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Explorer
Explorer
787 Views
Registered: ‎12-11-2017

Changing the AC701 VADJ... the hack-ish way

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Not so much a question as it is a gripe.

So the AC701 FMC header comes configured as VADJ=2.5V. I need 1.8V. I don't have the special TI USB to PMBus adapter or a Windows machine handy to run the their tool. And hack an XML file? Right.

What to do?

After weighing my options... the answer is... get out the soldering iron and swap the resistors R204 and R435 (version 2.0 schematic.) Then, VADJ comes up at 1.8V. About 2 minutes work and zero dollars in equipment.

 

Q: Is there any unintended effect of making this change, or should it be OK?

Xilinx people, I have two observations / feedback items:

 

(1) Why doesn't AC701 VADJ come this way from the factory? VC707 and VC709 are 1.8V by default (I know, the bigger Virtex parts can't do higher than 1.8, but still - should be somewhat consistent across platforms, yes?)

 

(2) There should be a way to do change VADJ without the headache of messing with PMBus. That's a huge investment in time just to make one adjustment. How about having that fancy TI controller read straps? I know that seems anachonistic, considering what PMBus can do, but in this case PMBus is an impediment, not a help.

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Explorer
Explorer
595 Views
Registered: ‎12-11-2017

Re: Changing the AC701 VADJ... the hack-ish way

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Followup: I can attest that this mod works fine - been using the board this way for months.

View solution in original post

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Xilinx Employee
Xilinx Employee
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Registered: ‎06-06-2018

Re: Changing the AC701 VADJ... the hack-ish way

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Hi @vortex1601,

 

Xilinx doesn't recommend to make any physical changes to the boards. It will void your board warranty.

 

Thanks and Regards,

Deepak D N

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Regards,
Deepak D N
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Explorer
Explorer
737 Views
Registered: ‎12-11-2017

Re: Changing the AC701 VADJ... the hack-ish way

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I (well, my employer) can live with that. I already had to grind down the SMB's next to the FMC header because they don't clear my test board.

Now, to my question, is there a side-effect of making that resistor change? I'm thinking no, but I'd like to confirm.

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Explorer
Explorer
596 Views
Registered: ‎12-11-2017

Re: Changing the AC701 VADJ... the hack-ish way

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Followup: I can attest that this mod works fine - been using the board this way for months.

View solution in original post

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77 Views
Registered: ‎08-08-2015

Re: Changing the AC701 VADJ... the hack-ish way

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@vortex1601, this has also worked for me. Good idea. Since they make it a pain to change FMC_ADJ_SEL[1:0] away from the default "00", simply make "00" the value that sets VADJ to 1.8V. Who needs warranties anyway?