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DM36X DDR ODT problem

micos
Prodigy 60 points

Hi,all,

When using  DM36X,I met a problem about DDR2.According to DM36X datasheet,DDR2 ODT pin should be pull down to ground,so I did.

But found it  work unstable,often crashed.And I pull up ODT pin to 1.8V,in this case,it works well with any crash.So can anyone explain this?

  • 0
    TI__Mastermind 27585 points

    Does your board already have series terminators on the DDR lines? If not, then the DDR should be set in reduced drive strength mode.

    ODT is not supported and should be pulled low on the memories.

    Jeff

  • 0 in reply to jc-ti
    Prodigy 60 points

    Hi,Jeff,

    On my board,there have 22ohm series terminators on DDR2 AB/DB line.

    There is no any problem in former version of this board with DDR2 ODT pulled down,After revised the PCB,the problem come out,and ODT must be pulled up so as to run well.In this revision of PCB,we did not move a line of DDR part,it's the same as former version.Is't prossible the problem of impedance matching?Any idea?

    Tks!

  • 0 in reply to micos
    TI__Mastermind 27585 points

    Can you list the changes made to the PCB to this revision from the previous working one? Does it still comply with the routing rules in the datasheet?

    Jeff

  • 0 in reply to jc-ti
    Prodigy 60 points

    This revision didnot make any changes about the DDR part,PCB routing is the same as previous revision.Is there any special routing rules of the DDR?We do it according normal DDR routing rules.

  • 0 in reply to micos
    TI__Mastermind 27585 points

    You mentioned the problem occurred after revising the PCB. What were those changes made?

    Yes there are very specific DDR routing rules found in the datasheet that must be followed.

    Jeff

  • 0 in reply to jc-ti
    Prodigy 60 points

    Jeff,

    Thanks for your reply.

    The change between two revisions is Nand flash connection,we correct the wrong connection of Nand flash in provious revision.But we don't use NAND in our application,just reserved for further use.So the board boot from Nor flash.

    I did a test in privours revsion which ODT was pulled down and work well,I change the ODT pulled down resistor to pulled up resistor,as a result,the board remain work well.So I got a conclusion:

    (1) Even DM36X DDR controller can't support ODT control,also datasheet recommend to pulled down ODT,pulled up DDR ODT is more safe than pulled it down

    (2) Make a redundant hardware design of pulled up/pulled down resistor will be acceptable

    Hope these will be helpful for someone met the similar problem.