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Visitor mvitamantys
Visitor
2,494 Views
Registered: ‎12-08-2011

XAPP 224 and constraints on Spartan


I'm using the XAPP 224 source code on a Spartan 6 device. I have a question about the constraints and the timing of the code provided with the XAPP.

Using the provided constraints (in particular RLOC / IOB false for the 4 FFs: ff_a0, ff_b0, ff_c0, ff_d0, plus the MAXSKEW constraint) the logic works on a real device even if I can't achieve the MAXSKEW constraint. I am using 25pS (as in the XAPP) and even if the 4 FFs are placed as closed as possible together (I checked it with PlanAhead), the skew I can achieve is more than 150pS. If I remove the RLOC and IOB false constraints, the skew is very good (7-10pS). But if I check with PlanAhead one ff_a0 is placed in the IOB, while the other 3 (ff_b0, ff_c0 and ff_d0) are placed far away from ff_a0 (and the design doesn't work).

My questions are:
- if the ff are placed so far away, why is the skew so good?
- is it possible to achieve very small maxskew (like 25pS used in the XAPP) using Spartan 6 device?

Note: I'm using a xc6slx25t-3fgg484 device and the design is running at 250MHz.

Regards,
Matteo

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2 Replies
1,113 Views
Registered: ‎08-29-2016

Re: XAPP 224 and constraints on Spartan

hello,sir ,i also came across the  problem ,

could u tell me how did u deal with the problem?

thank u for your advice

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516 Views
Registered: ‎03-07-2018

Re: XAPP 224 and constraints on Spartan

Did you solve this problem?

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