DIFF_TERM Question

Jul 03, 2007 3 Replies

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motty 19 years ago

For a Xilinx V4, there is an IBUFDS_DIFF buffer that you can use in the I/O pad. However, it appears that there is no differential termination resistor available for this setup. In XAPP861, it shows the buffer and the "optional DIFF_TERM" across the I and IB inputs. When I set the attribute to use the DIFF_TERM, Xilinx tools report that it is an invalid attribute. I assume that the tools are right and that for this particular buffer, there is not termination available. Could anyone give me a definitive answer?

Documentation and/or other info on this primitive is limited at best.

Also, there is a ULVDS standard supported in the I/O's but there isn't much info on that either (ie electrical specs).

Thanks in advance.

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motty 19 years ago

I swear every time I post here, I find some relevant info on Xilinx' site. It appears that the warning that the DIFF_TERM is not applicable is false. The diff term is applied to the IBUFDS_DIFF_OUT. However, the answer record did say that the warning would be fixed in 8.1 tools. I saw this problem in 8.2 tools.

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Bob 19 years ago

How are you setting the attribute? From what I recall, you have to add (something like) "DIFF_TERM = TRUE" in the UCF. I think that's different from the way you do it in V2Pro.

Also, the differential termination only works for banks where VCCO is 2.5V. What is the bank voltage (what are the other i/o standards defined for) in that bank?

Bob

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motty 19 years ago

The bank voltage is 2.5. I am using LVDS_25. I instantiate the buffer by hand and pass the attribute in there. The ISE has instantiation templates for most things. It doesn't have one for the IBUFDS_DIFF_OUT, but you can easily figure things out.

I'm just saying the tools give a warning and that is why I removed the attribute.

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