# Integrated Circuit Interfacing

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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

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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.

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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.

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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.

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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

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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.

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Open Collector/Drain Notation
• IEEE/ANSI standard notation.

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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

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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.

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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

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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.

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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.

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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
– 4000B cannot drive most of TTL series, except 74ALS
– 74AC/ACT, 74AHC/AHCT have current drive capability comparable to
the TTL series.

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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?

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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?

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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.

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01001000 01010101 01010011 01010011 01001001 01001110 01000000 01010101 01010100 01010000 00101111 00110000 00111001
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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.

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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.