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MSP Back Drive

Jeremy51164
Intellectual 780 points

Hello,

We have an application using an MSP that has a pin tied high (10k) to a separate 3.3V power supply that can become active before to the main 3.3V supply for the MSP is available.  The MSP appears to be drawing current from the separate supply, through the GPIO line it is attached to.  Could this damage the MSP?  I was hoping the MSP inputs would default to tri-state and not draw much current.  Is there a way to prevent this from happening?

Thank you,

Jeremy.

  • 0
    Guru 74080 points

    Hi Jeremy.

    When the MSP has no supply and you pull one pin high to another running supply, current is flowing into that pin, since there is a difference in the potential (the micro's Vcc is on GND-potential when not powered). With your 10k resistor a current of 3.3V / 10k = 330uA will flow into that pin. This can cause the Vcc rail voltage to raise depending on the circuitry that is connected to the MSPs Vcc line. But 330uA is no problem for the MSP itself. The clamping diodes can handle a current of about 2mA.

    Edit: Of course you have to subtract the voltage drop of the diode for the resulting current flowing into that pin, sorry. So it's less than 330uA.

  • 0 in reply to Dennis Eichmann
    Guru 23000 points

    Indeed the current isn’t a problem.

    But you will get a problem to Reset the ‘not’ powered MCU when his own power comes available. It’s better to avoid this situation or you need to create a RESET circuit.

  • 0 in reply to Leo Bosch
    Guru 227245 points
    A possible solution is to raise the resistor value (the MSP pins are high impedance when the MSP is powered, so a 100k resistor will do too, lowering the current by a factor of 10) and provide a load resistor between VCC and GND. This way, you actually form a voltage divider between the external supply and GND (through the clamp diode), which reduces the resulting fake-VCC. If you put a 100k between VCC and GND, this will (assuming no current draw by the MSP) result in 1.6V VCC. Given the fact that the MSP draws some current too, this will further lower the resulting voltage, keeping the MSP form starting. But maybe the other circuitry on VCC already serves as a load.
    I always use 100k pull-ups on inputs where needed. 10k is just a waste of energy when the pin becomes active, and price and size are the same. :)

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