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MSP430F6779A: Why is the leakage current of I/O so high (floating input)?

TinK
Intellectual 815 points
Part Number: MSP430F6779A


Tool/software:


I tested the current consumption when setting the MCU in LPM3 mode.

Configuration #1: unused pins to inputs and disable internal pull-up resistors (P4.1 to P4.6)
result: current consumption is 8 uA

Configuration #2: Set unused pins to low outputs (P4.1 to P4.6)
result: current consumption is 200 uA

Why is the leakage current I/O so high (expected 0.3 uA)?

  • 0
    Intellectual 815 points

    corrected

    Configuration #1: unused pins to inputs and disable internal pull-up resistors (P4.1 to P4.6)
    result: current consumption is 200 uA

    Configuration #2: Set unused pins to low outputs (P4.1 to P4.6)
    result: current consumption is 8 uA

  • 0
    Guru 24625 points

    Did you connect those input pins to Vss or Vcc? (see note)

    You never want to leave CMOS inputs floating which is why the recommendation in the principles of low power operation section is to set unused pins to outputs.

  • 0 in reply to David Schultz
    Intellectual 815 points

    No, I just measured the current of the Vcc pin.

    I would like to know why the current is so high when the I/O is configured as a floating input (i.e. noise or something).

    Is it possible to estimate this current (in case there is a floating input in the system)?

  • +1 in reply to TinK
    Guru 24625 points

    The current is high because the floating gates collect charge and can end up at a voltage where both the N and P channel FETs aren't completely off. Connecting Vcc to Vss.

    This is a well known characteristic of CMOS.

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