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DDR Drive Strength and Impedance

Charles Gervasi
Expert 2270 points

I have a question about DDRDRIVE[1:0].  It has two settings (high/low) in DDR2 mode to control output driver strength.  It is also described as "output driver impedance".  This makes sense because decreased drive strength is the same as increased output impedance. 

My question is if there is either a) anything to quantify "high" and "low" drive strength or b) any practical guidelines regarding what trace geometries or trace impedances lend themselves to each setting.  Is there any direct relationship between the value of the DDR_ZP resistor and drive strength? 

I'm driving 4-mil traces whose characteristic impedance works out to around 50 ohms with no external termination, mostly just under 1 inch long. I'm almost sure I need to use low drive strength, but I just wanted to see if there's more info out there.

When I search the memory controller user's guide, I also find VTPIO_CTL[3..5] contains drive strength control bits.  My understanding is that setting is not memory-controller specific and is not intended for user knob-twiddling in any case.

  • 0
    TI__Mastermind 27585 points

    DDRDRIVE controls the drive strength of the memories, while the DDR_ZP controls the drive strength of the controller.

    If all other DDR routing rules are followed, then DDRDRIVE should be set to "low" if no terminators are used and "high" if termination is used.

    DDR_ZP will be sampled during VTP calibration, and the control registers should not be modified outside of this process.

    Jeff

  • 0 in reply to jc-ti
    Expert 2270 points

    This answers my question regarding what to do.

    How does a register setting (DDRDRIVE) affect the drive strength of the memory chips?  Does the memory controller send it some configuration information including a memory IC output drive level?

    Thanks,

    CJ

  • 0 in reply to Charles Gervasi
    TI__Mastermind 27585 points

    Yes thats exactly right. The controller sends out the drive strength setting as a command to the memory. You can check the specific memory datasheet for more information.

    Jeff