Файл: Digital design with CPLD applications and VHDL (R. Dueck, 2000).pdf

518 C H A P T E R 1 1 • Logic Gate Circuitry

(i.e., size of the internal transistors) that allows more efficient packaging and faster operation. Low-voltage logic is particularly popular for battery-powered applications such as laptop computing or cell phones. Low voltage families typically operate at VCC 3.3 V or 2.5 V. Newer devices are available for VCC 1.8 V or 1.65 V.

Low-voltage CMOS families such as 74LVX or 74LCX can interface directly with TTL outputs if they are operated with a 3.0 V to 3.3 V power supply. These families are not really suitable for driving 5-volt TTL, as their noise margins are too small when they use a 3.0 V supply voltage.

If we wish to use a 74LS device to drive a 74HC device operating at a power supply voltage of less than 4.5 V, we can use a 74HC4049 or 74HC4050 buffer to translate the TTL logic level down to an appropriate value. The 74HC4049 is a package of six inverting buffers. The 74HC4050 has six noninverting buffers. These buffers can tolerate up to 15 V on their inputs. Their output voltages are determined by the value of their supply voltage.

Figure 11.15 shows an LSTTL-to-74HC interface circuit with a 74HC4050 buffer. Note that the interface buffer has the same power supply voltage as the load gate. Both sides of the interface are referenced to the same ground.

GND

FIGURE 11.15

74LS-to-74HC Interface Using a 74HC4050 Buffer

SECTION 11.6 REVIEW PROBLEM

11.6A 74LS00 driving gate is to be interfaced to a 74HC00 load using a 74HC4050 noninverting buffer. The 74HC00 has a power supply voltage of 2.5 V. What supply voltage should the 74HC4050 buffer have? Why?

11.7Internal Circuitry of TTL Gates

K E Y T E R M S

Cutoff mode The operating mode of a bipolar transistor when there is no collector current flowing and the path from collector to emitter is effectively an open circuit. In a digital application, a transistor in cutoff mode is considered OFF.

Saturation mode The operating mode of a bipolar transistor when an increase in base current will not cause a further increase in the collector current and the path from collector to emitter is very nearly (but not quite) a short circuit. This is the ON state of a transistor in a digital circuit.

TTL has been around for a long time. The first transistor-transistor logic ICs were developed by Texas Instruments around 1965. Since then, there have been many improvements in the speed and power consumption of these devices, but the basic logic principles remain largely unchanged. Even though they are seldom used in modern designs, it makes sense to examine the internal circuitry of standard TTL gates such as the 7400 NAND, 7402 NOR, and 7404 inverter because the internal logic concepts are similar to the more advanced types of TTL.

The most important parts of the circuit, as far as a designer or technician is concerned, are the input and output circuits, because they are the only parts of the chip to which we have access. It is to these points that we interface other circuits and where we make diag-

FIGURE 11.18

Example 11.13

Transistor as Inverter

Analyze the circuit to show that it acts as an inverter if a logic HIGH is defined as3 V and a logic LOW is defined as 0.5 V. Assume that b 100, and assume that VBE 0.7 V and VCE 0.2 V in saturation.

Solution We will analyze the circuit with two input voltages: 3 V (logic HIGH) and 0.5

V (logic LOW). These two conditions are shown in Figure 11.19.

FIGURE 11.19

Example 11.13

Voltage and Current Analysis of Inverter

High input. We must prove that VI 3 V is sufficient to saturate the transistor. Let us assume that this is true and find out if calculations confirm our assumption.

Figure 11.19a shows the circuit with VI 3 V. By Kirchhoff’s voltage law (KVL):

VI IBRB VBE, or

IB (VI VBE)/RB

If we assume that IB is sufficient to saturate the transistor, then:

IB (3 V 0.7 V)/22 k

105 A

bIB (100)(105 A) 10.5 mA

Collector current won’t increase beyond its saturated value, even if base current increases. Therefore, if the transistor is saturated, bIB will be larger than the current actually flowing in the collector-emitter path.

In saturation, the collector current can be calculated by KVL:

VCC IC RC VCE, or

IC (VCC VCE)/RC

IC (5 V 0.2 V)/470

10.2 mA

Since bIB IC, the transistor is saturated. Thus, an input voltage of 3 V will produce sufficient base current to saturate the transistor. The output is given by VO VCE 0.2 V, which is within the defined range of a logic LOW.

LOW input. Figure 11.19b shows the circuit with VI 0.5 V. By KVL:

VI IB RB VBE

VBE 0.5 V IB RB

Since VBE must be 0.6 V, the transistor is in cutoff mode. Thus, in the collector circuit:

VCC IC RC VCE

5 V (0)(470 ) VCE

VO VCE 5 V (logic HIGH)

Table 11.6 summarizes the operation of the circuit as an inverter.

Table 11.6 Input and Output of Single-Transistor

Inverter

TTL Open-Collector Inverter and NAND Gate

K E Y T E R M

Open-collector output A TTL output where the collector of the LOW-state output transistor is brought out directly to the output pin. There is no built-in HIGH-

state output circuitry, which allows two or more open-collector outputs to be connected without possible damage.

N O T E

The TTL gates (7405, 7401, 7404, 7400, and 7402) used in the following sections to illustrate TTL circuit principles are no longer in general use. They are from the original (“standard”) TTL family, which has been superceded by faster and more efficient devices. However, the standard TTL devices are easier to understand than devices from the newer TTL subfamilies, since their circuit structure is simpler. The operating principles are similar in both the standard and newer families, so we will use the standard devices to illustrate the general principles of TTL operation.

Figure 11.20 shows the circuit of the simplest TTL gate: a 7405 inverter with open-collector outputs. This circuit performs the same function as the single-transistor inverter we examined in Example 11.13. These circuits differ most obviously in their input circuitry. The inverter circuit in Example 11.13 has a resistor as its input; the 7405 inverter has a transistor,