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Original question:

About the "BALL RESET REL. STATE" on AM335x GPMC_ADn.

AM3354：GPMC_A9 Internal Pull Down Resistor

Ethan Tian
Intellectual 1872 points

Part Number: AM3354
Other Parts Discussed in Thread: OMAPL138

Hi experts

Could you pls tell me the internal PD resistor value of GPMC A9?

BR

Ethan

  • +1
    TI__Mastermind 30581 points

    Hi Ethan,

    There is no actual internal PD resistor. Instead there are weak FETs that pull the signal high or low, typically to control a floating input with no trace routed from the pin.

    See https://e2e.ti.com/support/processors-group/processors/f/processors-forum/253870/resistance-of-am335x-internal-pull-ups-pull-downs

    Refer to datasheet Table 5.7 DC Electrical Characteristics

    The equivalent resistance of the internal pull resistors can be estimated from Input leakage current with pullup enabled/ pulldown enabled. This parameter describe the input current with internal pullup or internal pulldown enabled in the opposite direction of the voltage applied to the input.

    II (pullup) = input current when a logic low is driven into the input with internal pull-up enabled - notice the input current is negative, so current is flowing from the internal pull-up and out of the IO

    II (pulldown) = input current when a logic high is driven into the input with internal pull-down enabled - the input current is positive, so current flows from the positive IO voltage through the internal pull-down to GND

    Lets say your IO voltage (DVDD) is 3.3V, then the estimated internal pull-up "resistance" is somewhere between 13.6kohm (3.3V / -243uA) and 173.7kohm (3.3V / -19uA) - there is process variation of the internal pull transistors from device to device, so you must account for the worst case. Notice also that these are not linear with voltage. Once the voltage starts getting pulled towards the pullup/pulldown, the "resistance" gets reduced.

    Note: this assumes VDD from your voltage regulator is ideal - 0% tolerance.

    Does it help your problem? Are you trying to override an internal pull resistor with a stronger external pull in order to keep < VIL or > VIH?

    Hope this helps,
    Mark

  • 0 in reply to Mark M
    TI__Mastermind 30522 points

    Hi Mark,

    Mark M said:
    Are you trying to override an internal pull resistor with a stronger external pull in order to keep < VIL or > VIH?

    #1. How to calculate external resistor value to do that? I found a formular in OMAPL138 errata to calculate the resistor to override the internal default state. Does it work for AM335x?

    #2. Follow the formular, if wants to opposite default LOW pin of 3.3V IO, the resistor should be: (3.3-2)/210uA=6.19Kohm.

    #3. There is not a resistor with that value, select >6.19K or <6.19K? if select lower one, then current will >MAX input leakage current, will it cause excessive wear to this pin?

  • 0 in reply to Tony Tang
    TI__Mastermind 30581 points

    Hi Tony,

    Choose a stronger resistor (lower resistance) to stay away a good margin away from the VIL/VIH threshold. Consider tolerances and any other component that may pull the signal.

    The damage to the pin will occur if the voltage is allowed to be between VIL and VIH for excessive time. Input leakage current with pulldown enabled describes the internal pull transistor when pulling from opposite rail.

    See response in this thread: e2e.ti.com/.../am572x-i-o-pullup-pulldown-resistor-value