18.104.22.168. default or OTHERS CASE Assignment
This assignment is especially important in one-hot encoding schemes where many combinations of the select lines are unused. Specifying a case for the unused select line combinations gives the synthesis tool information about how to synthesize these cases, and is required by the Verilog HDL and VHDL language specifications.
For some designs you do not need to consider the outcome in the unused cases, because these cases are unreachable. For these types of designs, you can specify any value for the default or OTHERS assignment. However, the assignment value you choose can have a large effect on the logic utilization required to implement the design.
To obtain best results, explicitly define invalid CASE selections with a separate default or OTHERS statement, instead of combining the invalid cases with one of the defined cases.
If the value in the invalid cases is not important, specify those cases explicitly by assigning the X (don’t care) logic value instead of choosing another value. This assignment allows your synthesis tool to perform the best area optimizations.
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