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mathieu
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Registered: ‎06-27-2018

Internal VRef for DDR4 on KCU105 board.

Hello

 

Background :


We are currently experiencing some issues with one of our custom board DDR3 interface, we suspect an issue with Vref as no pulld down resistors are populated (fortunately the pads are present and can be populated)

For our design, we looked at the KCU105 board schematics as a reference to implement our DDR3 interface.

If I understood well from the documentation (pg150-ultrascale-memory-ip.pdf and ug571-ultrascale-selectio.pdf) :

- DDR4 must use internal Vref 

- when using internal Vref settings the corresponding Vref pin must pulled down to ground with a 500 Ohm or a 1 kOhm resistor

 

Questions :

I would like to know why the resistor R686 and R695 on the schematics (see picture attached) seem not populated DNP (do not populate), which seems in contradiction with the comment on the schematics.

Did I miss something ?

Is the pull down resistor on Vref really necessary for DDR3 and DDR4 interface when using internal Vref ?

 

Thanks in advance for your help.

Best regards

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KCU105_DDR4_sheet_5.PNG
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rpr
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238 Views
Registered: ‎11-09-2017

Hi @mathieu 


Eval Boards are designed before the Device is characterized and so not recommend Eval Boards design. Please refer to UG583 http://www.xilinx.com/support/documentation/user_guides/ug583-ultrascale-pcb-design.pdf for Ultrascale Design Guidelines.

Regards
Pratap

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