An Incompletely specified function is a Boolean function that only define output values for a subset of its inputs - i.e. a Boolean function whos output is a don't care for at least one of its input combinations. Incompletely specified functions often make no guarantees as to the unspecified output whatsoever.
In many situations when working with combinational circuits, some combinations of inputs should not occur under normal working conditions. For circuits with such combinations, those combinations can be treated as either 0 or 1 depending on whichever yields a more simplified Boolean expression.
Boolean functions with don't care output terms are represented with a . For example consider the following function.
Note that depending on the final design of the circuit, the output for for example may be either 0 or 1.
- Main article: karnaugh_map#Don't cares
Karnaugh maps are especially suitable for minimizing logic circuts with don't care outputs because they directly translate to don't care cells on a k-map. If grouping don't care cells results in larger groups, then more variables may be eliminated resulting in smaller Boolean expressions.
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