Introduction to Digital Systems: Modeling, Synthesis, and Simulation Using VHDL

3.14 STANDARD LOGIC INTEGRATED CIRCUITS

An approach used widely until the mid-1980s was to connect multiple chips, each containing only a few logic gates. Standard chips are integrated circuits that are mass-produced to perform commonly needed electronic functions. They have proven to be very useful in circuit design over the years. Engineers and technicians can connect these components to parts of a larger circuit to accomplish a specific function. Some of the more common uses for standard chips are logic gates: AND, OR, NOT, NAND, NOR, XOR, and XNOR, a wide variety of which are available with different types of logic. Many of these have been created as chips in the 7400 or 4000 series (these part numbers are printed on the top of the chip casing). The external connections of the chip are called pins or leads. Two pins are used to connect V_CC and Gnd, which power the chip. The other pins are used to connect to the inputs and outputs of each individual gate.

Three main types of technology are used to build 7400 series chips. Chips that bear codes beginning with 74LS represent some of the older designs. They are made with TTL (transistor-transistor logic) and have a maximum frequency of about 35 MHz. Their input voltage is always around 5 V, and one of their outputs can drive up to ten 74LS inputs or fifty 74HCT inputs. This driving principle is called fan-out. 74HCT chips are compatible with TTL but are created using the technology known as CMOS (complementary metal-oxide semiconductor). ...