I ran into this frustrating warning:
WARNING:Pack:2515 - The LUT-1 inverter
"fmc_imageov_dvi_out_0/fmc_imageov_dvi_out_0/dvi_reset_n_t1_INV_0" failed to
join the OLOGIC comp matched to output buffer
"fmc_imageov_dvi_out_0/fmc_imageov_dvi_out_0/OBUFT_dvi_reset_n". This may
result in suboptimal timing. The LUT-1 inverter
fmc_imageov_dvi_out_0/fmc_imageov_dvi_out_0/dvi_reset_n_t1_INV_0 drives
multiple loads.
for multiple pins.
If you get this, you simply need to make sure you don't have any combinatorial logic in the T (enable signal obviously)
First saw something similar here:
http://forums.xilinx.com/t5/Implementation/Pack-2515-XST-kept-optimizing-my-output-enable/td-p/256978
It really sounds like the enable function itself is notcoming from a register, but from combinatorial logic that can't be pushed into the IOB. As longas the output enable comes directly from a register, then pushing it into the IOB should work.
Just make the change and register the signal.
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