Don't-care term

In digital technology, the do not care, or X means - that this value has no effect on the logic circuit.

• 2.1 restrictions

General

In the truth table Don't-Care - entries are typically used to avoid having to deal with all input states, or when an incompletely defined function occurs, the variable that is " 1" and may have "0". This is the case for certain input combinations, if the output has not been set for this. For example, if these input combinations are not possible or present in the circuit or are disabled when the block via the enable signal.

Also, in the logic synthesis and timing diagram Do not care values ​​can be used. In the function table ( truth table ) of the flip-flop provides a X is any condition that can optionally be "0" or "1". The flip-flop processes the data only if the respective trigger condition is fulfilled. At all time points at which there is no processing of the data, any logic states may be present at the inputs, without that they are involved in the processing. The respective flip-flops corresponding to the timing chart also includes in a corresponding time periods X when the logic state applied is not relevant.

Example D- flip-flop

Do not Cares in Karnaugh map

Practical circuit use

In a practical circuit using Don't- Cares have unique advantages. The entire circuit can be much more compact using Do not Cares in the corresponding circuits. For example, the entire circuit can be realized with a smaller number of individual gates or devices with less inputs. This can lead to more cost-effective solutions. At the same time more compact circuits are often faster, if less components must be connected in series.

Limitations

A circuit with Don't- Cares may only be used in the area for which the do not have Cares validity. The use of this circuit is then allowed if the domain compared to the original domain is further restricted. In the other case, in which the domain is expanded, then this circuit satisfies the requirements may no longer. For example, the circuit then in some cases no longer behave properly.