• 0
Sean Kelly

Cora DDR series termination resistors

Question

Posted (edited)

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

Edited by Sean Kelly

Share this post


Link to post
Share on other sites

8 answers to this question

Recommended Posts

  • 0
Posted (edited)

From ug585 pg. 310:

image.png.37e64690ffffcab05e0f9052e8fc0952.png

For DDR3 DCI calibrates the termination impedance and not the driver impedance. Series termination resistors is a form of drive impedance adaptation.

Edited by elodg

Share this post


Link to post
Share on other sites
  • 0

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.

Share this post


Link to post
Share on other sites
  • 0
Posted (edited)

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.

Edited by elodg

Share this post


Link to post
Share on other sites
  • 0

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.

Share this post


Link to post
Share on other sites
  • 0

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

Share this post


Link to post
Share on other sites
  • 0
Posted (edited)

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.

 

 

Edited by elodg

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now