AR# 40818

Description

It has been discovered that in Spartan-6 FPGA designs that utilize inputs implemented with split internal termination (UNTUNED_SPLIT_XX, where XX is 25, 50, or 75), no internal termination is enabled in hardware. Common indications that are observed include:

• unexpected reflections/poor signal integrity
• lower than expected power consumption
• lack of a bias on a signal

This issue does not impact bi-directional signals, so MIG/Memory interface-based designs are not typically affected.

Solution

For software versions prior to and including 13.2, the termination can be enabled by making a signal bi-directional with the 3-state control permanently enabled High (output disabled).

Note: the Save constraint must be utilized to avoid optimization back into an input.

This issue is resolved in ISE version 13.3. Later versions are free from this limitation.

Additional Notes:

Because the IN_TERM attribute is IOSTANDARD-independent, this issue can occur with many standards; most commonly this would be expected in SSTL, HSTL, and LVCMOS.

The issue is isolated to BitGen, so throughout the implementation process, the design would indicate that IN_TERM is appropriately enabled.

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Master Answer Records