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Processor VCC connected to Port Pin

Kyle Klumper
Intellectual 260 points
Other Parts Discussed in Thread: MSP430G2553

Hi there,

I'm using an MSP430G2553 processor on a circuit board that communicates to a separate, but identical, board through one of their port pins. I'm not using any communication protocol, the first board simply communicates to the other by sending a digital HIGH or LOW to it's input. The second board's port pin is set to internally pull-up.

The problem I'm seeing is that if the first board is unpowered (VCC floating), the pull-up from the second board is driving the first board's port pin high. This in turn pulls up VCC as well as other ports. I've tried tying the RST line of the first board low but it doesn't seem to change the behaviour.

Is this normal behavior? If so, is there a way to mitigate this issue?

Thanks,

Kyle

  • 0
    Guru 46710 points

    >Is this normal behavior?
    Yes. This happens to virtually ANY low power microcontroller with ESD protection diodes on I/O pins.

    >If so, is there a way to mitigate this issue?
    1. Add series resistor so slave simply does not receive enough current to run.
    2. add software detection of "power mode" - so CPU (running on slow clock) sense where it receives power from - IO pin or VCC. I am not yet sure how to implement such, but feel that it is doable.
    3. introduce some buffer which will pass signal only if slave VCC is on.

  • 0 in reply to Ilmars
    Intellectual 260 points
    OK, thanks for the quick response!

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