Saturday, December 15, 2012

Digital Integrated Circuits

Digital Integrated Circuits

Digital integrated circuits are all classified as being one of the following: (i) a small-scale integrated circuit (SSI), (ii) a medium-scale integrated circuit (MSI), (iii) a large-scale integrated circuit (LSI) and (iv) a very-large-scale integrated circuit (VLSI). These categories are based on the number of transistors within each 1C. An SSI 1C has up to 10 transistors, an MSI has between 10 and 100 transistors, LSI ICs have between 100 and 5000 transistors and, lastly, a VLSI circuit contains more than 5000 transistors.

Digital ICs are manufactured using different technologies and are members of various logic families. The three basic logic families are (a) transistor-transistor logic (TTL), (b) complementary metal oxide semiconductor (CMOS) and (c) emitter-coupled logic (ECL). All three logic families contain sub-families which have their relative advantages and disadvantages making one or another particularly suited for different applications. TTL and CMOS are much more commonly used than ECL.

ECL is the fastest logic family and it is used for high-speed circuits, such as mainframe computers, only. For all other digital circuits either TTL or CMOS is used. Standard TTL and 4000 series CMOS are the earliest versions of these logic families but neither of them is used for new designs now, although devices in these families are still available from manufacturers. TTL has developed first into low-power Schottky TTL and then into advanced Schottky and TTL advanced low-power Schottky TTL. The low-power versions offer a smaller power dissipation and the Schottky versions (which use Schottky diodes internally) offer a much increased speed of operation. The 4000 series CMOS devices are relatively slow to operate but have the advantage of a very small power dissipation. The pin connections of some common TTL and CMOS gates are given on page 172. The speed disadvantage of 4000 series CMOS has been overcome, while retaining its low power dissipation feature, with the modern replacements known as high-speed CMOS and advanced CMOS. For LSI and VLSI circuits either CMOS or NMOS is used, since devices using these technologies occupy less space in a silicon wafer and this factor allows many more devices to be packed into a given chip area. This is important with many of the complex circuits used today, such as microprocessors and large-capacity memories, which often contain many thousands of transistors.