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AM335x DDR2 Routing

Mark Norman
Prodigy 235 points


Hi,

I have a couple of queries regarding the DDR2 routing recommendations in the AM335x datasheet (sprs717d).

* Section 5.5.2.2.2.9 specifies for the Data net classes "No external termination resistors are allowed and ODT must be used for these net classes".  However, I noticed that all the dev boards (BeagleBone, AM335x EVM, etc) do use series termination resistors on these signals.  It will make the board a lot easier to route if I do not need these resistors however we have a very tight development schedule and can not afford to do a redesign if there is an issue.  Would you consider it safe to run without the termination resistors on the data nets?

* The example PCB stack-ups in the datasheet suggest that the DDR2 1.8V power plane can be used as a signal return plane however an app note by Analog Devices states that "Power planes cannot be used as signal return for the DDR2 interface" (Page 3 of http://www.analog.com/static/imported-files/application_notes/EE349_rev2.pdf).  Is this correct?

Kind Regards

Mark

  • 0
    TI__Guru**** 393215 points
    Hi Mark,
     
    On your first question: Please read the whole section 5.5.2.2.2.9. Table 5-48 calls for serial termination for frequencies <200MHz. That's why they are placed on the EVM.
    On your second question: TI supports only solutions that conform to the guidelines in the processor datasheet. However, I wasn't able to find the AM335X datasheet mentioning that the DDR2 power plane can be used as a signal return plane.
     
    Best Regards
    Biser
  • 0 in reply to Biser Gatchev-XID
    Prodigy 235 points

    Hi Biser,

    Thanks for your response.

    My comment about the planes was more an observation based on the datasheet which lists a 4-layer stack-up (minimum) where the bottom tracks are adjacent to a power plane.  Spraav0a Section 5.3 (Crosstalk) specifies that "Each signal routing layer should have an adjacent full ground plane to provide the shortest return current path."  Is this strictly a ground plane or can a power plane provide a sufficient return path?

    I noticed the Beaglebone is a 6-layer board which includes DDR routing on the bottom layer which is adjacent to a DDR2 1.8V power plane.  The AM335x EVM board is also 6-layers but has two ground planes which allows all DDR2 signals to be routed adjacent to a ground plane.

    Is there a recommended stack-up for a 8-layer PCB?  Is the multi-ground plane approach recommended or can the 1.8V power plane be used?

    Kind Regards

    Mark

  • 0 in reply to Mark Norman
    TI__Guru**** 393215 points
    Hi Mark,
     
    Definitely, the multi-ground plane approach is much better. The 1.8V plane can also be used, but it will be probably more difficult, as it's required to avoid routing signals across splits in the plane. The EVM board is also a 6-layer design, and the Starter Kit:
     
     
    is an 8-layer PCB (but still has the two GND planes next to external layers).
     
    Best Regards
    Biser
  • 0 in reply to Biser Gatchev-XID
    Prodigy 235 points

    Hi Biser,

    Thanks for the information.

    Kind Regards

    Mark