Additionally, users can define new attributes. They allow specifying precisely where warnings or errors are generated.
Attributes are a feature of VHDL that allow you to extract additional information about an object (such as a signal, variable or type) that may not be directly related to the value that the object carries. Table 3.
About VHDL Attributes A VHDL attribute associates a named value with a VHDL object.
For example, assume that the input As given in the above code, we can apply this range to the definition of the object (In my simulation code, line 17 has the assignment Note that specifying a smaller range does not always mean that we can represent the signal with a smaller number of bits. In the VHDL standard a set of predefined attributes is defined. Normally, we want a wire in a digital interface to have either the value '1' or '0'. Understanding VHDL Attributes . For example, consider the following declarations:The first two declarations require a three-bit representation even though the second declaration has a smaller range. Affectations littérales Literal assignments. The std_logic gives us a more fine-grained control over the resources in our design than the integer type, which we have been using in the previous tutorials.. For example, the following lines define the signal As shown in Figure 2, the integer data type is in the “standard types” category which is defined in the “standard” package from “std” library.
An attribute gives extra information about a specific part of a VHDL description. For example, if an intermediate operation is to add two integers with range 0 to 15, the result of this intermediate calculation will have enough bits to represent the largest possible value, which is 30 (for more details see Section 4.6.4 of this A subtype of a given type restricts the type range. Logic synthesis tools usually support the predefined attributes 'high, 'low, 'left, 'right, 'range, reverse_range, 'length and 'event. However, since this won’t be an optimal implementation, the synthesis software will perform some optimizations according to the nature of the utilized operators. As shown in Figure 2, “integer” has two predefined subtypes:The value integer’high represents the highest value of the integer. Predefined attributes can be contants, functions or signals. How many bits will be used to represent the integer signals in the above code? See Attributes (user-defined) for details. Predefined attributes are in the Predefined Attributes section attribute identifier : type_mark ; attribute enum_encoding : string; -- user defined type my_state is (start, stop, ready, off, warmup); attribute enum_encoding of my_state : type is "001 010 011 100 111"; signal my_status : my_state := off; -- value "100" attribute specification Read 3 answers by scientists with 3 recommendations from their colleagues to the question asked by Ayman Yousef on Jun 28, 2015
For example, from 200 ns to 300 ns, the inputs With the integer data type, we are not directly involved in the bit-level definitions; however, it’s clear that the implementation will use a number of bits to represent the defined signals. It should be noted that this attribute could be used only as a prefix for other attributes. The most common type used in VHDL is the std_logic.Think of this type as a single bit, the digital information carried by a single physical wire. This article will discuss the integer data type and its subtypes.We can use the integer data type to define objects whose value can be a whole number.