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DP83640: Why pull-up resistors and pull-down resistors are strongly recommended

Masanori Tachibana
Genius 5350 points
Part Number: DP83640
Other Parts Discussed in Thread: TLK110

Hello support team,

I have some questions about DP83640.

Looking at the circuit diagram of "TMS570LS31x Hercules Development Kit (HDK)", it is strongly recommended to connect pull-up resistors to CRS_DV of DP83640, and to connect pull-down resistors to RXD_3 and RXD_2.

[Questions]
1. Why is it strongly recommended to use pull-up and pull-down resistors?

2. Are these resistors necessary in the case of using the other PHYs (eg TLK110 etc.) as well?

Best regards,
M. Tachibana

  • 0
    TI__Mastermind 19780 points
    Hi Tachibana-san,

    I am not sure about the development of the Hercules kit, the engineers involved with the development are no longer available.

    If there is significant leakage from the IO of the Hercules, which is possible, that could put the strap voltage close to the threshold for the part. There is a very weak PU and PD resistor internal to the PHY, approx 15k-25k ohm. Or if the IO cell has a high capacitive load, it may take longer for the charge to drain off with the weak PU and PD internal to the PHY. It is recommended to place a strong PU or PD externally in those cases.

    I would use the recommendation to use external pulls for all PHYs with Hercules if that note is there.

    Best Regards,
  • 0 in reply to Rob Rodrigues
    Genius 5350 points

    Hello Rodrigues-san,

    I'm sorry for my late reply.

    All the pins of CRS_DV, RXD [0], RXD [1], RXD [2], RXD [3] of Hercules are internally pulled down at 20 μA.
    On the other hand, CRY_DV of the PHY is internally pulled up. And RXD [0], RXD [1], RXD [2] and RXD [3] are internally pulled down.

    In case the PHY is used by default condition, I think it is better to pull up CRS_DV with 2.2 kΩ of resistor. But, I think there is no need to pull down RXD [2] and RXD [3] externally.
    I speculate that these pull-down resistor are recommended to ensure the setting at power on.

    Best regards,
    M. Tachibana

  • 0 in reply to Masanori Tachibana
    TI__Mastermind 19780 points
    Hi Tachibana-san,

    Pulling up CRS_DV, if there is a weak pull-down on Hercules, is a very good idea.

    I would agree that default PD pins may not need the external resistors. Tho I do strongly recommend putting place holders for pull downs on RXD[3:0] in the case that some unexpected behavior is observed. The external pull downs would not need to be populated initially unless you find some issue with power up and reset.

    Do not forget that at reset, the straps are also sampled again.

    Best Regards,
  • 0 in reply to Rob Rodrigues
    Genius 5350 points
    Hello Rodrigues-san,

    Thank you for your comment.
    I understand, and feel relieved.
    We have already informed to customers as well.

    Sincerely,
    M. Tachibana