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As far as calibrated vs non-calibrated, is there a reason why you do not want to use the 50 ohm with calibration?
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I forgot to design in the RUP/RDN resistors in my current design and would need to make modifications to it. I don't like to make unneccesary changes so if the calibration is not needed then, perhaps, I could leave the resistors out.
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If you have access to a simulation tool, you can run a simulation using the DDR model and see what margins you can tolerate. The slower clock means that you can tolerate some ringing due to Z mismatch, but... depending on your signal Tr and Tf and your trace lengths, the ringing, while it may not cause data errors, it may reduce your MTBF because of specs violations.
So, regardless of clock speed, make sure your impedances are matched - a simulation tool helps tremendously here.
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In my experience, the termination impedance can vary quite a lot without affecting real-life signal integrity significantly. I have done SI simulations with Hyperlynx and a +/- 20% impedance mismatch in transmission lines or termination resistors is ok. What I don't know, however, is how much improvement the calibrated termination is compared to un-calibrated.
I guess I'll just design in the resistors, just to be safe!
Thanks.
Honored Contributor
