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Attributes :

Data Attributes :

The pre-defined, synthesizable data attributes in VHDL are,

  • data'LENGTH : Returns vector size.
  • data'RANGE : Returns vector range.
  • data'REVERSE_RANGE : Returns vector range in reverse order.
  • data'LOW : Returns lower array index.
  • data'HIGH : Returns upper array index.
  • data'LEFT : Returns leftmost array index.
  • data'RIGHT : Returns rightmost array index.

Signal Attributes :

Although the most signal attributes are used for simulation purposes only, the first two in the list bellow are synthesizable.
  • tick'EVENT : Returns true when an event occurs.
  • tick'STABLE : Returns true if no event has occurred.
  • tick'LAST_ACTIVE : Returns the time elapsed.
  • tick'LAST_VALUE : Returns the value before the last event.
  • tick'ACTIVE : Returns true.
  • tick'QUIET
  • tick'LAST_EVENT : Returns the time elapsed since last event.

User-defined Attributes :

The attributes discussed in previous section are of the type HIGH, RANGE, EVENT, etc. and are called as pre-defined. However, VHDL also has the construction of user defined attributes. To have a user-defined attribute, it can declared and specified. The syntax is the follows,

Attribute declaration :

  ATTRIBUTE attribute_test: attribute_test;

Attribute specification :

ATTRIBUTE attribute_test OF target_test: class IS value;
where :
	attribute_test any data type (BIT, INTEGER, STD_LOGIC_VECTOR)
	value: '0', 27, "00 11 10 01"