I have a Virtex II -5 and I would like to use the DDR registers in the IOBs. I read a few application notes involving these registers and it is always sais that the skew between two signals ouptut from DDR registers, especially if the IOBs are in the same bank, is negligible. What is negligible ? What if the IOBs are not in the same bank ? I read in a previous post that a reasonable 30% tracking could be expected between two IOBs but that this tracking was much much better when a same category of the device was considered. What is this tracking downto when considering IOBs ?