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nugentoffer
Contributor
Contributor
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Registered: ‎10-27-2010

Why need to use IOB constraint?

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Hello,

 

Could someone give me some pointers as to why you would need to use the IOB=true/false constraint? And what kind of situations?

 

Is there some advantage into moving some FF or other logic into an IO Buffer?

 

Thanks.

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gszakacs
Professor
Professor
4,102 Views
Registered: ‎08-14-2007

The most common reason is to help meeting the interface timing constraints.  Generally

the flip-flop in the IOB has the best setup, hold, and clock to output timing you can get

for any pin.  Also multiple IOB flip-flops on the same global clock net have very tight

skew timing, which makes them ideal for source-synchronous interfaces.

 

You don't HAVE to use the IOB constraint if you have timing constraints on the input

and output timing relative to the external clock pin.  Then the default is to automatically

push the flip-flops into the IOB when necessary to meet these constraints.

 

Regards,

Gabor

-- Gabor

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gszakacs
Professor
Professor
4,103 Views
Registered: ‎08-14-2007

The most common reason is to help meeting the interface timing constraints.  Generally

the flip-flop in the IOB has the best setup, hold, and clock to output timing you can get

for any pin.  Also multiple IOB flip-flops on the same global clock net have very tight

skew timing, which makes them ideal for source-synchronous interfaces.

 

You don't HAVE to use the IOB constraint if you have timing constraints on the input

and output timing relative to the external clock pin.  Then the default is to automatically

push the flip-flops into the IOB when necessary to meet these constraints.

 

Regards,

Gabor

-- Gabor

View solution in original post

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