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AM3874: DQS and DQSn processing of unused DDR3

shigehiro tsuda
Mastermind 9490 points
Part Number: AM3874
Other Parts Discussed in Thread: AM5728

Hi,

In section 9.13.4.2.4 of the AM3874 datasheet,it is recommended for unused DQS pullup to VDD and pulldown to DQSn to GND.
However, the description of the AM5728 datasheet is the reverse, qualitatively the reverse seems to be correct.
Is it incorrect?

If the datasheet of AM3874 is incorrect, could you tell me the concern with this pullup/pulldown?

In our customer product, only the DDR0 controller is used and the DDR1 controller is not used.
DQS is connect to 1.5V with 1 kΩ and DQSn is connect to GND with 1 kΩ on the DDR1 controller side.

If the internal pullup/pulldown setting is the opposite, could you tell me the operational concerns?

Best Regards,
Shigehiro Tsuda

  • 0
    TI__Guru* 86970 points
    Shigehiro, you are correct that the AM5728 datasheet method is correct (DQS pulled low and DQSn pulled high). Since the main purpose is to reduce noise on the signals, the fact that the customer connected in the opposite way should be OK. You just want to fix these signals to a certain voltage level so they don't float or are otherwise affected by noise.

    I would disable the internal pullup/pulldown setting, you will not be able to override the external pulls with internal pulls anyway. There may be a slight power difference, but i don't think there will be any operational concerns.

    Regards,
    James
  • 0 in reply to JJD
    Mastermind 9490 points

    Hi James,

    Thank you for quick reply.
    There was your answer that there was no problem with the opposite process, we were relieved.

    Best Regards,
    Shigehiro Tsuda