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Cora DDR series termination resistors


Sean Kelly

Question

I noticed that on the Cora dev kit there are series termination resistors on the address and control nets and wondering why.

Does not Bank 502 not have the ability to use DCI to create the 40 ohm output impedance?

Why are the VRP/VRN resistors 80 ohms instead of 40?

Thanks,

Sean

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Hello elodg, thank you for the help. 

So if I understand you correctly, Digilent feels that the drive needs impedance matching but Xilinx does not?

Why would all of your other dev kits with the Xilinx parts not need the series termination but the Zync does?  Any insight would be really appreciated.

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Each design is validated independently. Any tactic can be employed as long as the DDR3 specifications are met. Ideally the line is terminated at both ends, and drive impedance matches line impedance and input impedance. Some designs only employ parallel receiver termination, others only series drive impedance matching.  The Cora Z7 is the latter, the Xilinx dev kit is probably the former.

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I find that there being a need for a series resistor of that magnitude to match a 40 ohm TL to bring the signalling into compliance and yet Xilinx not thinking that DCI is needed to bring the output impedance closer to 40 ohms and hence not having a need for the 22 ohm part quite difficult to comprehend.  Especially since they have it for DDR2.

Any thoughts as to why they would have removed the option?  Please keep in mind my expertise is in PCB layout and not FPGAs so I am learning more about them.

 

By the way, is it possible to get the gerber files for the Cora dev kit?

Thanks

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DDR2/DDR3 designs work fine without driver impedance control, even in non-point-to-point topologies. Maybe Xilinx considered that receiver termination is enough to cover all use cases and the board designer should provide that. In our point-to-point topology, we could achieve compliance with a much simpler driver impedance adaptation.

We do not generally provide gerbers. If you think your use case is legitimate, contact us at https://store.digilentinc.com/further-assistance/ and we might be able to provide them under NDA.

 

 

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