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ISO7840: pull up and pull down of INx/OUTx pins on ISO784x

S.G.
Prodigy 40 points
Part Number: ISO7840

Hello,

What would be the correct approach to override the default state of OUTx pins in case of loss of Vcc1 and/or Vcc2?

Assuming the device is an ISO7840 would it be ok to add a weak pull down on the output of the desired OUTx pin to change the default state?

Kind regards

Spyros

  • 0
    TI__Mastermind 49495 points

    Hi Spyros,

    Thanks for reaching out. Yes, it is perfectly fine to have a pull-up or pull-down resistor at the output to leave the output at a defined state when the respective output supply voltage is OFF.

    Please note that these external pull-up and pull-down resistors are only effective when the output supply is OFF. When device output supply is ON, the outputs will be strongly driven to HIGH or LOW by the device and hence, the output takes the state accordingly irrespective of the weak external pull-up / pull-down.

    Let me know if you have any other questions, thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 40 points

    Hello Koteshwar

    Thank you for the prompt reply.

    I would like a small clarification when power supply is ON for Vcc1 and Vcc2.

    For an ISO7840 device is it possible or safe to bypass the default state of an output (high) by adding a weak pull down on the corresponding INx pin?

    kind regards

    Spyros

  • +1 in reply to S.G.
    TI__Mastermind 49495 points

    Hi Spyros,

    The input pins of ISO7840 have a weak internal pull-up / pull-down of 1.5MΩ, please refer to device I/O schematic in datasheet (also copied below). Hence, the internal pull-up / pull-down can be over driven by choosing a stronger pull-up / pull-down externally. For example, an external pull-up / pull-down resistor of 4.7kΩ is going to over drive the internal resistors and set the output according to the external resistor at the input pins.

    Please note that this is only applicable when the input supply is ON. If the input supply is OFF, the outputs are going to take the default state defined by the part number ('F' or 'non-F'). The devices are internally designed to return to default state when input supply is lost. This cannot be changed at the output. Thanks.


    Regards,
    Koteshwar Rao