See Also

A node, which is declared with a Node Declaration in the Variable Section, can be used to hold the value of an intermediate expression.

Node Declarations are especially useful when a Boolean expression is used repeatedly. The Boolean expression can be replaced with a descriptive node name, which is easier to read.

The boole2.tdf file shown below contains the same logic as boole1.tdf (shown in Implementing Boolean Expressions & Equations), but has only one output.

SUBDESIGN boole2
(
   a0, a1, b   : INPUT;
   out         : OUTPUT;
)
VARIABLE
   a_equals_2  : NODE;
BEGIN
   a_equals_2 = a1 & !a0;
   out = a_equals_2 # b;
END;

This file declares the node a_equals_2 and assigns the value of the expression a1 & !a0 to it. Using nodes can save device resources when the node is used in several expressions.

Both ordinary nodes (NODE keyword) and tri-state nodes (TRI_STATE_NODE keyword) can be used. NODE and TRI_STATE_NODE differ in that multiple assignments to them yield different results:

• Multiple assignments to nodes of type NODE tie the signals together by wired-AND or wired-OR functions. The default values for variables declared in Defaults Statements determine the behavior: a VCC default produces a wired-AND function; a GND default produces a wired-OR function. If a variable does not have a default value, a wired-OR function is used to tie the signals together.

• Multiple assignments to a TRI_STATE_NODE tie the signals to the same node.

• If only one variable is assigned to a TRI_STATE_NODE, it is treated as NODE.

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