ABSTRACT
Thus far the emphasis of this book has been on the design and analysis of gate circuits. However, the implementation of digital systems involves a number of other circuit considerations. First, most digital systems involve a mixture of circuits from different logic families. This necessitates interfacing between the different types of circuits with different current requirements and voltage levels by using level-shifting circuits.1-13 Second, it is often necessary to connect the outputs of multiple gates to a single node, as is the case with data, address, and control busses, and other signal lines. In some cases, all such outputs can remain active and the resulting level on the line is determined by the ORing or ANDing of the multiple outputs, which is referred to as wired logic.14 In other cases, it is necessary to allow only one of the outputs to be active at a particular time; this is achieved using transmission gates15 or tri-state logic gates.16-22
Many digital systems involve a mixture of circuits from different circuit families. This choice is made by the designer in order to optimize different subsystems with different requirements simultaneously. For example, if a CMOS processor may be connected to TTL bus drivers and NMOS memory circuits, the processor utilizes CMOS for high packing density, low power, and short on-chip propagation delays. The TTL bus drivers are used for high off-chip data rates and the NMOS memory circuits are used for maximum density and number of bits. In practice, because all conceivable circuit combinations have been used, interfacing the different types of circuits with different current requirements and voltage levels is necessary.
