Xilinx Synthesis Technology User Guide

XST User Guide
Note In combinatorial processes, if a signal is not explicitly assigned
in all branches of "if" or "case" statements, XST will generate a latch to
hold the last value. To avoid latch creation, assure that all assigned
signals in a combinatorial process are always explicitly assigned in all
paths of the process statements.
Different statements can be used in a process:
• Variable and signal assignment
• If statement
• Case statement
• For...Loop statement
• Function and procedure call
The following sections provide examples of each of these statements.
If...Else Statement
If...else statements use true/false conditions to execute statements. If
the expression evaluates to true, the first statement is executed. If the
expression evaluates to false (or x or z), the else statement is executed.
A block of multiple statements may be executed using begin and end
keywords. If ... else statements may be nested. Example 6-12 shows
the use of an If...else statement.
6-22 Xilinx Development System