Файл: Digital design with CPLD applications and VHDL (R. Dueck, 2000).pdf
ВУЗ: Не указан
Категория: Не указан
Дисциплина: Не указана
Добавлен: 13.06.2025
Просмотров: 8071
Скачиваний: 6
498 C H A P T E R 1 1 • Logic Gate Circuitry
•Draw the circuits of CMOS NAND and NOR gates and explain the operation of each.
•Design a circuit using a CMOS transmission gate to enable and inhibit digital and analog signals.
•Interpret TTL data sheets to distinguish between the various TTL families.
•Describe the use of the Schottky barrier diode in TTL gates.
•Calculate speed-power products from data sheets.
Our study of logic gates and flip-flops in previous chapters has concentrated on digital logic and has largely ignored digital electronics. Digital logic devices are electronic circuits with their own characteristic voltages and currents. No serious study of digital cir-
cuitry is complete without some examination of this topic.
It is particularly important to understand the inputs and outputs of logic devices as electronic circuits. Knowing the input and output voltages and currents of these circuits is essential, since gate loading, power dissipation, noise voltages, and interfacing between logic families depend on them. The switching speed of device outputs is also fundamental and may be a consideration when choosing the logic family for a circuit design.
Input and output voltages of logic devices are specified in manufacturers’ data sheets, which allows us to take a “black box” approach initially.
Later in the chapter, we will examine some basic digital circuits at a transistor level, since digital logic is based on transistor switching. Two major types of transistors, the bipolar junction transistor and the metal-oxide-semiconductor field effect transistor (MOSFET), form the basis of the major logic families in use today. Transistor-transistor logic (TTL) is based on the bipolar transistor. Complementary MOS (CMOS) is based on the MOSFET.
We will briefly study the operating characteristics of both bipolar transistors and MOSFETs and then see how these devices give rise to the electrical characteristics of simple logic gates.
11.1 Electrical Characteristics of Logic Gates
K E Y T E R M S
TTL Transistor-transistor logic. A logic family based on bipolar transistors.
CMOS Complementary metal-oxide semiconductor. A logic family based on metal-oxide-semiconductor field effect transistors (MOSFETs).
ECL Emitter coupled logic. A high-speed logic family based on bipolar transistors.
When we examine the electrical characteristics of logic circuits, we see them as practical, rather than ideal devices. We look at properties such as switching speed, power dissipation, noise immunity, and current-driving capability. There are several commonly available logic families in use today, each having a unique set of electrical characteristics that differentiates it from all the others. Each logic family gives superior performance in one or more of its electrical properties.
CMOS consumes very little power, has excellent noise immunity, and can be used with a wide range of power supply voltages.
TTL has a larger current-driving capability than CMOS. Its power consumption is higher than that of CMOS, and its power supply requirements are more rigid.
ECL is fast, making it the choice for high-speed applications. It is inferior to CMOS and TTL in terms of noise immunity and power consumption.
TTL and CMOS gates come in a wide range of subfamilies. Table 11.1 lists some of the TTL and CMOS variations of the quadruple 2-input NAND gate. All gates listed have
11.1 • Electrical Characteristics of Logic Gates |
499 |
Table 11.1 Part Numbers for a Quad 2-input NAND Gate in Different
Logic Families
Part |
||
Number |
Logic Family |
|
TTL |
74LS00 |
Low-power Schottky TTL |
74ALS00 |
Advanced low-power Schottky TTL |
|
74F00 |
Fast TTL |
|
CMOS |
74HC00 |
High-speed CMOS |
74HCT00 |
High-speed CMOS (TTL-compatible inputs) |
|
74LVX00 |
Low-voltage CMOS |
|
the same logic function but different electrical characteristics. Other gates would be similarly designated, with the last two or three digits indicating the gate function (e.g., a quadruple 2-input NOR gate would be designated 74LS02, 74ALS02, 74F02, etc.).
We will examine four electrical characteristics of TTL and CMOS circuits: propagation delay, fanout, noise margin, and power dissipation. The first of these has to do with speed of output response to a change of input. The last three have to do with input and output voltages and currents. All four properties can be read directly from specifications given in a manufacturer’s data sheet or derived from these specifications.
Figures 11.1 and 11.2 show how the input and output voltages and currents are defined in a 74XX00 NAND gate. This designation can be generalized to any logic gate input or output.
H |
L |
L |
H |
|
H |
L |
|||
VOL |
VOH |
|||
VIH |
VIL |
|||
FIGURE 11.1
Input/Output Voltage Parameters
H |
L |
L |
H |
H |
L |
IOL |
IOH |
IIH |
IIL |
FIGURE 11.2
Input/Output Current Parameters
The voltages and currents are designated with two subscripts, one that designates an input or output and another that indicates the logic level. For example, VOL is the voltage at the gate output when the output is in the logic LOW state. IIL is the input current when the input is in the LOW state.
These voltages and currents are specified in manufacturers’ published data sheets, which are usually available in print form in a data book or in an electronic format, such as Portable Document Format (pdf) on a CD or internet site.
Figure 11.3 shows a data sheet for a 74LS00 NAND gate, which also shows parameter values for a 54LS00 device. A 54-series device is manufactured to military specifications, which require a high range of environmental operating conditions. A 74-series device is suitable for general or commercial use. We will limit ourselves to the 74-series devices.
The voltage and current parameters indicated in Figures 11.1 and 11.2 are all shown in the 74LS00 data sheet. Some parameters are shown as typical values, as well as maximum or minimum. Typical values should be considered “information only” as device manufacturers