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AR# 15156

Virtex-II/Pro - SSO Guidelines: How do I account for a different impedance (reference resistor) used with LVDCI I/O?


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 example:

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.

For example:

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.

For more about handling SSO noise, please see (Xilinx XAPP689): Managing Ground Bounce in Large FPGAs.

AR# 15156
Date 12/15/2012
Status Active
Type General Article
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