An integrated circuit, abbreviated IC, is a type of circuit in which all the components are integrated on a single semiconductor chip of very small size. Integrated circuits are separated into related groups or families e.g. TTL and CMOS.

TTL

The term TTL stands for transistor-transistor logic which refers to the use of bipolar junction transistors in the circuit technology used to construct the gates at the chip level. TTL consists of a series of logic circuits e.g. standard TTL, LOW powered TTL and Schocttky TTL. The differences between the various types are in their performance characteristics, namely

The propagation delay time determines the speed that the IC can operate. The propagation delay time of a gate is the time interval between the application of an input pulse and the occurrence of the occurring output pulse.

The time interval between a specified point on the input pulse and the corresponding point on the output pulse which is changing from HIGH to LOW is denoted as t_PHL. Similarly, t_PLH is used to denote the interval when the output pulse is changing from LOW to HIGH. These points are highlighted in Figure 4-1 which illustrates the propagation delays associated with the inverter.

Figure 4-1 Propagation delays for Inverter

The Power dissipation of a logic gate is the product of the direct current (dc) voltage supply (V_CC) and the average supply current (I_CC). The average I_CC is determined based on a 50% duty cycle, i.e. the state is LOW 50% of the time and HIGH 50% of the time.

The fan-out of a gate is the maximum number of inputs of the same IC family series that the gate can drive while maintaining the its output levels within specified limits. In short, it specifies the maximum load that a given gate can handle. If this limit is exceeded that the operation of the gate degrades and become unpredictable.

CMOS

The term CMOS stands for complementary metal oxide semiconductor. Whether the circuit is constructed with TTL or CMOS technologies, the logic functions are the same. The difference occurs in the performance characteristics. Several CMOS technologies are available but they basically fall into two process technologies: the metal-gate CMOS (4000 series) and the silicon-gate (74C, 74HC and 74HCT series).

Table 4-1 lists the differences the TTC and CMOS families and Table 4-2 list the specifications of a IC member from each family.