Choosing a Means of Implementation
So far we have seen how to design both combinational and sequential circuits. These will, on paper, successfully perform many different functions but may well fail if the practicality of the hardware implementation issues are ignored. Ten years ago the choice of hardware options was limited; however, nowadays many choices exist for the designer’ some of which are more accessible than others. The aim of this chapter is to introduce the technology options that are available so that the appropriate selection can be made from a sound engineering basis.
As far as technology is concerned designers must choose the balance they require between the circuit speed of operation and its power consumption. The two choices available are typically either bipolar or Complementary Metal Oxide Semiconductor (CMOS). However, other more exotic high-speed options are available such as Emitter Coupled Logic (ECL) and Gallium Arsenide (GaAs). CMOS offers Low power consumption with moderate speeds. Alternatively, bipolar offers high speed but high power consumption. A combination of both is the ideal but was not available until only a few years ago. A mixed bipolar and CMOS technology (called BiCMOS) is now available and has an excellent combination of high speed and low power with the exception that this involves a more complex manufacturing procedure and hence is currently more expensive. As with most aspects of electronics technology the cost will certainly fall and BiCMOS may well be a low-cost technology option for the future.
The most common technology 10-15 years ago was bipolar (i.e. TTL (Transistor Transistor Logic) or ECL) but now CMOS is the preferred choice. Table 9.1 provides a comparison of logic families for various technology options. This table will provide a useful reference throughout this chapter. We shall start with a description of bipolar logic so that its limitations can be appreciated before moving to the more popular CMOS technology.