I.
PURPOSE
The purpose of this lab experiment was to serve as an introduction to physical
logic gates, used in this lab in the form of integrated circuit chips. This experiment
served as an introduction to equipment used for testing these integrated circuits, a
rather complicated-looking circuit with various switches, light bulbs, and wire
connections. This lab also introduced a program, Xilinx ISE, and along with that a
hardware description programming language, VHDL.
I.
INTRODUCTION
Logic gates are the basis of the heart of a computer, the processor, as well as most
other computer components. These gates are specific circuit elements, consisting
mostly of transistors. These elements perform all kinds of Boolean functions when
arranged correctly.
Logic gates only function with Boolean inputs. They work on the idea that each
input can only assume one of two possible values, a yes or a no, 1 or 0, true or false,
high or low, etcetera, depending on the context of the problem. Usually, in the basic
schematic of a logic gate, only a finite number of inputs and outputs are shown. For
example, in the logic gates shown in figure 1 of the Appendix section of this report,
each logic gate shows two inputs and one output. In the physical implementation,
however, there must be connections to each of an eternal voltage source and ground,
so that the logic gate can increase or decrease the current traversing the gate to the
output according to the function it serves.
Simple and complex circuits can both be modeled by forming a truth table, or a
chart showing the Boolean input variables, never more than three in this lab, as well
as the Boolean outputs of the resultant combination of logic gates. All the truth tables
formed in this lab were fairly simple, the most complex of which used three Boolean
variables and three logic gates. The inputs of a truth table are determined by first
counting the number of Boolean variables present at the input. From this, it is easy to
calculate the total number of possible values for the inputs, using the fact that the
number of rows in a truth table is equal to 2
n
, where n is the number of Boolean input
variables. Therefore, if A and B are Boolean input variables, there will be four total
possible combinations, and four rows in a truth table for a Boolean function of A and
B.
Physically, these logic gates appear as integrated circuit (IC) chips, usually with