• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 99 subscribers
  • Views 1902 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DDR VTT termination

Peter T-Innova
Intellectual 640 points
Other Parts Discussed in Thread: TPS51200

Hello.

Again, following TMDSSK3358, the pcb people took away the DDR3 VTT terminations with the two following arguments:

"One reason is that our DDR memory circuit is located near MCU and these resistors are needed if the routed trace length is over 6.3 cm. Other reason (found in Micron design guide) “The external parallel termination resistors that were required for DDR2 are no longer required with DDR3 JEDEC-compliant modules because the VTT terminating resistors are built onto the module”"

1) Does this seem to be ok?

2) If looking in AM335x datasheet figure 5.46 (DDR3 with termination) and figure 5.47 (DDR3 without termination) there seems to be some small change in the DDR_VREF circuitry. Now, we are using the TPS51200 as VTT Regulator according to SSK3358. But *if* the terminations are dropped, should we then need to make some change to the DDR_VREF circuitry provided from SSK3358?

Tnx

  • 0
    TI__Guru 62835 points

    We recently updated the AM335x Data Sheet to include the non-VTT option for DDR3.

    We agree it is possible to design a product using a single memory device with point-to-point topology without VTT resistors, but we recommend system performance be verified with signal integrity analysis using specific PCB details while designing the PCB.

    AM335x does not support DDR3 modules.  It only supports single 16-bit DDR3 or dual 8-bit DDR3 devices.  So the comment about VTT resistors being built into the module does not apply.

    Most VTT power sources also provide a reference voltage source for the DDR input buffers (DDR_VREF).  DDR_VREF would be connected to this source when using VTT because it is free once you have the VTT power source.   However, the lowest cost option when not using VTT would be a simple voltage divider to source DDR_VREF.

    Regards,
    Paul

  • 0 in reply to peaves
    Intellectual 640 points

    Yes, right. I didn't pay enough attention to the word "modules". It seems that the statement is taken from "tn4108_ddr3_design_guide.pdf" which holds true for modules.

    Ok, I will forward Your answer to pcb team, thanks.