Sean Kelly Posted July 2, 2020 Share Posted July 2, 2020 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 Link to comment Share on other sites More sharing options...
elodg Posted July 2, 2020 Share Posted July 2, 2020 From ug585 pg. 310: For DDR3 DCI calibrates the termination impedance and not the driver impedance. Series termination resistors is a form of drive impedance adaptation. Link to comment Share on other sites More sharing options...
Sean Kelly Posted July 2, 2020 Author Share Posted July 2, 2020 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. Link to comment Share on other sites More sharing options...
elodg Posted July 2, 2020 Share Posted July 2, 2020 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. Link to comment Share on other sites More sharing options...
Sean Kelly Posted July 2, 2020 Author Share Posted July 2, 2020 So basically the validation/simulation found the driving impedance to be ~18 ohms on the Zync Bank 502 outputs? Link to comment Share on other sites More sharing options...
elodg Posted July 2, 2020 Share Posted July 2, 2020 What you are saying is a tad too big of a leap. All I can say is that 22-ohm series resistor added to the driving impedance met the design requirements. Link to comment Share on other sites More sharing options...
Sean Kelly Posted July 2, 2020 Author Share Posted July 2, 2020 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 Link to comment Share on other sites More sharing options...
elodg Posted July 2, 2020 Share Posted July 2, 2020 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. Link to comment Share on other sites More sharing options...
Sean Kelly Posted July 2, 2020 Author Share Posted July 2, 2020 Thank you for the help and insight. Link to comment Share on other sites More sharing options...
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Sean Kelly
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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