Integrated Circuits

LOGIC GATES

Unit –II

➢ All the logic elements and functions that have been discussed are generally available in integrated circuit (IC) form.

➢ Digital systems have incorporated ICs for many years because of their small size, high reliability, low cost, and low power consumption.

➢ A monolithic integrated circuit (IC) is an electronic circuit that is constructed entirely on a single small chip of silicon.

➢ All the components that make up the circuit—transistors, diodes, resistors, and capacitors—are an integral part of that single chip.

➢ Fixed-function logic and programmable logic are two broad categories of digital ICs.

➢ In fixed-function logic devices, the logic functions are set by the manufacturer and cannot be altered.

Integrated Circuits

➢ Figure shows a cutaway view of one type of fixed-function IC package with the circuit chip shown within the package. Points on the chip are connected to the package pins to allow input and output connections to the outside world.

IC Packages

➢ Integrated circuit (IC) packages are classified according to the way they are mounted on printed circuit boards (PCBs) as either through-hole mounted or surface mounted.

➢ The through-hole type packages have pins (leads) that are inserted through holes in the PCB and can be soldered to conductors on the opposite side.

➢ The most common type of through hole package is the dual in-line package (DIP) shown in Figure.

Pin Numbering

Classifications of ICs

➢ Fixed-function digital ICs are classified according to their complexity.

❖ Small-scale integration (SSI) describes ICs that have up to ten equivalent gate circuits on a single chip, and they include basic gates and flip-flops.

❖ Medium-scale integration (MSI) describes integrated circuits that have from 10 to 100 equivalent gates on a chip. They include logic functions such as encoders, decoders, counters, registers, multiplexers, arithmetic circuits, small memories, and others.

❖ Large-scale integration (LSI) is a classification of ICs with complexities of from more than 100 to 10,000 equivalent gates per chip, including memories.

❖ Very large-scale integration (VLSI) describes integrated circuits with complexities of from more than 10,000 to 100,000 equivalent gates per chip.

❖ Ultra large-scale integration (ULSI) describes very large memories, larger microprocessors, and larger single-chip computers. Complexities of more than 100,000 equivalent gates per chip are classified as ULSI.

Integrated Circuit Technologies

➢ The types of transistors with which all integrated circuits are implemented are either MOSFETs (metal-oxide semiconductor field-effect transistors) or bipolar junction transistors.

➢ A circuit technology that uses MOSFETs is CMOS (complementary MOS).

➢ One type of fixed function digital circuit technology uses bipolar junction transistors and is sometimes called TTL (transistor-transistor logic).

➢ BiCMOS uses a combination of both CMOS and bipolar.

➢ All gates and other functions can be implemented with either type of circuit technology.

SSI and MSI circuits are generally available in both CMOS and bipolar.

➢ LSI, VLSI, and ULSI are generally implemented with CMOS because it requires less area on a chip and consumes less power.

Types of Logic Families

Unipolar

MOS CMOS

Hybrid

BiCMOS Bipolar

Saturated

RTL DTL HTL I²L TTL

Unsaturated

ECL

Schottky TTL

Basic Operational Characteristics and Parameters

1. DC Supply Voltage

➢ The nominal value of the dc supply voltage for TTL (transistor-transistor logic) devices is +5 V.

➢ CMOS (complementary metal-oxide semiconductor) devices are available in different supply voltage categories: +5 V, +3.3 V, 2.5 V, and 1.8 V.

➢ The dc supply voltage is connected to the V_CC pin of an IC package, and ground is connected to the GND pin.

➢ Both voltage and ground are distributed internally to all elements within the package.

2. Logic Levels

➢ The voltages used to represent a 1 and a 0 are called logic levels.

➢ Ideally, one voltage level represents a HIGH and another voltage level represents a LOW.

➢ In a practical digital circuit, however, a HIGH can be any voltage between a specified minimum value and a specified maximum value.

➢ Likewise, a LOW can be any voltage between a specified minimum and a specified maximum.

➢ There can be no overlap between the accepted range of HIGH levels and the accepted range of LOW levels.

Fig: Logic level ranges of voltage for a digital circuit.

Logic Levels

CMOS Logic Levels

➢ There are four different logic-level specifications: V_IL, V_IH, V_OL, and V_OH.

➢ For CMOS circuits, the ranges of input voltages (V_IL) that can represent an acceptable LOW (logic 0) are from 0 V to 1.5 V for the +5 V logic and 0 V to 0.8 V for the 3.3 V logic.

➢ The ranges of input voltages (V_IH) that can represent an acceptable HIGH (logic 1) are from 3.5 V to 5 V for the 5 V logic and 2 V to 3.3 V for the 3.3 V logic, as indicated in Fig.

➢ The ranges of values from 1.5 V to 3.5 V for 5 V logic and 0.8 V to 2 V for 3.3 V logic are regions of unpredictable performance, and values in these ranges are unacceptable.

➢ Therefore, CMOS gates cannot be operated reliably when the input voltages are in these unacceptable ranges.

CMOS Logic Levels

CMOS Logic Levels

➢ The ranges of CMOS output voltages (V_OL and V_OH) for both 5 V and 3.3 V logic are also shown in Figure.

➢ Notice that the minimum HIGH output voltage, V_OH(min), is greater than the minimum HIGH input voltage, V_IH(min).

➢ Also, notice that the maximum LOW output voltage, V_OL(max), is less than the maximum LOW input voltage, V_IL(max).

CMOS Logic Levels

TTL Logic Levels

➢ The input and output logic levels for TTL are given in Figure.

➢ There are four different logic level specifications: V_IL, V_IH, V_OL, and V_OH.

3. Noise Immunity

➢ Noise is unwanted voltage that is induced in electrical circuits and can present a threat to the proper operation of the circuit.

➢ Wires and other conductors within a system can pick up stray high-frequency electromagnetic radiation from adjacent conductors in which currents are changing rapidly or from many other sources external to the system.

➢ Also, power-line voltage fluctuation is a form of low-frequency noise.

➢ In order not to be adversely affected by noise, a logic circuit must have a certain amount of noise immunity.

➢ This is the ability to tolerate a certain amount of unwanted voltage fluctuation on its inputs without changing its output state.

➢

➢ For example, if noise voltage causes the input of a 5 V CMOS gate to drop below 3.5 V in the HIGH state, the input is in the unacceptable region and operation is unpredictable.

➢ Thus, the gate may interpret the fluctuation below 3.5 V as a LOW level, as illustrated in Figure (a).

➢ Similarly, if noise causes a gate input to go above 1.5 V in the LOW state, an uncertain condition is created, as illustrated in Figure (b).

Figure: Illustration of the effects of input noise on gate operation.