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schematic

simulate this circuit – Schematic created using CircuitLab

I am working on MC56F8006 DSC . On power up or reset the default state of all GPIOs is inputs with internal pullups enabled. I am driving IGBT three phase bridge via PWM chanel GPIO-A0-A5. But if i power up DSC IGBT driver will gate logic '1' from my Port-A and all IGBTs will turn ON making a short circuit across supply.

So my doubt is even if my GPIO is configured as input and that input is connected to NOT gate output of NOT gate is connected to cathode of LED and anode is tied up to 3.3V. So on power up that LED will glow? If yes then on power up all igbts will turn on and short circuit will take place

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  • \$\begingroup\$ Controllers have high-impedance inputs, so when configured as such the pins will act like high-Z. So with an input pin driving an inverter I would venture to say the output of that gate is indeterminant without pull-up/down resistor. When connected directly to an input pin you are not really driving it at all, so the gate input is left floating. \$\endgroup\$ – sherrellbc Aug 5 '14 at 12:19
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I guess the two obvious choices would be:

  1. Use external pull-down resistors to ensure a correct state on those lines after a reset
  2. Ensure that the bridge only gets powered up after the MCU is set up / initialized.

External pull-downs are probably the easier way. But there may be other reasons to do (2).

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The LED will not glow if you use an external Pull-down. Else, the input port is in high impedence or floating.

I also agree with the og1L's response on this. After power-up, the part could take up to a couple of micro-seconds to configure the input ports. This time is engh to short your supply if the port defaults as an output port. You must wait for the MCU to initialize.

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  • \$\begingroup\$ Hi, the default(power on) configuration is as input port. My concern is that after power up and before initialization of port as PWM output '1' logic at input pin (because of internal pull up) will drive all igbts in 3ph bridge causing dead short circuit. \$\endgroup\$ – chatty85 Aug 5 '14 at 14:30

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