What Is a Logic IC?

A logic IC is an integrated circuit that performs one or more digital logic functions.

Digital systems normally work with two primary logic states:

• Logic LOW
• Logic HIGH

These states are represented by voltage ranges rather than by one exact voltage.

Logic ICs can perform operations such as AND, OR, NOT, NAND, NOR and XOR, as well as more complex functions such as counting, storage, decoding and data selection.

Digital Logic

Digital logic uses combinations of binary states to process information.

A simplified representation is:

LOW  = 0
HIGH = 1

Logic gates process these states according to defined logical rules.

Logic Gates

The fundamental logic gates are:

• AND
• OR
• NOT
• NAND
• NOR
• XOR
• XNOR

These gates can be combined to create increasingly complex digital circuits.

AND Gate

An AND gate produces a HIGH output only when all of its inputs are HIGH.

Boolean expression:

Y = A · B

OR Gate

An OR gate produces a HIGH output when at least one input is HIGH.

Boolean expression:

Y = A + B

NOT Gate

A NOT gate inverts its input.

Boolean expression:

Y = NOT A

NAND Gate

A NAND gate is an AND gate followed by an inverter.

Its output is LOW only when all inputs are HIGH.

Y = NOT(A · B)

NAND gates are particularly important because universal logic can be constructed using NAND gates.

NOR Gate

A NOR gate is an OR gate followed by an inverter.

Its output is HIGH only when all inputs are LOW.

Y = NOT(A + B)

NOR gates can also be used to construct complete logic systems.

XOR Gate

An XOR gate produces a HIGH output when the inputs are different.

XNOR Gate

An XNOR gate produces a HIGH output when the inputs are equal.

Logic IC Families

Logic ICs are manufactured using different semiconductor technologies. Common logic families include:

• TTL
• CMOS
• BiCMOS
• Other specialized logic technologies

TTL Logic

TTL stands for Transistor-Transistor Logic. Traditional TTL logic uses bipolar transistors as the primary switching elements.

The classic 74xx series contains many widely used TTL logic devices.

Examples include:

• 7400
• 7402
• 7404
• 7408
• 7432
• 7486

CMOS Logic

CMOS stands for Complementary Metal-Oxide-Semiconductor. CMOS logic uses complementary MOSFET structures.

CMOS logic is known for very low static power consumption and very high input impedance.

Common CMOS logic families include the 4000 series and many modern 74-series CMOS families.

TTL vs CMOS

74 Series Logic ICs

The 74-series is one of the most widely recognized logic IC families.

Different generations and technologies exist, including:

• 74xx
• 74LS
• 74HC
• 74HCT
• 74AC
• 74ACT

The letters identify different electrical technologies and characteristics. They should not be treated as interchangeable without checking the datasheets.

4000 Series Logic ICs

The CMOS 4000 series contains a large range of digital logic devices.

Examples include:

• CD4001
• CD4011
• CD4013
• CD4017
• CD4026
• CD4040
• CD4060
• CD4066

These devices are commonly encountered in older electronic equipment, timers, counters and control circuits.

Logic IC Voltage Levels

A logic input does not normally recognize only one exact voltage. Instead, the manufacturer specifies voltage ranges for LOW and HIGH states.

The actual thresholds depend on the logic family and supply voltage.

Always use the datasheet values when connecting different logic families.

Logic HIGH

A logic HIGH is a voltage within the input's guaranteed HIGH-recognition range.

The exact voltage depends on the logic family.

Logic LOW

A logic LOW is a voltage within the input's guaranteed LOW-recognition range.

A voltage between the guaranteed LOW and HIGH regions may result in unpredictable operation.

Logic Noise Margin

Noise margin describes how much unwanted voltage can be added to a logic signal before it could be interpreted incorrectly.

Good noise margins improve reliability when signals travel through electrically noisy environments.

Logic IC Inputs

Logic inputs are generally designed to recognize HIGH and LOW voltage levels.

Unused inputs should not normally be left floating when the logic family requires defined input states.

Floating inputs can result in unpredictable behavior and increased susceptibility to noise.

Logic IC Outputs

A logic output drives the input of another digital circuit or a suitable load.

Important output specifications include:

• Output HIGH voltage
• Output LOW voltage
• Source current
• Sink current
• Propagation delay
• Rise time
• Fall time

Propagation Delay

A logic gate does not respond instantaneously.

