Interface - Declare Lower-level Sources for Hierarchical Designs

The interface keyword is used for hierarchical designs to pass signal names from lower-level sources to the top-level sources that use those signals. To create a hierarchical design, you need the following

In the top-level source:

An interface declaration for the lower-level source.

One or more functional_block statements instantiating the lower-level source for use in the top-level source.

In the lower-level source:

An interface declaration of the signals that can be used by top-level sources, and an optional default value for the input signals.