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What are VHDL attributes and how do I use them?

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Attributes are a useful VHDL mechanism that allow you to extract additional information of an object. To use an attribute we write the name of the object we want to apply the attribute on followed by an apostrophe and the attribute name. Attributes are widely used when writing generic VHDL code.

Example :

constant width_of_some_input : integer := some_input ' length ; -- "some_input" maybe an entity port of unconstrained length. By using the ' length attribute we are able to extract its length.

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Attributes are a way of attaching additional information to an entity in VHDL. Attributes are assigned to entities through attribute specifications.

When an attribute specification is done, the declared attribute name is used for reference. While declaring an attribute, a type identifier is mentioned to define the type of user-defined attribute. A simple syntax is given below for attribute declaration:

attribute att_name : type;

For assigning an attribute to the desired entity, attribute specification is done as depicted by the simplified syntax below:

attribute att_name of item : item_class is expression;

In the above code snippet, 'expression' specifies the attribute value.

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