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DP83826I: Bootstrap threshold

Oba
Genius 17295 points
Part Number: DP83826I

Hello,

I have a question for DP83826.
DP83826 datasheet recommends to use 2.49Kohm for both pull-down and up of strap configuration.
And the internal pull-down/up is 10Kohm(+-25%).

When configuring Mode 0 in a PULL-UP pin with recommended 2.49Kohm, the input voltage can be calculated as:

3.3V * 2.49 / (2.49+10) =0.658V

 

On the other hand, Low level bootstrap threshold is defined as “0.6V” max in the below table.



It means using 2.49Kohm can’t achieve this threshold.

Are these threshold values really correct?

 

Regards,

Oba

  • 0
    TI__Genius 16030 points

    Hello Oba,

    I understand your concern. I see that 2.49K is the value mentioned in table 8 but for more accurate values table highlighted by you should be followed (As mentioned in the foot note of table 8). 2.49K is a valid value for mode0/pull-up but "boot strap DC characteristics" table has extra buffer added on VIH and VIL for more margins.

    --

    Regards,

    Vikram

  • 0 in reply to Vikram Sharma
    Expert 2031 points

    Hi Vikram,

    Do you mean Table 9-5?  I agree with Oba that the suggested value of 2.49k is wrong, based on the VIL spec. Would you please explain why 2.49k is a valid value for mode0/pull-up?  Why should I have confidence in calculating a resistor value based on the VIL spec if the suggested resistor value doesn't meet the spec?  Shouldn't the datasheet be changed to reflect a suggested resistor value that will work under all conditions?

    Regards,

    Greg

  • 0 in reply to Greg
    TI__Genius 16030 points

    Greg, Oba,

    Yes. I understand your point and confusion because of mismatch in these tables. I am checking with team on how to bridge the gap between two tables.

    --

    Regards,

    Vikram

  • 0 in reply to Vikram Sharma
    TI__Genius 16030 points

    Greg, Oba,

    Which pin's strap is under discussion here?

    --

    Regards,

    Vikram

  • 0 in reply to Vikram Sharma
    Expert 2031 points

    Hi Vikram,

    Thank you for checking with the team.  Your question is a good one.  I don't think there is a pin on this device that applies to the discussion!  The only configuration pin that has a pullup is ModeSelect.  But the datasheet says this pin should be shorted to ground for BASIC mode, which is essentially a pulldown resistor of zero Ohms.  Any pins that I intend to strap will use an external pullup resistor.  I joined the discussion because I didn't think Oba's question was sufficiently answered.

    I looked through all of the pins that have strapping options, and I don't see one that has an internal pullup resistor.  All of them have pulldown resistors, and the Active type is Output.  Since they're outputs, it is unlikely there are any other low-impedance loads on those signals, making the analysis easy.  If there are, the threshold table should be used to ensure the spec is met.

    I just made the realization that this device is capable of VDDIO ranging from 1.8V to 3.3V.  I am using 3.3V.  At 1.8V, the threshold is met for both pullups and pulldowns, even taking into account the 10k internal resistor could be as low as 7.5k.  I believe the 2.49k resistors were chosen with a VDDIO of 1.8V in mind.  2.49k does not work for 3.3V with an external pulldown, but the design should not require any external pulldown resistors.

    I'm going to ask a question I don't think I need to know the answer to.  But I'm going to ask because the datasheet doesn't make sense to me.  I will be using ENHANCED Mode, and all of the default values in the tables for Enhanced Mode are 0.  All of the strap pins have internal pulldown resistors, so I would expect the default values to be 0.  Is that what "DEFAULT" means in the tables?  BASIC Mode is different, however.  For example, Strap7 in Table 9-12 shows a default value of 1.  Why is this?  In BASIC Mode, all of the strap pins have pulldown resistors.  I don't understand what DEFAULT means.

    Thank you and regards,

    Greg

  • 0 in reply to Greg
    TI__Genius 16030 points

    Greg,

    So to summarize the discussion so far : 

    1. You will be using ENHANCED mode and there is no pull-down to ground strap requirement.

    2. You are curious about the meaning of "Default" in the strap table for BASIC mode as for some of the strap pins "Default" has been captured as 1.

    I will get back to you with clarification about point 2.

    --

    Regards,

    Vikram

  • 0 in reply to Vikram Sharma
    Expert 2031 points

    Vikram,

    Yes, that is correct.  I'm sorry for changing the subject to the meaning of "Default".  If you want me to open up a new question I will.

    Thanks and regards,

    Greg

  • 0 in reply to Greg
    TI__Genius 16030 points

    No worries. I am checking for the clarification with the team.

    --

    Regards,

    Vikram

  • 0 in reply to Vikram Sharma
    Expert 2031 points

    Hi Vikram.  Have you gotten clarification from the team?

    Thanks,

    Greg

  • 0 in reply to Greg
    TI__Genius 16030 points

    Here is the clarification : 

    1. Enhanced mode : No use-case of a pull-down resistor

    2. Basic mode : No use-case of a pull-down resistor. Straps with default as 1, can be pull-down to 0 by shorting them directly to ground. Basic mode is supposed to be used for drop in replacement of other PHY in the market and that's how the strapping is defined for that part.

    So in summary as we discused earlier there is no functionality impact because of VIL value of boot straps.

    --

    Regards,

    Vikram

  • 0 in reply to Vikram Sharma
    Expert 2031 points

    Thank you Vikram.  The concept of Basic Mode being a drop-in replacement makes a lot of sense, but I'm still unsure about what "Default" means.  I would assume that the electrical properties of the pins do not change depending upon the state of the ModeSelect pin.  In fact, the datasheet explicitly says that all of the strap pins have pulldown resistors in Basic mode (pages 9 and 10 of the datasheet).  Therefore I would conclude that "Default" means "Industry Standard Default", and has nothing to do with the default electrical properties of the pin.  It just means that if you want to make your circuit board accept PHYs from other vendors, you should use pullup resistors to achieve a default of Logic 1.  Do you agree?  Or does the register get loaded with the inverse of the pin state at powerup?  I could see that as an another meaning for "Default".  For example, if a pin is unconnected it will be pulled down by the internal pulldown resistor, meaning the register will be loaded with a Logic 1 at powerup?  I don't think the datasheet makes any mention of inversion, but that could be another explanation for this.

    Regards,

    Greg

  • 0 in reply to Greg
    TI__Genius 16030 points

    Hello Greg,

    In this context of basic mode, default = 1 means that strap will be sampled as 1 without any pull-up attached to the pin. Now how that is getting done in Basic mode (whether there is a change in the IO or change in the logic) is based on internal implementation and is something that we may not be able to share here.

    To summarize this current thread at a logical point, I will close it here for future reference by others. Thanks for your valuable inputs on this one. 

    --

    Regards,

    Vikram