The time between a change at the input and the corresponding change at the output is called propagation delay.

Input:
───────┐
       └────────

        delay
          ↓

Output:
──────────┐
          └──────

Propagation delay becomes particularly important in high-speed digital systems.

Logic IC Fan-Out

Fan-out describes how many standard logic inputs an output can reliably drive.

The actual fan-out depends on the output current capability and the input current requirements of the connected devices.

Tri-State Logic

A tri-state output can operate in three states:

• HIGH
• LOW
• High impedance

The high-impedance state effectively disconnects the output from the signal line.

Tri-state outputs are useful when multiple devices share a common data bus.

Open-Collector and Open-Drain Outputs

Some logic devices use open-collector or open-drain outputs.

These outputs normally require an external pull-up resistor when a HIGH logic level is required.

VCC
 │
 Rpullup
 │
 ├──────── Output
 │
Logic IC
 │
GND

These outputs can be useful for interfacing and for combining signals in certain logic arrangements.

Pull-Up Resistors

A pull-up resistor connects a logic signal to a positive supply so that the signal assumes a defined HIGH state when no device is actively pulling it LOW.

The resistor value must be selected according to the logic family, switching speed and connected load.

Pull-Down Resistors

A pull-down resistor connects a logic signal to ground so that the signal assumes a defined LOW state when no active device drives it HIGH.

Buffers

A logic buffer reproduces a logic signal while providing improved drive capability or isolation between circuit sections.

Buffers can be used to:

• Increase drive capability
• Isolate circuit sections
• Drive longer traces
• Improve signal integrity

Inverters

A logic inverter changes a LOW input into a HIGH output and a HIGH input into a LOW output.

A common example is the 7404 or one of its modern equivalents.

Logic IC Counters

Counter ICs advance through a sequence of digital states in response to clock pulses.

They are used in:

• Frequency dividers
• Digital clocks
• Event counters
• Timing circuits
• Display drivers

Logic IC Flip-Flops

Flip-flops are digital storage elements capable of retaining a binary state.

Common types include:

• SR flip-flop
• JK flip-flop
• D flip-flop
• T flip-flop

Flip-flops are fundamental building blocks for counters, registers and sequential logic.

Logic IC Latches

A latch is a digital storage element that can hold a logic state.

Unlike an edge-triggered flip-flop, a latch is generally level-sensitive.

Multiplexers

A multiplexer, or MUX, selects one of several input signals and connects the selected signal to an output.

Input 0 ──┐
Input 1 ──┤
Input 2 ──┤──► MUX ───► Output
Input 3 ──┘
       │
   Select lines

Multiplexers are used extensively in digital data-routing systems.

Demultiplexers

A demultiplexer performs the reverse function of a multiplexer.

It directs one input signal to one of several outputs according to the select inputs.

Decoders

A decoder converts a binary input code into one of several output states.

Applications include:

• Display driving
• Memory selection
• Address decoding
• Control systems

Encoders

An encoder performs the reverse type of operation by converting one of several active inputs into a binary code.

Encoders are used in keyboards, control panels and digital interfaces.

Logic IC Registers

Registers are groups of storage elements used to hold multiple bits of digital information.

They can be used for:

• Temporary data storage
• Serial-to-parallel conversion
• Parallel-to-serial conversion
• Data shifting
• Digital processing

Shift Registers

A shift register moves stored bits from one stage to another in response to clock pulses.

Common configurations include:

• Serial-in serial-out
• Serial-in parallel-out
• Parallel-in serial-out
• Parallel-in parallel-out

Clock Signals

Sequential logic circuits often operate according to a clock signal.

     ┌───┐   ┌───┐   ┌───┐
─────┘   └───┘   └───┘   └──

The rising or falling edge of the clock can be used to trigger digital operations depending on the circuit.

Schmitt Trigger Logic ICs

A Schmitt-trigger input provides hysteresis.

This means that different threshold levels are used for transitions from LOW to HIGH and HIGH to LOW.

Schmitt triggers are useful for cleaning up slowly changing or noisy signals.

Logic IC Oscillators

Logic gates can be combined with resistors and capacitors to create simple oscillators.

Schmitt-trigger gates are particularly useful for this type of circuit.

The resulting signal can be used as a clock or timing signal in simple digital circuits.

