The Virtex-II Platform FPGA User Guide and the Virtex-II Pro Platform FPGA User Guide (Design Considerations -> Using Single-Ended SelectIO-Ultra Resources -> SSO Guidelines) indicates that the SSO for LVDCI has 50 ohms impedance (25 ohms for LVDCI_DV2).
Refer to the "Virtex-II Pro and Virtex-II Pro X FPGA User Guide" at:
Refer to the "Virtex-II Platform FPGA User Guide" at:
How do I calculate SSO for other impedance values if I use a different reference resistor value?
The drive strength progresses in line with the resistor: Higher impedance = less drive strength = more SSO per power/ground pair for the same ground bounce.
For a 65 ohm reference resistor using LVDCI_18 and an FG package, the calculation can be performed as follows:
(SSO at 65 ohms) = 65/50 * (SSO at 50 ohms) = 130% * (SSO at 50 ohms).
From the table, SSO for LVDCI_18 at 50 ohms = 13. Based on the calculation, you can have 16 SSO at 65 ohms.
Alternately, you can keep the same number of SSOs and have 33% less ground bounce.
For the LVDCI_DV2 I/O standard, use half of the number from your calculation.
SSO for LVDCI_DV2_18 25 ohms = 1/2 of LVDCI_18 at 50 ohms.
Similarly, SSO for LVDCI_DV2_18 32.5 ohm = 1/2 of LVDCI_18 at 65 ohms = 1/2 * 16 = 8.