VLSI related question

Discussion in 'General Electronics Chat' started by uzicohen10, May 7, 2012.

  1. uzicohen10

    Thread Starter New Member

    May 7, 2012
    i have a question related to chips.
    should there be any problem to drive a signal into a powered off chip?
    that is, the chip is turned off (no power) but some of the digital signals that connected to it are toggling, for example due to communication to another chip that share the same bus.
  2. @android


    Dec 15, 2011
    Most probably there will not be any problem. It'll be like giving signal at the base of transistor which is not biased.
  3. crutschow


    Mar 14, 2008
    Depending upon the IC it can conduct current from it's input protection diodes to the V+ supply pins. I've seen CMOS circuits that will still operate without the power supply on when the clock is a low impedance source, such as a bench pulse generator due to this. The diode and power line capacitors act as rectifier to generate a plus voltage on the power bus. Rather a surprise when you first see it.

    So the input can look like a low impedance to the driving signal. Whether that's a problem or not depends upon the characteristics of the source and whether the source signal is still used by other powered inputs. If it's a problem you can add a series resistor between the source and each input to limit the current.