In the Spartan-6 FPGA Data Sheet v3.0 Table 34, why do higher Drive options sometimes have larger SSO values
The SSO numbers in Table 32 assume a common IOSTANDARD drive strength for all of the pins, including both the aggressors and the victim.
Higher-drive victims have more noise margin, and therefore, are sometimes better. As a consequence, this table is optimal for those cases where the whole bank is populated by the same IOSTANDARD. For example, LVCMOS33 Slow in Bank 0/2 increases fromone SSO per Vcco/GND pair at 16 mA drive totwo at 24 mA drive.
In general, for a given pin, reducing the Drive level and/or the Slew rate of other pins in the bank will improve the SSO characteristics.
For all other cases, PlanAhead software 12.4 and later versions handle mixed standards.