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Xilinx Synthesis Technology User Guide

Xilinx Synthesis Technology User Guide

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Multiple Wait Statements Descriptions<br />

VHDL Language Support<br />

Sequential circuits can be described with multiple wait statements in<br />

a process. When using XST, several rules must be respected to use<br />

multiple wait statements. These rules are as follows:<br />

• The process must only contain one loop statement.<br />

• The first statement in the loop must be a wait statement.<br />

• After each wait statement, a next or exit statement must be<br />

defined.<br />

• The condition in the wait statements must be the same for each<br />

wait statement.<br />

• This condition must use only one signal—the clock signal.<br />

• This condition must have the following form:<br />

"wait [on ] until [('EVENT |<br />

not 'STABLE) and ] = ;"<br />

Example 6-22 uses multiple wait statements. This example describes<br />

a sequential circuit performing four different operations in sequence.<br />

The design cycle is delimited by two successive rising edges of the<br />

clock signal. A synchronous reset is defined providing a way to<br />

restart the sequence of operations at the beginning. The sequence of<br />

operations consists of assigning each of the four inputs: DATA1,<br />

DATA2, DATA3 and DATA4 to the output RESULT.<br />

XST <strong>User</strong> <strong>Guide</strong> 6-31

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