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gjacques
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Registered: ‎05-06-2011

IBUFDS Attributes

For the IBUFDS attribute DIFF_TERM, what represents FPGA internal differential termination used (TRUE or FALSE)?

 

 

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eteam00
Instructor
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Registered: ‎07-21-2009

This is a mess!!

 

If you look in the Spartan-6 SelectIO User Guide UG381 (v1.4), page 23, it is mostly clear that "DIFF_TERM=TRUE" means 'yes, include input termination'.  Sadly, if you look at theSpartan-6 Libraries Guide for HDL UG615 (v12.4), page 118 "DIFF_TERM" does not appear in the list of available attributes for IBUFDS.  But DIFF_TERM does appear in the Verilog and VHDL example templates.

 

That's mess #1.

 

If you look in the Virtex 6 SelectIO User Guide UG361 (v1.3), page 32, it is mostly clear that "DIFF_TERM=TRUE" means 'yes, include input termination'.  Sadly, if your look at the Virtex-6 Libraries Guide for HDL UG623 (v13.1), page 151 "DIFF_TERM" does not appear in the list of available attributes for IBUFDS.  But DIFF_TERM does appear in the Verilog and VHDL example templates.

 

That's mess #2.

 

Bottom line:  "DIFF_TERM=TRUE" means 'yes, include input termination'.

 

Bottom line #2:  Just because you find an answer in one doc does not mean you can stop looking for another answer to the same question.  Sigh....

 

This probably warrants a webcase and a CR.

UPDATE:  I've opened webcase # 876157, referencing this thread.

 

-- Bob Elkind

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gjacques
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Registered: ‎05-06-2011

Thanks, I spent at least 2 hours searching for the answer and I could not find a single Xilinx document for any Silicon part that explicitly defined DIFF_TERM attibute TRUE/FALSE.  I too have opened a web case.

 

 

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eteam00
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Registered: ‎07-21-2009

I too have opened a web case.

The webcase you have opened is redundant and spurious.

 

The meaning of DIFF_TERM=TRUE/FALSE is already known, you already have the information you need.

The purpose of the webcase is to clean up the docs to make them more consistent and explicitly clear.

 

-- Bob Elkind

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eteam00
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Registered: ‎07-21-2009

Under Webcase #876157, Change Request (CR) #612035 has been created to clean up the docs involved in this 'mess'.

 

I have asked for a direct answer to the questions below, but have not (yet) received a response.

What does DIFF_TERM="TRUE" mean?  Are different values of diff termination available?

-- Bob Elkind

SIGNATURE:
README for newbies is here: http://forums.xilinx.com/t5/New-Users-Forum/README-first-Help-for-new-users/td-p/219369

Summary:
1. Read the manual or user guide. Have you read the manual? Can you find the manual?
2. Search the forums (and search the web) for similar topics.
3. Do not post the same question on multiple forums.
4. Do not post a new topic or question on someone else's thread, start a new thread!
5. Students: Copying code is not the same as learning to design.
6 "It does not work" is not a question which can be answered. Provide useful details (with webpage, datasheet links, please).
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mcgett
Xilinx Employee
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Registered: ‎01-03-2008

> What does DIFF_TERM="TRUE" mean?  Are different values of diff termination available?

 

Setting the value to TRUE enables the 100 ohm termination between the P and N sides of a differential pair.

Setting the vlaue to FALSE disables to the 100 ohm termination between the P and N sides of a differential pair.

 

No other options are available.

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eteam00
Instructor
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Registered: ‎07-21-2009

Thanks, Ed.

 

Response from Webcase support tech matches Ed's description, exactly.  Here are some diagrams to accompany the description (which I suspect are excerpted from a Spartan-6 doc).

 

diffterm_false.png

 

diffterm_true.png

 

-- Bob Elkind

SIGNATURE:
README for newbies is here: http://forums.xilinx.com/t5/New-Users-Forum/README-first-Help-for-new-users/td-p/219369

Summary:
1. Read the manual or user guide. Have you read the manual? Can you find the manual?
2. Search the forums (and search the web) for similar topics.
3. Do not post the same question on multiple forums.
4. Do not post a new topic or question on someone else's thread, start a new thread!
5. Students: Copying code is not the same as learning to design.
6 "It does not work" is not a question which can be answered. Provide useful details (with webpage, datasheet links, please).
7. You are not charged extra fees for comments in your code.
8. I am not paid for forum posts. If I write a good post, then I have been good for nothing.
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mcgett
Xilinx Employee
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Registered: ‎01-03-2008

Those diagrams are correct.

The first wih DIFF_TERM = FALSE shows an external 100 ohm terminator and nothing within the IBUFDS.

The second with DIFF_TERM = TRUE shows no external termination and an internal Rdt terminator within the IBUFDS.
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