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Contributor
Contributor
505 Views
Registered: ‎12-03-2018

PS_DDR_VRN/VRP termination resistors

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Hi,

I am designing a board with a Zynq XC7Z035-FBG676 using DDR3L memory.  I'm having trouble finding what resistor values I should use for the PS_DDR_VRN and PS_DDR_VRP pins.  I did a previous design with a Zynq 007 and DDR3L and I used 80.6 ohms on these pins and 40.2 ohm pullup resistors for the address pins.

The Xilinx ZC706 eval board has an XC7Z045 and uses 80.6 and 40.2 ohm resistors.  However, the memory is DDR3...not DDR3L.

My confusion comes from another reference design that I am looking at where the values for VRN/VRP are 240 ohms and the pullup resistors are 120 ohms.

Thanks,
Dan

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1 Solution

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Observer nima_taie
Observer
462 Views
Registered: ‎04-23-2018

Re: PS_DDR_VRN/VRP termination resistors

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Per MiniZed Development board from Avnet, the VRP/VRN are 80.6 Ohms for a DDR3L device on a ZYNQ 7Z007S. Hope this helps. 

5 Replies
Observer nima_taie
Observer
463 Views
Registered: ‎04-23-2018

Re: PS_DDR_VRN/VRP termination resistors

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Per MiniZed Development board from Avnet, the VRP/VRN are 80.6 Ohms for a DDR3L device on a ZYNQ 7Z007S. Hope this helps. 

Contributor
Contributor
460 Views
Registered: ‎12-03-2018

Re: PS_DDR_VRN/VRP termination resistors

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Thanks for the reply.  I've since learned that the 120 ohm termination resistors were from an Analog Devices reference design.  They state in their documentation that they found the higher values worked just fine and reduced power consumption.  I'm not comfortable designing my board based on a 3rd party's findings and have decided to go with the Xilinx recommended values that you mentioned.

 

Thanks,

Dan

 

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Observer nima_taie
Observer
456 Views
Registered: ‎04-23-2018

Re: PS_DDR_VRN/VRP termination resistors

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Wise choice on your part! Good luck!
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Moderator
Moderator
310 Views
Registered: ‎11-28-2016

Re: PS_DDR_VRN/VRP termination resistors

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Hello @edadangtri ,

Please follow the PS DDR PCB design guidelines covered in UG933 linked here starting on page 53:
http://www.xilinx.com/support/documentation/user_guides/ug933-Zynq-7000-PCB.pdf

DDR3 and DDR3L are functionally identical except for a decrease in the I/O voltage from 1.5V to 1.35V.  This has no impact on trace impedances.
The guidelines state to use 80-Ohm resistors on VRP/VRN and still terminate the CAC bus signals with a 40-ohm resistor to VTT.
ps_resistors.PNGddr3_rterm.PNG

Contributor
Contributor
305 Views
Registered: ‎12-03-2018

Re: PS_DDR_VRN/VRP termination resistors

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Thanks for the info.  This confirms my decision to follow the Xilinx guidelines.

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