• State TI Thinks Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 2 answers
  • Subscribers 32 subscribers
  • Views 316 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.

DP83869HM: External bootstrap pulldown in addition to internal pulldown?

Ross Petty
Intellectual 300 points
Part Number: DP83869HM


For bootstrapping pins that contain internal weak pulldowns (such as the RX_D* pins), and you wish for them to be Mode 0, is it still necessary to install an external 2.49k Rlo resistor?

  • 0
    TI__Mastermind 27212 points

    Hi Ross, 

    For the 4-level straps (which the RX_D pins are), no external resistor is required for mode 0, since they do have an internal pulldown resistor. 

    For 2-level straps, mode 0 requires an external pulldown of 2.49kohm. 

    This information is shown in Table 9 and 10 of the datasheet. 

    Thanks,

    David

  • 0 in reply to David Creger
    Intellectual 300 points

    I agree that the table shows an external pulldown. However, the table also shows what the target voltage threshold is for a 0 versus a 1. So my thought was, if it's voltage based, an it has an internal pulldown, then connecting no external resistors would still get pulled down to a low enough voltage via the internal pulldown. It also labels them as "ideal" resistors instead of "required" resistors, so there was a bit of an interpretation to make.

  • 0 in reply to Ross Petty
    TI__Mastermind 27212 points

    Hi Ross, 

    You are correct. It is voltage based, so as long as the voltage at the pin is less than 0.093xVDDIO for the 4 level straps, then the strap will be in mode 0. There are some factors that may affect this voltage unexpectedly such as temperature, leakage current, tolerances, etc., but as long as you can ensure the voltage stays below that threshold, then it will be fine. Here is an app note that contains more information on this. 

    As you noticed, I misworded my previous response. Our resistor values are recommended, but you do not have to use the exact values as long as you can ensure the voltage stays within the target range listed in the table. 

    Thanks,

    David

  • 0 in reply to David Creger
    Intellectual 300 points

    "For the 4-level straps (which the RX_D pins are)". My interpretation from Table 11 is that RX_D0 and RX_D1 are 4-level straps, representing PHY_ADD[1:0] and PHY_ADD[3:2], but from Table 12, RX_D3 and RX_D2 are 2-level straps, representing OPMODE_1 and OPMODE_2. Is this correct?

  • 0 in reply to Ross Petty
    TI__Mastermind 27212 points

    Hi Ross, 

    My mistake, your interpretation is correct. From section 9.5.1 of the datasheet: "RX_D0 and RX_D1 pins are 4 level strap pins. All other strap pins have two levels."

    Thanks,

    David