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MSP430F6723: what is the actual impedance/resistance of an input pin without internal pull-up/down resistor?

Zhiyong Li (FT)
Expert 2710 points
Part Number: MSP430F6723

Hi,

We need to keep one of the input pins high by default, when an external device is connected, that input pin will be driven low by the MCU on external device, as part of the hand-shake process. To reduce power consumption as much as possible, we would like to use as large a pull-up resistor as possible. An old post from distant past recommended no more than 220K. That still results in ~15uA current flowing through pull-up resistor. We tried 3.9M external pullup resistor, so far it seems working. Also the voltage at the input pin measures around 2.7V when VCC is around 3.6V, higher than the threshold for input high. But the current flowing through the pull-up resistor amounts to about 0.25uA, a lot higher than stated leakage current 50nA.

My question is: is there a limit or recommended range of value for external pullup resistors? With or without need for interrupt? Although the current 3.9M resistors seem working for all practical purpose, I would like to know if there is a small chance or in some edge cases they won't.

Please advise, thanks.

ZL

  • 0
    TI__Intellectual 1100 points

    Hi ZL,

    In my opinion, as the pull-up resistor you uses increases, the voltage in the pin will decrease when the pin is not driven low by external device. Because we could consider the external device connected to the pin as an equivalent resistor, the value of pull-up resistor have an influence on the voltage of the pin.  As a result, if you choose a larger pullup resistor (larger than 3.9M), the pin voltage may be low than the threshold for input high. And the actual resistor value is related to you external device. 

    So it's not be recommended to use too large pull-up resistor.

  • 0 in reply to Pengfei Xie
    Expert 2710 points

    Hi,

    Thanks for your reply. At moment, the external devices are mostly built around other MSP430 MCUs such as MSP430I20xx, MSP430FR24XX. Are there any numbers that we can look up in datasheet of both MSP430F67 and other MCUs and that we can use to calculate the voltage at the input pin?

    Best,

    ZL

  • 0 in reply to Zhiyong Li (FT)
    TI__Intellectual 1100 points

    Hi Zhiyong,

    Sorry I can't find any numbers about the calculation of IO voltage in datasheet. But the voltage is apparently depends on the current and the value of the pullup resistor. In some cases, we want to use a small resistor to give larger drive current , and sometimes we want to use a larger resistor to get less power consumption. A way for you to make sure of the IO voltage is by measuring. And in this way you may get a estimated value of external devices. And then you could change your pullup resistor to adjust the IO voltage to a reasonable value.

    Regards,

    Pengfei Xie

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