April 16, 2011

Combinational Logic Circuit

          Multiple gate functions can be combined to form more complex or different Boolean logic functions. Wiring together multiple gates are used to build a complex logic function that only outputs a specific value when a specific combination of True and False inputs are passed to it is known as ‘‘combinatorial logic’’. The output of a combinatorial logic circuit is dependent on its input; if the input changes then the output will change as well.
          When I wrote the preceding paragraph, I originally noted that combinatorial logic circuits produce a ‘‘True’’ output for a given set of inputs. This is incorrect, as there will be some cases where you will require a False output in your application. I made the definition a bit more ambiguous so that you do not feel like the output has to be a single, specific value when the input consists of the required inputs. It is also important to note that in a combinatorial logic circuit, data flows in one direction and outputs in logic gates cannot be used as inputs to gates which output back to themselves. These two points may seem subtle now, but they are actually critically important to the definition of combinatorial logic circuits and using them in applications. 

          An example of a combinatorial circuit is shown in figure above. In this circuit, I have combined three AND gates, a NOR gate, a NOT gate and an XOR gate to produce the following logic function:

 This combinatorial circuit follows the convention that inputs to a gate (or a chip or other electronic component) are passed into the left and outputs exit from the right. This will help you ‘‘read’’ the circuit from left to right, something that should be familiar to you.
          While seeing a series of logic gates, like the one in figure above seems to be overwhelming, you already have the tools to be able to work through it and understand how it works. In the previous section, I noted that gates could be connected by passing the output of one into an input of another; a combinatorial circuit (like figure) is simply an extension of this concept and, being an extension, you can use the same tools you used to understand single gates to understand the multiple gate operation.

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