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Registered: ‎03-03-2017

Vivado 2019.1 7-series device "sub-optimal placement for BUFG-BUFG and MMCM-BUFG"

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I have been struggling all day trying to get a design to pass on an Artix-7 75T device that passes successfully on an Artix-7 100T device with the same package.   I am using Vivado 2019.1.   The main failure seems to be BUFG-BUFG sub-optimal placements problems.   I guess what I don't understand is the fact that both the 100T and 75T devices should have the same number of BUFGs so I wouldn't think I would be struggling so much with this.

I assume I can open the synthesis and select all BUFGs and place them manually.   Can somebody help with the commands to a) find and select all BUFG objects, and b) how do I place them into the Device view?

Thanks in advance.

Tim

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Registered: ‎01-22-2015

Re: Vivado 2019.1 7-series device "sub-optimal placement for BUFG-BUFG and MMCM-BUFG"

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Hi Tim,

     I assume I can open the synthesis and select all BUFGs and place them manually.

For the Artix-7 (xc7a75t and xc7a100t) there are 32 BUFG and they are all grouped together in the center of the die.   Usually, when a BUFG is needed, we let the tools select one and then (almost never) get a suboptimal placement warning.  The suboptimal placement warnings can occur when using LOC constraints to select a specific BUFG. 

So, if you have LOC constraints on your BUFG then an easy fix might be to get rid of the LOC constraints and see if the tools can pick all the right BUFG for you.

The trouble with LOCing your own BUFG is that they are divided into two groups (16 in top-half and 16 in bottom-half).  As shown in Table 1-1 of UG472 there are restrictions when connecting a BUFG in the one half to a BUFG in the other half.  Similarly, there are restrictions when connection a CMT (eg. MMCM) in one half to a BUFG in the other half.

Cheers,
Mark

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244 Views
Registered: ‎01-22-2015

Re: Vivado 2019.1 7-series device "sub-optimal placement for BUFG-BUFG and MMCM-BUFG"

Jump to solution

Hi Tim,

     I assume I can open the synthesis and select all BUFGs and place them manually.

For the Artix-7 (xc7a75t and xc7a100t) there are 32 BUFG and they are all grouped together in the center of the die.   Usually, when a BUFG is needed, we let the tools select one and then (almost never) get a suboptimal placement warning.  The suboptimal placement warnings can occur when using LOC constraints to select a specific BUFG. 

So, if you have LOC constraints on your BUFG then an easy fix might be to get rid of the LOC constraints and see if the tools can pick all the right BUFG for you.

The trouble with LOCing your own BUFG is that they are divided into two groups (16 in top-half and 16 in bottom-half).  As shown in Table 1-1 of UG472 there are restrictions when connecting a BUFG in the one half to a BUFG in the other half.  Similarly, there are restrictions when connection a CMT (eg. MMCM) in one half to a BUFG in the other half.

Cheers,
Mark

View solution in original post