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Integrated Circuit Interfacing

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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 III : Interfacing
                                          8-11, 8-12, 8-15 to 8-17



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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                                             Objectives
      • Understand and handle some of the issues when interfacing
        different logic families
      • To use open-collector / open-drain circuits in interfacing.




                                                                                                        IC/INTERFACE -   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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                                             Example 1
      • There are situations when several devices must share a single wire to
        transmit a signal to some destination device as shown in figure below.
      • What is wrong with the circuit shown below.

                                  A                                      Z



                                  B


      • Conventional CMOS and TTL outputs should never be connected together.
      • When one output is HIGH and the other is LOW, we have HIGH/LOW
        conflict. Which one will win?
      • They both lose.


                                                                                                        IC/INTERFACE -   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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                                   Example 1 (cont’d)
      • Gate A output HIGH, Gate B output LOW. We have a conduction path.
      • CMOS: Output voltage is VDD/2—invalid. Lots of current will damage
        CMOS output after a period of time.
      • TTL: Transistor with lower ON resistance will draw current far greater than
        it is rated to handle.




                                                                                                        IC/INTERFACE -   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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                                   Example 1 (cont’d)
      • Gate A output HIGH, Gate B output LOW. We have a conduction path.
      • CMOS: Output voltage is VDD/2—invalid. Lots of current will damage
        CMOS output after a period of time.
      • TTL: Transistor with lower ON resistance will draw current far greater than
        it is rated to handle.



               VON = 0.2 V
               VR ≥ 5 – 0.4 – 0.4
                   ≥ 4.2 V
               IR ≥ 4.2 V / 130
                  ≥ 32.3 mA




                                                                                                        IC/INTERFACE -   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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                  Solution 1: Open Collector/Drain
      • Solution is to remove active pull-up transistor from the gate’s output. The
        gates may only pull-down voltage output.
      • Must use external pull-up resistor to replace the missing internal pull-up
        transistor.




                                                                                                        IC/INTERFACE -   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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                     Open Collector/Drain Notation
      • IEEE/ANSI standard notation.




                                                                                                        IC/INTERFACE -   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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                                     OC/OD Example
      •   LED needs 20 mA for normal brightness.
      •   What value of RS should we use?
      •   Q1: Which state of A will turn LED ON?
      •   Q2: Can the IC handle the required current?
      •   Q3: RS = ?



                                                                          A




                                                                                                        IC/INTERFACE -   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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                                         IC Interfacing
      • Interfacing means connecting the output(s) of one circuit or
        system to the input(s) of another circuit or system that has
        different electrical characteristics.
      • Often direct connection cannot be made because of the
        difference in the electrical characteristics of the driver circuit
        that is providing the output signal and the load circuit that is
        receiving the signal.
      • An interface circuit is a circuit connected between the driver
        and the load. It conditions the output signal so that it is
        compatible with the requirement of the load.




                                                                                                        IC/INTERFACE -   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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                      IC Voltage/Current Characteristics
                                   Input/Output Voltage Levels (in Volts) with VDD = VCC = +5 V.

                                              CMOS                                                          TTL
Parameter     4000B      74HC     74HCT      74AC      74ACT      74AHC     74AHCT       74        74LS     74AS   74ALS         74F
 VIH (min)    3.5        3.5        2.0        3.5       2.0        3.85      2.0        2.0        2.0     2.0        2.0        2.0
VIL (max)     1.5        1.0        0.8        1.5       0.8        1.65      0.8        0.8        0.8     0.8        0.8        0.8
VOH (min)     4.95       4.9        4.9        4.9       4.9         4.4      3.15       2.4        2.7     2.7        2.5        2.5
VOL (max)     0.05       0.1        0.1        0.1       0.1        0.44       0.1       0.4        0.5     0.5        0.5        0.5
    VNH       1.45       1.4        2.9        1.4       2.9        0.55      1.15       0.4        0.7     0.7        0.5        0.5
    VNL       1.45       0.9        0.7        1.4       0.7        1.21       0.7       0.4        0.3     0.3        0.3        0.3


                                          Input/Output Current Levels with VDD = VCC = +5 V.

                                        CMOS                                                        TTL
                                74HC/      74AC/         74AHC/
Parameter       4000B            HCT        ACT           AHCT           74           74LS          74AS      74ALS            74F
 IIH (max)       1 µA            1 µA       1 µA          1 µA          40 µA         20 µA         20 µA      20 µA         20 µA
  IIL (max)      1 µA            1 µA       1 µA          1 µA         1.6 mA        0.4 mA        0.5 mA     100 µA         0.6 mA
IOH (max)       0.4 mA          4 mA       24 mA          8 mA         0.4 mA        0.4 mA         2 mA      400 mA          1 mA
 IOL (max)      0.4 mA          4 mA       24 mA          8 mA         16 mA          8 mA         20 mA       8 mA          20 mA




                                                                                                                       IC/INTERFACE -   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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                                  TTL driving CMOS
      • 74HCT, 74ACT, AHCT family: OK.
      • Current requirement: OK
      • Voltage requirement
            – LOW state: OK
            – HIGH state: NOT COMPATIBLE
      • Solution: External pull-up resistor as interface circuit to increase TTL
        voltage output.




                                                                                                      IC/INTERFACE -   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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                             TTL driving HV-CMOS
      • Some CMOS IC’s can be operated at +5 V < VDD < +15 V.
      • If CMOS operates at VDD > 5 V, interfacing would become slightly more
        difficult.
      • E.g. IC’s used with Relays (usually 12 V) or 12-Volt-motor.
      • Most TTL outputs cannot be connected to pull-up resistors with more than 5V.
      • Open-collector interface is used.




                                                                                                      IC/INTERFACE -   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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                                  CMOS driving TTL
      • Case 1: HIGH State
            – CMOS output voltage is adequate.
            – CMOS output current is adequate.


      • Case 2: LOW State
            – CMOS output voltage is adequate.
            – TTL input current ranges from 100 µA to 1.6 mA
            – 74HC and 74HCT: can sink up to 4 mA. Reliable to drive at least 2 TTL
              loads from any series.
            – 4000B cannot drive most of TTL series, except 74ALS
            – 74AC/ACT, 74AHC/AHCT have current drive capability comparable to
              the TTL series.


                                                                                                      IC/INTERFACE -   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 2
      • What’s wrong with the circuit?




                                                                                                      IC/INTERFACE -   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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                                             Example 3
      • What’s wrong with the circuit?




                                                                                                      IC/INTERFACE -   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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                          A Special Interface Circuit
      • Analogue Voltage Comparator.
      • Interfaces analogue output to the input of a digital system.
      • Read Section 8-18 on your own (3-pages).




                                                                                                      IC/INTERFACE -   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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                                              Summary
      • All digital logic devices are similar in nature but very much
        different regarding the details of their characteristics.
      • Understanding the fundamental operations inside TTL and
        CMOS integrated circuits are essential in designing functional
        and reliable digital systems.
      • Understanding the issues when interfacing digital circuits are
        even more important.
      • Now, we will proceed to study MSI circuits that are commonly
        used in digital systems design.




                                                                                                      IC/INTERFACE -   17
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Description: This article will explain how the IC or CMOS interface in circuit. How TTL will driving CMOS and how CMOS driving TTL. The concept open collector and open drain also will be explained.