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Xilinx Synthesis Technology User Guide

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Counters<br />

HDL Coding Techniques<br />

XST is able to recognize counters with the following control signals:<br />

• Asynchronous Set/Clear<br />

• Synchronous Set/Clear<br />

• Asynchronous/Synchronous Load (signal and/or constant)<br />

• Clock Enable<br />

• Modes (Up, Down, Up/Down)<br />

• Mixture of all of the above possibilities<br />

HDL coding styles for the following control signals are equivalent to<br />

the ones described in the “Registers” section of this chapter:<br />

• Clock<br />

• Asynchronous Set/Clear<br />

• Synchronous Set/Clear<br />

• Clock Enable<br />

Moreover, XST supports both unsigned and signed counters.<br />

XST <strong>User</strong> <strong>Guide</strong> 2-35

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