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                                            TTL Issues
      •   Unconnected inputs.
      •   Unused inputs.
      •   Tied-together inputs.
      •   E.g. You need 2-input NAND but you have 3-input NAND.
      •   E.g. You need inverters but you have 3-input NAND and 2-input NOR.
      •   So you have one of the following cases, as illustrated by the figure.




                                                                                                           IC/TTL -   16
01001000 01010101 01010011 01010011 01001001 01001110 01000000 01010101 01010100 01010000 00101111 00110000 00111001
01000110 01000001 01010111 01001110 01001001 01011010 01010101 01000001 01011010 01001101 01000001 01000100 01001001
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               Tied-Together Inputs Affect Fan-out
                          Calculations
      • When TTL inputs on the same gate are connected together, they will
        represent a load that is the sum of the load currents of each individual input.
      • Except, for NAND and AND gates. The LOW-state input load will be the
        same as a single input, no matter how many inputs are tied together.
                                                            • Total current through both
                                                              inputs goes through resistor R1.

                                                                    VCC − VON (diode) − VON (Q4 )
                                                            • IR1 =
                                                                                 R1
                                                                       5 − 0.7 − 0.2
                                                                     ≈
                                                                            4k
                                                                     ≈ 1mA

                                                                                                           IC/TTL -   17
01001000 01010101 01010011 01010011 01001001 01001110 01000000 01010101 01010100 01010000 00101111 00110000 00111001
01000110 01000001 01010111 01001110 01001001 01011010 01010101 01000001 01011010 01001101 01000001 01000100 01001001
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                                 Example 2 (Fan-out)
      • What is the total # of input loads
            – When X is at HIGH logic?
            – When X is at LOW logic?


      • Assume all logic gates are from Schottky
        family with the following characteristics
            – IIH = 20 µA
            – IIL = 0.4 mA
            – Determine total load current on gate 1
              on both HIGH and LOW states.


      • How can we further reduce the load current?

                                                                                                           IC/TTL -   18
01001000 01010101 01010011 01010011 01001001 01001110 01000000 01010101 01010100 01010000 00101111 00110000 00111001

				
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Description: The meaning of Transistor-Transistor Logic or TTL logic family. After that, we will learned about current sinking and current sourcing in TTL family members. Next, the concept of schottky Diode, TTL loading and Fan-Out.