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Xilinx Synthesis Technology User Guide

Xilinx Synthesis Technology User Guide

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XST <strong>User</strong> <strong>Guide</strong><br />

Figure 3-1 Grouping through Incremental <strong>Synthesis</strong><br />

LEVA<br />

incremental_synthesis=true<br />

LEVA_1<br />

my_add<br />

RESYNTHESIZE<br />

VHDL Flow<br />

TOP<br />

my_sub<br />

LEVB<br />

incremental_synthesis=true<br />

LEVA_2 my_and my_or<br />

X9858<br />

For VHDL, XST is able to automatically recognize what blocks were<br />

changed and to resynthesize only changed ones. This detection is<br />

done at the file level. This means that if a VHDL file contains two<br />

blocks, both blocks will be considered modified. If these two blocks<br />

belong to the same group then there is no impact on the overall<br />

synthesis time. If the VHDL file contains two blocks that belong to<br />

different groups, both groups will be considered changed and so will<br />

be resynthesized. <strong>Xilinx</strong> recommends that you only keep different<br />

blocks in the a single VHDL file if they belong to the same group.<br />

Use the RESYNTHESIZE constraint to force resynthesis of the blocks<br />

that were not changed.<br />

3-12 <strong>Xilinx</strong> Development System

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