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TM4C1294NCPDT: Using Pull Down Resistor w/ ADC Input?

Terence D
Intellectual 755 points
Part Number: TM4C1294NCPDT

For GPIO input pins there is of course the option to configure the pin to use internal weak pull-up or pull-down resistors.  This is easy to configure using TivaWare's Peripheral Driver Lib's GPIOPadConfigSet() function.  I'm wondering if it's also possible to do the same with an ADC input pin?

I'd like to configure the analog input of this pin to weak pull-down so that when nothing is connected it reads a value of zero.  Looking through the Peripheral Driver Library I'm not seeing an option to do so.  Just wanted to make sure I'm not overlooking something.

  • 0
    TI__Guru**** 190846 points

    Hi Terence,

      This is not possible. The AINx goes directly to the ADC module.  It will not work even if you attempt to enable a weak pulldown on a GPIO pin that is muxed with the AIN pin.

  • 0 in reply to Charles Tsai
    Intellectual 755 points

    Okay, thank you, Charles.

  • 0 in reply to Terence D
    Prodigy 225 points

    Hi Terence,

    I tried using the pull-up in combination with ADC on the very similar MSP432E401, see e2e.ti.com/.../708007. It works. For some special pins you have to do an unlock procedure before, see . When I remember correctly there is no need to activate the digital enable bit (DEN), but I can cross check if it is helpful for you. I'm quite sure that it works for TM4C1294NCPDT, too. Its data sheet is very, very similar to MSP432E4. So if it gives you design opportunities, just try. Nevertheless pull-up or pull-down are deactivated after a reset. Thus you might need an external pull-up or pull-down resistor at all.

  • 0 in reply to Sven Probst
    Guru 117855 points

    Greetings,

    After (first) Finding - then Emplacing my (not too frequently worn) 'diplomat's cap' - may I note:

    "The practice of 'attempting to deploy' such pull-down resistor - as a 'fix' for an (otherwise) floating (and at risk) ADC input is neither:

    • 'best'
    • or even 'recommended' practice.

    The fact that it (may) appear to work is illusory - and a proper, 'ADC input impedance match' should be one's objective.   Usually - a 'simple Op-Amp' may serve this role - and it provides a 'protective/sacrificial' buffer for the MCU - as well.

    As Vendor's Charles earlier well noted - "No provision for (either) MCU's internal 'pull-up or pull-down' resistors exist" - and that for 'good reason.'

    You can find the, "input impedance specification for these MCUs" in the 'cheap-seats' (i.e. my seats - far back in the MCU manual - the dreaded 'Specification Pages')    (Read these pages ONLY when wide-awake - you've been warned!)  

    In addition - Your 'Forum Search Bar' (high atop this forum page) should provide (many) postings - which 'build your knowledge & understanding' of the MCU's  (proper) ADC input treatment.    It is 'doubted' that the MSP device differs significantly in its ADC input implementation - and the, 'It Works' award is most likely (pardon) both premature & an 'over-promotion.'

    Normal & Customary ADC Design practice - does not 'fall back upon' such use of 'pull-down components' - which stray so far - from the ADC's 'Correct Input Circuit requirement...'