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THVD1400: Impact of internal pull-up/pull-down on shorted Re/De inputs

Nitish Patel
Prodigy 10 points
Part Number: THVD1400

Tool/software:

Hello,

Inputs 'Re_bar' and 'De' are internally tied to Vcc and GND respectively but the 'Re/De' short is pulled down to GND.

The effect of  'DIR' driving a '1' or a '0' is clear.  What is the impact if 'DIR' is tristate i.e. floating? I would expect the device that the device is in Rx mode.

Ques 1: What are the values of the internal pullup/pulldowns on RE and DE?

Ques 2: If DIR is tristated, and the external 10K is not placed, what is the state of the chip: RX or TX or both disabled or indeterminate?

Also Fig 10.1 shows two pull-down resistors, one each for Re and De. This too suggests a default RX mode - a good thing! but 10.1 schematically differs from Fig 8.5 but functionally identical.

Thank you in advance

  • 0
    Guru 314780 points

    1. They can be computed from the input leakage currents, i.e., the minimum value is 5 V / 5 µA = 1 MΩ.

    2. Both disabled; see tables 7-1/7-2.

    3. /RE and DE can share a pull-down if they are controlled by the same DIR signal. If your MCU wants to control them separately, then they need separate pull-up/-down resistors.

  • 0 in reply to Clemens Ladisch
    TI__Guru 69280 points

    Thanks Clemens, appreciate your help.

    Nitish,

    If you have any additional questions please let me know.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Thanks Clemens. I just want to be 100% sure. 

    Your answer "2) Both disabled ..." refers to the schematic in Fig 8.5? 

    My simplistic analysis is shown below. The THVD is in blue and the micro's DIR pin is tristated. Re/De should be a mid-point of 5V (pull-up == pull-down=1M) which implies pins are at 2.5V But VIH(min) is 2V. So I would have assume DIR tristated will be interpreted as a '1' and hence function as a 'TX'. 

    In case you want to know why this is an issue for me - I have a need to drive two independent lines from one data source/sink. I have to use two THVDs. Driving Re and De independently implies 4 pins. If you are correct about both being disabled, I need 2 pin instead of 4.    

    Thanks in advance. I missed your email so sorry about not responding earlier. 

  • +1 in reply to Nitish Patel
    TI__Guru 69280 points

    Hi Nitish,

    The diagram you have set up looks correct. The answer can vary since the resistors have quite a bit of tolerance. I generally hear designers say that it could be as wide as +/-30% since these resistors are not trimmed.

    I would probably estimate that in most cases the logic seen by the pins would be considered a logic high so the driver would likely be enabled while the receiver would likely be disabled. 

    If your goal is to control the pins using only 2 pins (one to enable either DE or RE and one to disconnect the two when tristated), you could set up some kind of FET that can break the connections between RE/DE when the DIR pin is tristate which would let the DE and RE go into a disabled state. 

    -Bobby

  • +1 in reply to Nitish Patel
    Guru 314780 points

    When /RE and DE are shorted together, then the internal resistors will indeed form a divider, resulting in an intermediate voltate.

    To prevent this, use an external pull-down resistor that is stronger than the internal ones, as shown in figure 8-5.

  • 0 in reply to BOBBY
    Prodigy 10 points

    Thanks Bobby. 

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Thanks Clemens.