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evaluation board explanation of drive strengths for controller and memory and channel impedance

paul kerstetter
Intellectual 455 points
Other Parts Discussed in Thread: OMAP-L138

My question involves the use of LPDDR memory and the TMS320C6X (C6748/OMAP-L138) dsp evaluation board design.  In the design there is the use of 22 ohm series resistors and I believe that they are used near the dsp controller for the DQ lines.  A 50 ohm characteristic impedance is used for the DQ transmission line.  So far I've been told that the drive strengths used for the developement board for both the controller and memory are full strenght.  I believe this equates to 25 ohms output impedance for the drivers.  So the 22 ohms makes sense for transmission in one direction but not the other when considering the correct termination for the transmision line should be about 50 ohms.  What am I missing?  Does full strenght mean that both the controller and the memory have driver output impedances of 25 Ohms?  Im not to familiar with LPDDR interfaces but I do know that a matched transmission line is what is desired for high speed signals.

  • 0
    TI__Guru* 84110 points

    Paul,

    Welcome to the TI E2E forum. You will find many good answers here and in the TI Home Page documents and in the TI Wiki Pages. You can search all of these for helpful information and browse this forum for the questions others have asked. Our new E2E red Search box near the upper-right corner has improved functionality, and it is the key to getting the most out of the forum as we include more and more answers to questions like yours.

    The DDR2/mDDR interface is a very critical one to build correctly. TI supports these interfaces by providing the design rules, such as in Section 6.11 of the OMAP-L138 datasheet. As long as you use memories that meet the specified JEDEC standard and you follow the board design and layout rules, you will be sure it will work. We do not support detailed analysis of the components on the board or why the rules are specified.

    This does not answer your question, but I hope it helps you understand our position and how we can most easily help you to be successful.

    Regards,
    RandyP

     

    If you need more help, please reply back. If this answers the question, please click  Verify Answer  , below.

     

  • 0 in reply to RandyP
    Intellectual 455 points

    I talked with micron about this and they weren't sure what TI is doing on the evaluation board either.  Anyway, we have the ability to simulate the channels through the use of IBIS models and hyperlnyx so this should answer the question I hope.