If you need a refresher on how to use a logic probe, see the description in the FED 101 manual, available on the ECE Department Laboratory web page,. The logic probe is an invaluable tool that greatly simplifies the testing and debugging of digital logic circuits. Students are also expected to have experience with gate delay and effect of fan-out on the speed of digital circuits.
Students are required to use Karnaugh map and Boolean algebra techniques to simplify the given Boolean functions prior to implementing them. Some basic, commonly used logic circuits, such as multiplexer (MUX), demultiplexer (DEMUX), and adder, are introduced. This lab is designed to enable students to design and analyze combinational logic circuits. Some commonly used laws and relations are listed in Table 1 below.Ĭombinational logic is a very important part of digital systems. These laws and relations are effective in simplifying Boolean functions because there is no need to have the truth tables of the functions. The other method used to simplify Boolean functions is the basic laws and relations in Boolean algebra. Detailed descriptions on how to use Karnaugh mapping to simplify Boolean functions can be found in Chapter 2, Contemporary Logic Design †. There are two ways of simplifying a Boolean function. In VLSI design other criteria such as propagation delay and interconnect regularity may lead to designs which do not minimize total gates. In order to keep the number of the logic gates that implement the Boolean functions minimum, these Boolean functions must be simplified before they are implemented in discrete logic circuits. Quite often, the given Boolean functions to be implemented in combinational logic circuits are not simplified (minimized). The logic function that a combinational logic circuit represents is usually expressed using a Boolean function, or a set of Boolean functions. No matter how complex a combinational logic circuit might possibly be, it will always be comprised of basic logic gates such as NAND gates, NOR gates, inverters and so on. Sequential logic circuits will be the topic of your next lab. In other words, the outputs of a sequential logic circuit are functions of its present inputs and the previous state of its memory elements. Therefore, the outputs of sequential logic circuits depend not only on the present values of the inputs but also on the input's previous values. On the other hand, a sequential logic circuit consists of not only logic gates but also memory elements. In other words, the combinational logic circuits do not have the ability to memorize their past. A combinational logic circuit consists of logic gates whose outputs at any time are determined directly from the present combination of inputs without regard to previous inputs.
One is combinational logic circuits, the other is sequential logic circuits. Logic circuits for digital systems can generally be classified into two categories.
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