1. chandu13

    Thread Starter Member

    Oct 15, 2008
    22
    0
    Hay
    I am using the LPC2114 micro controller, on RESET all GPIO pins of the controller will be INPUTS & the logic on the pins is 0 (low state).
    If the micro controller burns or fail or die then what will be the status of the GPIOs

    Regards
    chandu
     
  2. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    The GPIO for any modern microcontroller is a complex structure. When something burns, you never really know which part will fail.

    From practical experience, I have blown either the upper or lower transistor when the device was configured as an output. This is because I was either trying to drive a short high or pull a low-impedance high line low.

    You have less to worry about when it is an input. If the input is high impedance, these can tolerate a lot of overvoltage because the input diodes clamp the signal.

    Steve
     
  3. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    For the protection diodes to work for large overvoltage conditions you need to limit the current with a series resistance. There is usually an absolute maximum input current specification in the datasheet.

    (* jcl *)
     
  4. chandu13

    Thread Starter Member

    Oct 15, 2008
    22
    0
    Hay thanks for the reply

    The micro controller burns or fails or dies because
    1. The GND pin & Vcc pin short
    2. Applying high voltage to the Vcc
    3. Applying high voltage to the GPIOs (INPUT mode)

    I am controlling C2H2&N20 solenoids using controller, when controller pin high (1) solenoid will ON if controller pin low (0) solenoid will OFF
    When the controller is RESET both solenoids will be OFF.
    If micro controller burns or fails or dies then what will be the status off the solenoids
    Regards
    chandu
     
  5. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    John,

    I was ambiguous in my statement, I meant if the source impedance is high, then the clamp diodes should handle higher voltage. You're right though, in regards to the current limitation.

    We told you. The transistor that is responsible for sourcing or sinking will no longer work. Sometimes both of them get destroyed. Depending on the failure mode, the transistor could be stuck open or shut. Which results in a steady low or high signal.

    Steve
     
  6. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    These diodes are never supposed to pass any current. In fact I have seen Microchip's document mentioned that current through these protection diodes are out of spec. of the PIC. They don't even put a max. current value for them in the datasheet.

    At least this is my understanding. Higher input voltages can be dealt with using clamp diodes and current limiting resistors.
     
  7. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Many microcontrollers allow it, so it probably varies by manufacturer. I have even seen zero-crossing detectors straight from 120V into the analog inputs through a high impedance.

    Steve
     
  8. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    I have located the document that warns against doing so for PICs by Microchip. Funny enough some of their application notes are doing just that.

    But the document is quite recent so maybe it doesn't apply retrospectively.

    Edited: A definitive answer from Microchip regarding this issue is presented here in this thread: http://forum.allaboutcircuits.com/showthread.php?p=107510#post107510
     
    Last edited: Dec 27, 2008
  9. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    That note warns against passing *sustained* currents through the diodes. ESD
    is transient. Usage of the diodes will vary by application.

    Unless a manufacturer specifies the ESD performance of the protection diodes
    I would use an external TVS (e.g. Littelfuse SP72x).

    (* jcl *)