Logic IC Applications

• Digital clocks
• Frequency counters
• Timers
• Control systems
• Computers
• Embedded electronics
• Digital displays
• Keyboards
• Communication systems
• Industrial automation
• Consumer electronics
• Measurement equipment
• Motor controllers

Testing Logic ICs

A multimeter can perform some basic checks, but a complete functional test usually requires applying known logic signals and observing the outputs.

Useful equipment includes:

• Digital multimeter
• Logic probe
• Oscilloscope
• Function generator
• Logic analyzer
• Stable power supply

Checking the Power Supply

Always check the IC's supply voltage first.

An incorrect supply can cause apparently mysterious logic failures.

Check:

• VCC
• GND
• Supply ripple
• Supply current

Testing a Logic Gate

A basic gate can be tested by applying known input states and checking the output against its truth table.

For example, an AND gate should behave as:

00 → 0
01 → 0
10 → 0
11 → 1

If one or more combinations produce incorrect results, the IC or surrounding circuit may be faulty.

Logic IC Faults

Floating Logic Inputs

A floating digital input is an input that is not connected to a defined logic HIGH or LOW level.

Floating inputs can produce unpredictable results.

Where required, use an appropriate pull-up or pull-down arrangement to establish a defined state.

Logic IC Decoupling

Digital ICs can generate rapid current transients when their internal switches change state.

A small bypass capacitor placed close to the supply pins can help reduce supply disturbances.

Poor decoupling can cause:

• False triggering
• Unstable operation
• Logic errors
• Oscillation
• Interference with nearby circuits

Logic IC Replacement

When replacing a logic IC, verify more than the part number.

• Logic function
• Pinout
• Supply voltage
• Logic thresholds
• Output drive capability
• Propagation delay
• Input compatibility
• Package
• Operating temperature

TTL to CMOS Replacement

A TTL device cannot automatically be replaced by a CMOS device merely because both have the same logic function.

Important differences can include:

• Input thresholds
• Output voltage levels
• Input current
• Output current
• Supply voltage
• Switching speed

The interface between the surrounding circuits must be checked carefully.

How to Select a Logic IC

1. Identify the required logic function.
2. Determine the supply voltage.
3. Determine the required number of gates or channels.
4. Check input voltage requirements.
5. Check output drive capability.
6. Check propagation delay.
7. Check switching frequency.
8. Check package type.
9. Check operating temperature.
10. Check compatibility with the existing logic family.

Logic IC Package Types

Logic ICs are available in many packages.

• DIP
• SOIC
• TSSOP
• SSOP
• QFN
• QFP
• SOT packages

The package must be compatible with the PCB or adapter being used.

Common Logic IC Numbers

Logic IC vs Microcontroller

Advantages of Logic ICs

• Fast operation
• Simple implementation
• Low cost
• Reliable hardware operation
• No software required for basic functions
• Available in many logic families
• Large selection of functions

Limitations of Logic ICs

• Limited flexibility compared with programmable devices
• Different families may not be directly compatible
• Propagation delay increases at high speeds
• Input and output voltage limits must be respected
• Incorrect floating inputs can cause unpredictable operation
• Large digital systems may require many individual ICs

Common Logic IC Design Mistakes

• Using the wrong supply voltage
• Leaving inputs floating
• Exceeding output-current ratings
• Connecting incompatible logic families
• Ignoring propagation delay
• Ignoring noise margins
• Failing to provide supply decoupling
• Assuming all 74-series ICs are electrically identical
• Ignoring the IC pinout
• Using an incorrect replacement device

Key Points

• Logic ICs process binary digital signals.
• AND, OR, NOT, NAND, NOR, XOR and XNOR are fundamental logic functions.
• TTL and CMOS are important logic technologies.
• The 74-series contains many common digital logic devices.
• The 4000 series contains many CMOS logic devices.
• Logic HIGH and LOW are voltage ranges rather than necessarily exact voltages.
• Noise margin is important for reliable digital operation.
• Propagation delay limits high-speed operation.
• Fan-out determines how many inputs an output can reliably drive.
• Tri-state outputs allow multiple devices to share a bus.
• Counters, flip-flops, registers, multiplexers and decoders are common logic IC functions.
• Unused inputs should normally be given defined logic states.
• Supply decoupling is important for reliable operation.
• Always check voltage levels and pinouts before replacing a logic IC.