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Thread: Picaxe Draws Current Through I/O's

  1. #1
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    Default Picaxe Draws Current Through I/O's

    As the title states, I'm concerned about the pic drawing current through I/O's when powered off.

    In my specific situation I'm reading an analog value and my picaxe is a "piggyback" to another micro controller. Once I turn off the picaxe it'll draw current from the analog input thus killing the signal to the main micro controller. This also happens when reading a digital signal.

    For the digital signal I can use a diode as the forward voltage drop should not affect the picaxe. But a diode will not work for the analog signal nor will a resistor as the analog signal can become very low voltage.

    Any ideas?

    thanks, Zander

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    without knowing more about the circuit, and the reason for switching off the picaxe and not the other controller I would sugget leaving it on or putting in sleep or nap mode so that current draw is minimal.

    but I suspect that there are going to be myriad of questions to come like but not limited to
    What picaxe?
    what is the other controller?
    what do you mean by piggyback?
    what signal to there other controller?
    what is the analogue signal?
    can we have a circuit diagram?

  3. #3
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    Quote Originally Posted by oracacle View Post
    without knowing more about the circuit, and the reason for switching off the picaxe and not the other controller I would sugget leaving it on or putting in sleep or nap mode so that current draw is minimal.

    but I suspect that there are going to be myriad of questions to come like but not limited to
    What picaxe?
    what is the other controller?
    what do you mean by piggyback?
    what signal to there other controller?
    what is the analogue signal?
    can we have a circuit diagram?
    What I've described is not related to my circuit but about how the picaxe operates with voltage.

    Zander

    for example:
    I've built a simple blinkey circuit with the necessary programming resistors and such. What i've observe is when the picaxe is grounded it will pull voltage through any of the I/O pins IF power is NOT supplied to V+. By doing so, I can supply power from any I/O pin and have the LED blink.

    I've tested this on the 14M2, 20M2 and 20X2.

  4. #4
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    Quote Originally Posted by ZanderPIC View Post
    What i've observe is when the picaxe is grounded it will pull voltage through any of the I/O pins IF power is NOT supplied to V+. By doing so, I can supply power from any I/O pin and have the LED blink.
    That is correct. It is a common condition known as "Phantom Powering" which applies to some extent to most microcontrollers.

    Most I/O pins will have diode clamps internally which allow power supplied to an I/O pin to reach the internal V+ rail and can power the chip.

    If a micro pulls its inputs towards 0V when not powered - and I have not checked if a PICAXE does - then digital inputs will be affected as much as analogue inputs are. A diode won't be enough to prevent a signal being pulled towards 0V. Usually current limiting resistors would be used but it is usually recommended not to have any IC un-powered if it is being driven by any signal.

    If an IC does need to be un-powered it will probably depend on exactly what the circuit and signals are to determine what would be the most appropriate means of avoiding problems.

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    Quote Originally Posted by hippy View Post
    That is correct. It is a common condition known as "Phantom Powering" which applies to some extent to most microcontrollers.

    Most I/O pins will have diode clamps internally which allow power supplied to an I/O pin to reach the internal V+ rail and can power the chip.

    If a micro pulls its inputs towards 0V when not powered - and I have not checked if a PICAXE does - then digital inputs will be affected as much as analogue inputs are. A diode won't be enough to prevent a signal being pulled towards 0V. Usually current limiting resistors would be used but it is usually recommended not to have any IC un-powered if it is being driven by any signal.

    If an IC does need to be un-powered it will probably depend on exactly what the circuit and signals are to determine what would be the most appropriate means of avoiding problems.
    Thanks for the detailed description for why this happens. Ideally the picaxe and main micro controller would be powered together. I was interested in adding a fail-safe to prevent the pic from disrupting the main micro controller if the pic would fail.

    I've put more thought into protection and maybe a simple NPN transistor with the base to V+ so if the pic loses power then the transistor will open.

    Thanks, Zander

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    It's an annoying'feature' of microcontrollers. In the past, I've added a dual opto-isolator to my circuit to overcome the problem.

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