Chapter 11 - Slightly More Complex Functions

Abstract

The primitive logic functions NOT, AND, OR, NAND, NOR, XOR, and XNOR can be connected together to build slightly more complex functions. A single signal carrying one bit of binary data is known as a scalar entity. A set of signals carrying similar data can be gathered together into a group known as a vector. A key advantage of vector notation is that it allows all of the signals comprising the vector to be easily referenced in a single statement. A multiplexer uses a binary value, or address, to select between a number of inputs and to convey the data from the selected input to the output. A decoder uses a binary value, or address, to select between a number of outputs and to assert the selected output by placing it in its active state. There is a special category of gates called tri-state functions whose outputs can adopt three states: 0, 1, and Z. The tri-state buffer’s symbol is based on a standard buffer with an additional control input known as the enable. A more sophisticated function called a D-type latch can be constructed by attaching two AND gates and a NOT gate to the front of an RS latch. In the case of a D-type flip-flop, the data appears to be loaded when a transition, or edge, occurs on the clock input, which is therefore said to be edge-sensitive. Counter functions are also commonly used in digital systems. The number of states that the counter will sequence through before returning to its original value is called the modulus of the counter.