Logic gates whose input has hysteresis are often known as a Schmitt trigger. Figure 5-22 shows a 7414 hex Schmitt trigger inverter.

Figure 5-22 7414 Hex Schmitt Trigger Inverter

Note in Figure 5-22 that there is a small hysteresis curve inside each inverter symbol. This indicates that the inverters are the Schmitt triggers.

5.5 Other TTL Logic Families

In 1971 a major advance in TTL logic occurred with the introduction of TTL devices that incorporate Schottky diodes. They are based on the property of aluminum to act much like a p-type semiconductor when in contact with n-type silicon. The Schottky diode acts like an ordinary p-n diode except that it has a faster response time and the voltage drop is about 0.3 volts instead of 0.6 volts. When a Schottky diode is connected between the base and the collector of a bipolar transistor, the transistor is prevented from going into saturation. The Schottky diode/transistor combination, known as a Schottky transistor, has a significantly faster switching speed. Schottky TTL logic devices have part numbers 74SXXX and give three times the speed of standard TTL using only twice the power.

By increasing the resistor sizes, low-power Schottky TTL was developed giving the same speed as standard TTL, but using only 1/5 the power. These devices, whose part numbers are in the format 74LSXXX, were the standard TTL logic parts for many years. In 1980, more sophisticated Schottky-type logic circuits using smaller, higher performance transistors were developed by Texas Instruments. These are the advanced Schottky and advanced low-power Schottky logic families. Their part numbers are 74ASXXX and 74ALSXXX respectively.

5.6 CMOS Logic Gates

Around the same time that the original TTL circuits using bipolar transistors were introduced, a line of logic circuits using CMOS (complementary metal-oxide semiconductor) technology became available. A line of TTL-compatible CMOS ICs have part numbers 74XXX. TTL series pinouts are also available with part numbers 74CXXX.

CMOS logic circuits have two significant advantages over TTL. In the first place, CMOS circuits operate with very low power dissipation. A CMOS input requires virtually no current to remain at a given logic level. In fact, the entire circuit draws insignificant current when it is not switching between logic levels. In CMOS, power is

consumed only during switching, while bipolar logic power dissipation is only weakly dependent on the switching rate. At low switching rates, CMOS provides huge savings in power dissipation.

A second advantage of CMOS is the smaller size of the circuits. Since no resistors and only two simple types of transistors are needed, the resulting logic gates require less area on a silicon wafer than their bipolar counterparts. The combined advantages of less power consumption and less area make CMOS the choice for VLSI (very large scale integration) integrated circuits such as microprocessors.

However, there are also significant drawbacks to CMOS which have prevented it from completely replacing bipolar logic. One of them is that CMOS circuits have slower switching speeds and propagation delays compared to bipolar circuits. The original CMOS logic gates had switching speeds that were about five to ten times slower than the 74XXX bipolar logic gates. High-speed CMOS, introduced in 1980, have improved processing technology and smaller transistor sizes, resulting in higher switching speeds and improved output drive current capability.

The CMOS 74HCT parts are completely TTL-compatible and can be freely intermixed with bipolar TTL parts. The 74HC series, on the other hand, have a logic transition threshold of 2.5 V when using a 5-volt power supply, compared to the 1.4 volts TTL standard. Since CMOS outputs have 5-volt and 0-volt logic levels, a 2.5-volt threshold provides better noise immunity than TTL; however, 74HC series parts cannot be mixed with standard TTL parts. For this reason, in mixed circuits, it is preferable to use the 74HCT parts.

An advanced CMOS technology family was introduced in 1985 having part numbers 74ACXXX. The TTL-compatible versions have part numbers 74ACTXXX. These new ICs have about double the speed of HC and HCT with yet another increase in drive power. The result is that the propagation delays for 74ACT parts approach those of bipolar TTL, although they are not quite equal to the fastest TTL families. To further increase CMOS speeds manufacturers turned to a process known as BiCMOS, which uses a mixture of bipolar and MOS transistors on the same chip. By strategically placing bipolar transistors at critical points in the circuit, the switching speed can be improved with only a small increase in power dissipation. The most popular BiCMOS logic family is the 74FCTXXX (fast CMOS) series of logic ICs.

Still another drawback to CMOS logic is that the circuits are susceptible to static electricity. The static discharge of the human body in a dry environment can destroy a CMOS transistor. Although protective diodes on CMOS circuit inputs provide some protection to static breakdown, all CMOS circuits are susceptible. For this reason ICs and circuits boards should be stored in conductive pouches and not handled until you have discharged yourself by touching a good electrical ground.