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TMS320F28377D: Enabling Weak Pullup for Bonded GPIO's that are Pulled-Down Externally in hardware.

Jeffrey Andich1
Prodigy 20 points
Part Number: TMS320F28377D

Hi, 

We've had an ongoing question/discussion in software on how to handle GPIO lines that are currently unused, but externally pulled-down in hardware. 

Our current guidance is to configure those GPIO lines as inputs, and enable weak pullup within the GPIO channel. So long as the pulldown is ?much? stronger than the internal weak pullup for the GPIO, the input will not "...float to mid-rail voltage and cause wasted shoot-through current on the input buffer..."  (TI TRM for TMS320F28377D, spruhm8i).  But I've seen some posts where programmers prefer not to enable a weak pullup for GPIO lines that are either pulled-up or pulled-down in hardware. 

What is TI's typical guidance for unused GPIO lines that are already pulled-down in hardware?  Does it matter if a weak internal pullup is enabled for the corresponding GPIO channel, so long as the pulldown is always strong enough to overcome the weak internal pullup?  Are there different use cases to consider?

Thanks!

Jeff

  • 0
    TI__Mastermind 42195 points

    Jeff,

    please see table 7-6 "Connections for Unused Pins" in the datasheet. https://www.ti.com/lit/gpn/tms320f28379d 

    The table lists 3 viable options for unused GPIOs.

    1. No connection (input mode with internal pullup enabled)
    2. No connection (output mode with internal pullup disabled)
    3. Pullup or pulldown resistor (any value resistor, input mode, and with internal pullup disabled)

    The idea is that you don't want the buffer sitting at a mid-rail voltage as you indicated. The three methods above describe how this can be done. There is no need to have the pull-up enabled if you have an external pull-up, and certainly no need to have the internal pull-up enabled if you have an external pull-down resistor. 

    Regards,
    Cody 

  • 0 in reply to Cody Watkins
    Prodigy 20 points

    Thanks Cody!

    This greatly helps!

    The reference you provided was a lot clearer on this than everything else we've looked at thus far..

    If any of our team has any follow-on questions, will post them here.  I'll wait a day or two and then mark resolved if there are no further questions during this period.

    Regards,

    Jeff

  • 0 in reply to Jeffrey Andich1
    TI__Mastermind 42195 points

    Jeff,

    Not a problem, I'll await your confirmation.

    If the question isn't closely related please start a new thread though, that will ensure you get the expert for that question as soon as possible.

    Regards,
    Cody