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ddr2 layout doc question

Joaquin Gonzalez
Expert 1200 points

Hi to everybody,

I have a question regarding doc SPRAAL6A (DDR2 pcb layout). On page 5 it says that parammeter Zo must be between 50-75 Ohms and the impedance control must be from Zo-5 to Zo+5. Does this mean that if DDR_ZN and DDR_ZP resistances are 200 ohm (as in the reference design), then Zo is 50 and, therefore, yo have to design all your DDR signal traces so that they all have an impedance bettween 45 and 55 ohms?

Thanks in advance,

Nuba.

  • 0
    TI__Guru*** 125430 points

    The datasheet specifies that DDR_ZN and DDR_ZP should be terminated with 200 ohm pullups/downs.

    The layout app note specifies that Zo (single ended trace impedance) can be between 50 and 75 ohms and specified by item 12 of the PCB Stackup Specifications.

    I don't understand how you reached your conclusion.  Those seem like independent requirements to me.

  • 0 in reply to Brad Griffis
    Expert 1200 points

     

     

     

    Thanks for the reply.

    But in the datasheet it says  : " DDR_ZN, DDR_ZP:

    Required to set the DDR2 output impedance. Connected by way of

     a 200-ohm resistor to power and ground (see

    Figure 3). The resistor should be chosen to be

    4 times the desired impedance of the output buffer. By changing the size of the resistor, the

    DDR2 outputs can be tuned to match the board load, if necessay".

    So if you place 200 resistor it means you want to tune the impedance to 50, so your Zo should be around 50 if you want a better perfromance of your system. This is how these parammeter are related to me. Am I wrong?

    On the other hand, What would parammeter 13 on PCB Stackup Specification mean? That your traces can be between 45 and 55 ohms and still will work?

  • 0 in reply to Joaquin Gonzalez
    TI__Mastermind 41665 points

    Nuba said:
    So if you place 200 resistor it means you want to tune the impedance to 50, so your Zo should be around 50 if you want a better perfromance of your system. This is how these parammeter are related to me. Am I wrong?

    I believe you are correct, the 200ohm resistor spec in the datasheet is assuming you have a typical single ended impedance of 50ohm. EDIT: note that the resistor should only be 200ohm, other values are not supported.

    Nuba said:
    On the other hand, What would parammeter 13 on PCB Stackup Specification mean? That your traces can be between 45 and 55 ohms and still will work?

    That sounds right as well, you have a +-5ohm tolerance on your impedance control.

  • 0 in reply to Joaquin Gonzalez
    TI__Mastermind 41665 points

    An additional clarification on this, you should ignore the comment in the DDR manual (SPRUEM4a) about the DDR_ZN and DDR_ZP resistors being adjusted for changing the impedance of the output buffer, please only set these resistor values to 200ohm as shown in the datasheet, changing the value of the DDR_ZN and DDR_ZP resistors is not supported. The DDR manual should be updated in its next revision to remove this statement.

  • 0 in reply to Bernie Thompson TI
    TI__Guru*** 125430 points

    Furthermore the layout app note is correct that the trace impedance can be in the range of 50-75 ohms.  Thanks for bringing the error in the DDR2 User Guide to our attention.  So to summarize, you should use 200 ohm resistors on DDR_ZN and DDR_ZP and your trace impedance can be 50-75 ohms.

  • 0 in reply to Brad Griffis
    Expert 1200 points

    Thanks for these answers.

     

    Nuba