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Reflection and Termination

Roibert , 10-23-2017, 12:52 PM
Dear forum

​In a famous book wrote by Bogatin, i read that we can consider a transmission line as a capacitor, if we respect this limit:

​ Lenght (inch) < 2 * RiseTime (ns)

​the means, we can trascurate the reflection ,so, we dont need any adaption and termination resistence.

In mm become:

​ Lenght (mm) < 50 RiseTime(ns).

This means, if we are working to 100MHZ, so that RT=1ns circa (a approximation), we have

Lenght < 50 mm= 5cm

So question is:

​-If I have to connect a DDR ,(witch tipical works at 100MHZ) ,can we avoid terminations (so saving space )
if we put DDR and uP less than this distance ? (5cm) .

​Many Thanks for your time.

robertferanec , 10-24-2017, 11:22 AM
I have routed several memory interfaces with no series nor parallel termination - they work oki. It depends not only on distance, it depends also on topology. For example, if you use balanced T-branch topology, you may be able to use it with no termination (up to 500MHz, maybe even more). Search for "imx6 design guide", it may help you (pages 44 and 45, 2 and 4 Gigabyte recommendations: ): https://cache.nxp.com/docs/en/user-g...6DQ6SDLHDG.pdf
Roibert , 10-24-2017, 12:36 PM
Thanks very much for reply..Very useful information.

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