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The Constant Statement allows you to substitute a meaningful symbolic name (a constant) for a number, text string, or value of an arithmetic expression (where the value is a number or text string). When you use a constant in a Text Design File (.tdf), the Compiler replaces the constant with the number, text string, or value assigned to the constant in the Constant Statement.
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The Compiler evaluates each arithmetic operator in the arithmetic expression for the evaluated function, and reduces the expression to a value (a number or text string). The Compiler does not generate logic for the expression. |
The following examples show Constant Statements:
CONSTANT UPPER_LIMIT = 130; CONSTANT BAR = 1 + 2 DIV 3 + LOG2(256);
The Constant Statement has the following characteristics:
The Constant Statement begins with the keyword
CONSTANT, followed by a symbolic name, an equals symbol (=),
and a number (including a radix, if necessary), text string,
or arithmetic expression.
The statement ends with a semicolon (;).
Once a constant is declared, you can use it to represent the number, text string, or value
throughout a TDF. For example, you can use the UPPER_LIMIT constant shown above to represent the decimal number 130 anywhere in the TDF where you declare the constant.
In arithmetic expressions, you can use previously defined constants, evaluated functions, or parameters to define constants. Example
The value of the arithmetic expression that you use to define a constant can be a text string. These text strings can be the values of previously defined parameters, constants, or evaluated functions.
Constant Statements must conform to the following rules:
A constant can be used in a TDF only after it is defined in the TDF, or in an AHDL Include File (.inc) that is included in the TDF.
Each constant name must be unique in a TDF.
The constant name cannot contain spaces. Use underscores to separate the words in the name and improve readability.
The Constant Statement can be used any number of times in a TDF.
The Constant Statement must be placed outside all other AHDL sections.
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