multiple power sources on CMOS?

Discussion in 'The Projects Forum' started by Gadersd, Apr 21, 2014.

  1. Gadersd

    Thread Starter Member

    Dec 8, 2012
    98
    1
    I have a micro controller connected to a CMOS shift register. The shift register is powered by a separate battery than the micro controller. The micro controller powers on before the shift register. Would the floating vcc on the shift register cause problems when the micro controller powers on?
     
  2. BobTPH

    Active Member

    Jun 5, 2013
    783
    114
    Not if you set all the outputs going to the CMOS SR to low until the SR is powered. If one is high, it might end up powering the CMOS chip through protection diodes.

    Bob
     
  3. crutschow

    Expert

    Mar 14, 2008
    13,022
    3,235
    You can also put a resistor (1kΩ-10KΩ) is series with the SR inputs to limit any current during the time the SR is not powered.
     
  4. Gadersd

    Thread Starter Member

    Dec 8, 2012
    98
    1
    Would it damage the chip if the micro controller outputs a high signal to it before it turns on?
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,022
    3,235
    Not if the current is limited by a resistor as I suggested.
     
  6. Gadersd

    Thread Starter Member

    Dec 8, 2012
    98
    1
    I would like to place a 10K pull down resistor on the shift register input and a 10k series resistor from the input to the micro controller. I am worried that this won't supply the 4.1 volts needed to turn on the input. What can I change to make this work?
     
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